1.
Hubble Deep Field
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The Hubble Deep Field is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the whole sky, which is equivalent in angular size to a tennis ball at a distance of 100 metres. The image was assembled from 342 separate exposures taken with the Space Telescopes Wide Field, by revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe. Three years after the HDF observations were taken, a region in the celestial hemisphere was imaged in a similar way. The similarities between the two regions strengthened the belief that the universe is uniform over large scales and that the Earth occupies a region in the Universe. A wider but shallower survey was made as part of the Great Observatories Origins Deep Survey. In 2004 a deeper image, known as the Hubble Ultra-Deep Field, was constructed from a few months of light exposure. The HUDF image was at the time the most sensitive astronomical image ever made at visible wavelengths, the Medium Deep Survey used the Wide Field and Planetary Camera 2 to take deep images of random fields while other instruments were being used for scheduled observations. At the same time, other dedicated programs focused on galaxies that were known through ground-based observation. All of these studies revealed substantial differences between the properties of galaxies today and those that existed several years ago. Up to 10% of the HSTs observation time is designated as Directors Discretionary Time, a special Institute Advisory Committee recommended that the WFPC2 be used to image a typical patch of sky at a high galactic latitude, using several optical filters. A working group was set up to develop and implement the project, the field selected for the observations needed to fulfill several criteria. It had to be at a high latitude, because dust. These criteria restricted the field of potential target areas and it was decided that the target should be in Hubbles continuous viewing zones —the areas of sky which are not occulted by the Earth or the moon during Hubbles orbit. Twenty fields satisfying these criteria were identified, from which three optimal candidate fields were selected, all within the constellation of Ursa Major. The field that was selected is located at a right ascension of 12h 36m 49. 4s. The area is approximately 1/28,000,000 of the area of the sky. Once a field had been selected, a strategy had to be developed
2.
Astronomy
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Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets, while the phenomena include supernovae explosions, gamma ray bursts, more generally, all astronomical phenomena that originate outside Earths atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole, Astronomy is the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, during the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs can play an active role, especially in the discovery. Amateur astronomers have made and contributed to many important astronomical discoveries, Astronomy means law of the stars. Astronomy should not be confused with astrology, the system which claims that human affairs are correlated with the positions of celestial objects. Although the two share a common origin, they are now entirely distinct. Generally, either the term astronomy or astrophysics may be used to refer to this subject, however, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics, some titles of the leading scientific journals in this field includeThe Astronomical Journal, The Astrophysical Journal and Astronomy and Astrophysics. In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye, in some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye, most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the model of the Universe, or the Ptolemaic system. The Babylonians discovered that lunar eclipses recurred in a cycle known as a saros
3.
Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
4.
Galaxy
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A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally milky, Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxys center of mass. Galaxies are categorized according to their morphology as elliptical, spiral. Many galaxies are thought to have holes at their active centers. The Milky Ways central black hole, known as Sagittarius A*, has a four million times greater than the Sun. Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. The space between galaxies is filled with a gas having an average density of less than one atom per cubic meter. The majority of galaxies are organized into groups, clusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Heras breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing a baby, she pushes the baby away, some of her milk spills and. In the astronomical literature, the capitalized word Galaxy is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380, See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, all of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044. One of the arguments to do so is that these impressive objects deserve better than uninspired codes, for instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major
5.
Local Group
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The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies and its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group covers a diameter of 10 Mly and has a binary distribution, the group itself is a part of the larger Virgo Supercluster, which in turn may be a part of the Laniakea Supercluster. The three largest members of the group are the Andromeda Galaxy, the Milky Way and the Triangulum Galaxy, the larger two of these spiral galaxies each have their own system of satellite galaxies. The Triangulum Galaxy may or may not be a companion to the Andromeda Galaxy, pisces Dwarf is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either. The membership of NGC3109, with its companions Sextans A, the term The Local Group was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. He also identified IC10 as a possible Local Group member, by 2003, the number of known Local Group members had increased from his initial 12 to 36. Smiths Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth and 8,000 light years from the disk of the Milky Way galaxy HVC 127-41-330, updated Information on the Local Group. The Publications of the Astronomical Society of the Pacific
6.
Supernova remnant
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A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by a shock wave, and consists of ejected material expanding from the explosion. In either case, the supernova explosion expels much or all of the stellar material with velocities as much as 10% the speed of light. These ejecta are highly supersonic, assuming a typical temperature of the medium of 10,000 K. Therefore, a shock wave forms ahead of the ejecta. One of the best observed young supernova remnants was formed by SN 1987A, the youngest known remnant in our galaxy is G1. 9+0.3, discovered in the galactic center. An SNR passes through the stages as it expands, Free expansion of the ejecta. This can last tens to a few hundred years depending on the density of the surrounding gas, sweeping up of a shell of shocked circumstellar and interstellar gas. This begins the Sedov-Taylor phase, which can be modeled by a self-similar analytic solution. Strong X-ray emission traces the strong waves and hot shocked gas. Cooling of the shell, to form a thin, dense shell surrounding the hot interior and this is the pressure-driven snowplow phase. The shell can be seen in optical emission from recombining ionized hydrogen. The dense shell continues to expand from its own momentum and this stage is best seen in the radio emission from neutral hydrogen atoms. Merging with the interstellar medium. There are three types of supernova remnant, Shell-like, such as Cassiopeia A Composite, in which a shell contains a central pulsar wind nebula, mixed-morphology remnants, in which central thermal X-ray emission is seen, enclosed by a radio shell. The thermal X-rays are primarily from swept-up interstellar material, rather than supernova ejecta, examples of this class include the SNRs W28 and W44. Supernova remnants are considered the source of galactic cosmic rays. The connection between cosmic rays and supernovas was first suggested by Walter Baade and Fritz Zwicky in 1934 and this hypothesis is supported by a specific mechanism called shock wave acceleration based on Enrico Fermis ideas, which is still under development
7.
Stellar kinematics
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In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space. The subject of stellar kinematics encompasses the measurement of velocities in the Milky Way. Stellar kinematics is related to but distinct from the subject of stellar dynamics, the component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects, once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed. This is the actual motion relative to the Sun or the local standard of rest. The Suns motion with respect to the LSR is called the solar motion. The peculiar motion of the Sun with respect to the LSR is = km/s, with statistical uncertainty km/s, the stars in the Milky Way can be subdivided into two general populations, based on their metallicity, or proportion of elements with atomic numbers higher than helium. Among nearby stars, it has found that population I, higher metallicity stars have generally lower velocities than older. The latter have elliptical orbits that are inclined to the plane of the Milky Way, comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars share a common point of origin in giant molecular clouds. Within the Milky Way, there are three components of stellar kinematics, the disk, halo and bulge or bar. The halo may be subdivided into an inner and outer halo, with the inner halo having a net prograde motion with respect to the Milky Way. Depending on the definition, a high-velocity star is a star moving faster than 65 km/s to 100 km/s relative to the motion of the stars in the Suns neighbourhood. The velocity is sometimes defined as supersonic relative to the surrounding interstellar medium. The three types of high-velocity stars are, runaway stars, halo stars and hypervelocity stars, a runaway star is one that is moving through space with an abnormally high velocity relative to the surrounding interstellar medium. The proper motion of a star often points exactly away from a stellar association, of which the star was formerly a member. In the second scenario, an explosion in a multiple star system can result in the remaining components moving away at high speed. Although both mechanisms are possible, astronomers generally favour the supernova mechanism as more common in practice
8.
Galaxy group
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The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies. The Milky Way galaxy is part of a group of galaxies called the Local Group, Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs and their mass is approximately 1013 solar masses. The spread of velocities for the galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger, Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a range between those of the very large elliptical galaxies and clusters of galaxies. In the local universe, about half of the groups exhibit diffuse X-ray emissions from their intracluster media and those that emit X-rays appear to have early-type galaxies as members. The diffuse X-ray emissions come from zones within the inner 10-50% of the virial radius. There are several subtypes of groups, Compact groups are small groups in closely grouped in a small area. They typically have around 5 galaxies in close proximity relatively isolated from other galaxies, the first discovered, Stephans Quintet, was discovered in 1877. Though Stephans Quintet itself is a group of 4 and a foreground galaxy. Astronomer Paul Hickson created a catalogue of such groups in 1982, Compact groups of galaxies readily show the effect of dark matter, as the visible mass is greatly less than that needed to dynamically bind the galaxies into a bound group. Compact galaxy groups are also not dynamically stable over Hubble time, fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge, the physical process behind this galaxy-galaxy merger is dynamical friction. The time-scales for dynamical friction on luminous galaxies suggest that groups are old. Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies, fossil groups may still contain unmerged dwarf galaxies, but the more massive members of the group have condensed into the central galaxy. The closest fossil group to the Milky Way is NGC6482, proto-groups are groups that are in the process of formation. They are the form of protoclusters
9.
Galaxy cluster
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They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium, the ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as clusters, which are structures of stars within galaxies, or with globular clusters. Small aggregates of galaxies are referred to as groups of galaxies rather than clusters of galaxies, the groups and clusters can themselves cluster together to form superclusters. Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, a very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters. Galaxy clusters typically have the properties, They contain 100 to 1,000 galaxies, hot X-ray- emitting gas. Details are described in the Composition section, the distribution of the three components is approximately the same in the cluster. They have total masses of 1014 to 1015 solar masses and they typically have a diameter from 2 to 10 Mpc. The spread of velocities for the galaxies is about 800–1000 km/s. There are three components of a galaxy cluster. They are tabulated below, Stars, Star clusters, Galaxies, Galaxy clusters, Super clusters Abell catalogue Intracluster medium List of Abell clusters
10.
Supercluster
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A supercluster is a large group of smaller galaxy clusters or galaxy groups, which is among the largest-known structures of the cosmos. The Milky Way is part of the Local Group galaxy cluster and this supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The number of superclusters in the universe is estimated to be 10 million. Galaxies are grouped into clusters instead of being dispersed randomly, clusters of galaxies, in turn, are grouped together to form superclusters. Typically, superclusters contain dozens of individual clusters throughout an area of space about 150 million light-years across, unlike clusters, most superclusters are not bound together by gravity. The component clusters are generally shifting away from each due to the Hubble flow. The Milky Way galaxy falls within the Local Group, which is a poor, poor clusters may contain only a few dozen galaxies, as compared to rich clusters with hundreds or even thousands. The Local Group is near the Local Supercluster, which has a diameter of 100 million light-years, the Local Supercluster contains a total of about 1015 times the mass of the Sun. The biggest cluster in the universe is called the Great Attractor. Its gravity is so strong that the Local Supercluster, including the Milky Way, is moving in a direction towards it at a rate of several hundred kilometers per second, the biggest supercluster outside of the local universe is the Perseus-Pegasus Filament. It contains the Perseus supercluster and it spans about a billion light-years, research has tried to understand the way superclusters are arranged in space. Maps are used to display the positions of 1.6 million galaxies, three-dimensional maps are used to further understand the positions of these superclusters. To map them three-dimensionally, the position of the galaxy in the sky as well as the galaxys redshift are used for calculation, the galaxys redshift is used with Hubbles law to determine its position in three-dimensional space. It was discovered from those maps that superclusters of galaxies are not spread uniformly across the universe, maps reveal huge voids where there are extremely few galaxies. Some dim galaxies or hydrogen clouds can be found in some voids, the voids themselves are often spherical but the superclusters are not. They can range from being 100 million to 400 million light-years in diameter, the pattern of sheets and voids contains information about how galaxy clusters formed in the early universe. Those groups and clusters and additional isolated galaxies in turn form even larger structures called superclusters and their existence was first postulated by George Abell in his 1958 Abell catalogue of galaxy clusters. He called them second-order clusters, or clusters of clusters and these are the largest known structures to date
11.
Galaxy filament
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In physical cosmology, galaxy filaments are the largest known structures in the universe. They are massive, thread-like formations, with a length of 50 to 80 megaparsecs h−1 that form the boundaries between large voids in the universe. Filaments consist of gravitationally bound galaxies, parts wherein many galaxies are very close to one another are called superclusters. In the standard model of the evolution of the universe, galactic filaments form along and it is thought that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts baryonic matter, and it is this matter that astronomers see forming long. Discovery of structures larger than superclusters began in the late-1980s, in 1987, astronomer R. Brent Tully of the University of Hawaiis Institute of Astronomy identified what he called the Pisces–Cetus Supercluster Complex. In 1989, the CfA2 Great Wall was discovered, followed by the Sloan Great Wall in 2003, filament subtype of filaments have roughly similar major and minor axes in cross-section, along the lengthwise axis. A short filament, detected by identifying an alignment of star-forming galaxies, in the neighborhood of the Milky Way and the Local Group was proposed by Adi Zitrin and Noah Brosch. The reality of this filament, and the identification of a similar, the galaxy wall subtype of filaments have a significantly greater major axis than minor axis in cross-section, along the lengthwise axis. A wall was proposed to be the embodiment of the Great Attractor. It is sometimes referred to as the Great Attractor Wall or Norma Wall and this suggestion was superseded by the proposal of a supercluster, Laniakea, that would encompass the Great Attractor, Virgo Supercluster, Hydra-Centaurus Superclusters. A wall was proposed in 2000 to lie at z=1.47 in the vicinity of radio galaxy B3 0003+387, a wall was proposed in 2000 to lie at z=0.559 in the northern Hubble Deep Field. Large quasar groups are some of the largest structures known and they are theorized to be protohyperclusters/proto-supercluster-complexes/galaxy filament precursors. Kevin A. Pimbblet, Pulling Out Threads from the Cosmic Tapestry, pictures of the filamentary network The Universe Within One Billion Light Years with List of Nearby Superclusters
12.
Active galactic nucleus
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Such excess non-stellar emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy, the radiation from an AGN is believed to be a result of accretion of matter by a supermassive black hole at the center of its host galaxy. Numerous subclasses of AGN have been defined based on their observed characteristics, early photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, in 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. Galaxies observed as part of study included NGC1068, NGC4151, NGC3516. Active galaxies such as these are known as Seyfert galaxies in honor of Seyferts pioneering work, the development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87, the 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, a major breakthrough was the measurement of the redshift of the quasar 3C273 by Maarten Schmidt, published in 1963. Shortly afterward, optical spectra were used to measured the redshifts of a number of quasars including 3C48. The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars, Accretion of gas onto a supermassive black hole was suggested as the source of quasars power in papers by Edwin Salpeter and Yakov ZelDovich in 1964. Today, AGN are a topic of astrophysical research, both observational and theoretical. For a long time it has argued that an AGN must be powered by accretion of mass onto massive black holes. AGN are both compact and persistently extremely luminous, thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole. In the standard model of AGN, cold material close to a black hole forms an accretion disc, dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The radiation from the accretion disc excites cold atomic material close to the black hole, a large fraction of the AGNs radiation may be obscured by interstellar gas and dust close to the accretion disc, but this will be re-radiated at some other waveband, most likely the infrared. Some accretion discs produce jets of twin, highly collimated, the direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low, there exists a class of radiatively inefficient solutions to the equations that govern accretion. The most widely known of these is the Advection Dominated Accretion Flow, radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc
13.
Quasar
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A quasar is an active galactic nucleus of very high luminosity. A quasar consists of a black hole surrounded by an orbiting accretion disk of gas. As gas in the accretion disk falls toward the black hole, quasars emit energy across the electromagnetic spectrum and can be observed at radio, infrared, visible, ultraviolet, and X-ray wavelengths. The most powerful quasars have luminosities exceeding 1041 W, thousands of greater than the luminosity of a large galaxy such as the Milky Way. Quasars are found over a broad range of distances. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, as of 2011, the most distant known quasar is at redshift z=7.085, light observed from this quasar was emitted when the Universe was only 770 million years old. Because quasars are distant objects, any light which reaches the Earth is redshifted due to the expansion of space. In early optical images, quasars appeared as point sources, indistinguishable from stars, with infrared telescopes and the Hubble Space Telescope, the host galaxies surrounding the quasars have been detected in some cases. These galaxies are normally too dim to be seen against the glare of the quasar, most quasars, with the exception of 3C273 whose average apparent magnitude is 12.9, cannot be seen with small telescopes. The luminosity of some quasars changes rapidly in the optical range, because these changes occur very rapidly they define an upper limit on the volume of a quasar, quasars are not much larger than the Solar System. This implies a high power density. The mechanism of brightness changes probably involves relativistic beaming of astrophysical jets pointed nearly directly toward Earth, the highest redshift quasar known is ULAS J1120+0641, with a redshift of 7.085, which corresponds to a comoving distance of approximately 29 billion light-years from Earth. Since light cannot escape the black holes, the energy is actually generated outside the event horizon by gravitational stresses. Central masses of 105 to 109 solar masses have been measured in quasars by using reverberation mapping. The matter accreting onto the hole is unlikely to fall directly in. Quasars may also be ignited or re-ignited when normal galaxies merge, in fact, it has been suggested that a quasar could form as the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3–5 billion years. More than 200,000 quasars are known, most from the Sloan Digital Sky Survey, all observed quasar spectra have redshifts between 0.056 and 7.085. Applying Hubbles law to these redshifts, it can be shown that they are between 600 million and 28.85 billion light-years away
14.
Radio galaxy
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Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process, the observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large elliptical galaxies, radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work has been done on the effects of objects on the intergalactic medium, particularly in galaxy groups. The radio emission from radio-loud active galaxies is synchrotron emission, as inferred from its smooth, broad-band nature. This implies that the plasma contains, at least, electrons with relativistic speeds. Since the plasma must be neutral, it must also contain either protons or positrons, there is no way of determining the particle content directly from observations of synchrotron radiation. A sister process to the radiation is the inverse-Compton process, in which the relativistic electrons interact with ambient photons. This has been used to argue that many sources are actually quite near the minimum-energy condition. In the latter case the electrons must have energies in excess of 1 TeV in typical magnetic field strengths. Again, polarization and continuum spectrum are used to distinguish the synchrotron radiation from other emission processes, jets and hotspots are the usual sources of high-frequency synchrotron emission. It is hard to distinguish observationally between the synchrotron and inverse-Compton radiation, making them a subject of ongoing research, processes, collectively known as particle acceleration, produce populations of relativistic and non-thermal particles that give rise to synchrotron and inverse-Compton radiation. Fermi acceleration is one plausible particle acceleration process in radio-loud active galaxies, radio galaxies, and to a lesser extent, radio-loud quasars display a wide range of structures in radio maps. The most common structures are called lobes, these are double, often fairly symmetrical. A significant minority of low-luminosity sources exhibit structures usually known as plumes which are more elongated. Some radio galaxies show one or two long narrow features known as coming directly from the nucleus and going to the lobes. Since the 1970s, the most widely accepted model has been that the lobes or plumes are powered by beams of high-energy particles and magnetic field coming from close to the active nucleus. The jets are believed to be the manifestations of the beams
15.
Supernova
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This causes the sudden appearance of a new bright star, before slowly fading from sight over several weeks or months. Supernovae are more energetic than novae, in Latin, nova means new, referring astronomically to what appears to be a temporary new bright star. Adding the prefix super- distinguishes supernovae from ordinary novae, which are far less luminous, the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. It is pronounced /ˌsuːpərnoʊvə/ with the plural supernovae /ˌsuːpərnoʊviː/ or supernovas, only three Milky Way naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies using telescopes. The most recent directly observed supernova in the Milky Way was Keplers Supernova in 1604, Supernovae may expel much, if not all, of the material away from a star, at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the interstellar medium, and in turn, sweeping up an expanding shell of gas and dust. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis, Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of primary cosmic rays. They are also potentially strong galactic sources of gravitational waves, in the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time, in Latin, Nova means new, referring astronomically to what appears to be a temporary new bright star. Adding the prefix super- distinguishes supernovae from ordinary novae, which are far less luminous, the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. It is pronounced /ˌsuːpərnoʊvə/ with the plural supernovae /ˌsuːpərnoʊviː/ or supernovas, the earliest recorded supernova, SN185, was viewed by Chinese astronomers in 185 AD. The brightest recorded supernova was SN1006, which occurred in 1006 AD and was described in detail by Chinese, the widely observed supernova SN1054 produced the Crab Nebula. Johannes Kepler began observing SN1604 at its peak on October 17,1604 and it was the second supernova to be observed in a generation. There is some evidence that the youngest galactic supernova, G1. 9+0.3, occurred in the late 19th century, neither supernova was noted at the time. In the case of G1. 9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed, the situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of especially high extinction, before the development of the telescope, there have only been five supernovae seen in the last millennium
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Intergalactic star
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An intergalactic star, also known as a rogue star, is a star not gravitationally bound to a galaxy. The common belief that stars exist only in galaxies was disproven in 1997 with the discovery of intergalactic stars, the first to be discovered were in the Virgo cluster of galaxies, where some one trillion are now surmised to exist. The missing baryon problem is identified by many theories of formation and evolution of the universe, which point out that there should be many more baryons than those that scientists have found. The way these stars arise is still a mystery, but the most common theory is that the collision of two or more galaxies can toss some stars out into the vast empty regions of intergalactic space. Although stars normally reside within galaxies, they can be extracted by gravitational forces when galaxies collide and it is commonly believed that these intergalactic stars may also have come from extremely small galaxies, since it is easier for stars to escape a smaller galaxys gravitational pull. Another theory states that stars can be ejected from their galaxy by supermassive black holes, the star that has been ejected from the galaxys center is travelling at high speeds to escape the gravitational pull of the galaxy, and is now called a hypervelocity star. In 1997, the Hubble telescope discovered a number of intergalactic stars in the Virgo cluster of galaxies. Later in the 1990s scientists discovered another group of stars in the Fornax cluster of galaxies. Recently astronomers have noticed a faint glow that may be a product of intergalactic stars, as CIBER soared above the atmosphere, it looked at five different regions in space for about a minute each, gathering as many particles of cosmic light as possible. The light must be coming from something closer and more modern, some Vanderbilt astronomers report that they have identified more than 675 stars at the edge of the Milky Way, between the Andromeda Galaxy and the Milky Way. They argue that these stars are hypervelocity stars that were ejected from the Milky Ways galactic center and these stars are red giants with a high metallicity indicating an inner galactic origin, since stars outside the disks of galaxies tend to have low metallicity and are older. But the speeds found are only estimated minimums, as in reality their speeds may be larger than the speeds found by the researchers. One of the exiles is moving in the direction of the constellation Ursa Major at about 1.25 million mph with respect to the galaxy. The other is headed toward the constellation Cancer, outbound at 1.43 million miles per hour and 180,000 light-years away, some recently discovered supernovae have been confirmed to have exploded hundreds of light years from the nearest star or galaxy. Although the precise mass of these stars cannot be known, it is estimated that together make up 10 percent of the mass of the Virgo cluster of galaxies. This means that, most likely, these stars collectively have a larger mass than any one of the 2500 galaxies that form the Virgo cluster. The first intergalactic stars were discovered in the Virgo cluster of galaxies and these stars form a massive group approximately 300,000 light years away from the nearest galaxy. Approximately 675 rogue stars have been discovered at the edge of our galaxy, blue straggler HE 0437-5439 Intergalactic dust Rogue planet, or interstellar planet Stellar kinematics
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Observable universe
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There are at least two trillion galaxies in the observable universe, containing more stars than all the grains of sand on planet Earth. Assuming the universe is isotropic, the distance to the edge of the universe is roughly the same in every direction. That is, the universe is a spherical volume centered on the observer. Every location in the Universe has its own universe, which may or may not overlap with the one centered on Earth. The word observable used in this sense does not depend on modern technology actually permits detection of radiation from an object in this region. It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth, in practice, we can see light only from as far back as the time of photon decoupling in the recombination epoch. That is when particles were first able to emit photons that were not quickly re-absorbed by other particles, before then, the Universe was filled with a plasma that was opaque to photons. The detection of gravitational waves indicates there is now a possibility of detecting signals from before the recombination epoch. The surface of last scattering is the collection of points in space at the distance that photons from the time of photon decoupling just reach us today. These are the photons we detect today as cosmic microwave background radiation, however, with future technology, it may be possible to observe the still older relic neutrino background, or even more distant events via gravitational waves. It is estimated that the diameter of the universe is about 28.5 gigaparsecs. The total mass of matter in the universe can be calculated using the critical density. Some parts of the Universe are too far away for the light emitted since the Big Bang to have had time to reach Earth. In the future, light from distant galaxies will have had time to travel. This fact can be used to define a type of cosmic event horizon whose distance from the Earth changes over time, both popular and professional research articles in cosmology often use the term universe to mean observable universe. It is plausible that the galaxies within our observable universe represent only a fraction of the galaxies in the Universe. If the Universe is finite but unbounded, it is possible that the Universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies and it is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different
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Galaxy formation and evolution
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Galaxy formation is hypothesized to occur, from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. Because of the inability to conduct experiments in space, the only way to “test” theories. Explanations for how galaxies formed and evolved must be able to predict the properties and types of galaxies. Edwin Hubble created the first galaxy classification scheme known as the Hubble tuning-fork diagram and it partitioned galaxies into ellipticals, normal spirals, barred spirals, and irregulars. These groups divide into blue star-forming galaxies that are more like spiral types, spiral galaxies are quite thin, dense, and rotate relatively fast, while the stars in elliptical galaxies have randomly-oriented orbits. The majority of mass in galaxies is made up of matter, a substance which is not directly observable. The majority of giant galaxies contain a black hole in their centers. The black hole mass is tied to the host galaxy bulge or spheroid mass, metallicity has a positive correlation with the absolute magnitude of a galaxy. This is not the case instead the tuning fork diagram shows an evolution from simple to complex with no temporal connotations intended, astronomers now believe that disk galaxies likely formed first, then evolved into elliptical galaxies through galaxy mergers. The earliest stage in the evolution of galaxies is the formation, when a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like arm structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter, however, at present, olin Eggen, Donald Lynden-Bell, and Allan Sandage in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud. The distribution of matter in the universe was in clumps that consisted mostly of dark matter. These clumps interacted gravitationally, putting tidal torques on each other that acted to give them some angular momentum, as the baryonic matter cooled, it dissipated some energy and contracted toward the center. With angular momentum conserved, the matter near the center speeds up its rotation, then, like a spinning ball of pizza dough, the matter forms into a tight disk. Once the disk cools, the gas is not gravitationally stable and it breaks, and these smaller clouds of gas form stars. Since the dark matter does not dissipate as it only interacts gravitationally, observations show that there are stars located outside the disk, which does not quite fit the pizza dough model. It was first proposed by Leonard Searle and Robert Zinn that galaxies form by the coalescence of smaller progenitors, known as a top-down formation scenario, this theory is quite simple yet no longer widely accepted. More recent theories include the clustering of dark matter halos in the bottom-up process and this still results in disk-like distributions of baryonic matter with dark matter forming the halo for all the same reasons as in the top-down theory
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Observational cosmology
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Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors. This was precipitated by observations that established the size and the dynamics of the cosmos that could be explained by Einsteins General Theory of Relativity. In its infancy, cosmology was a science based on a very limited number of observations. Distance measurements in astronomy have historically been and continue to be confounded by considerable measurement uncertainty, Hubble used the 100-inch Hooker Telescope at Mount Wilson Observatory to identify individual stars in those galaxies, and determine the distance to the galaxies by isolating individual Cepheids. This firmly established the spiral nebula as being well outside the Milky Way galaxy. Determining the distance to island universes, as they were dubbed in the media, established the scale of the universe and settled the Shapley-Curtis debate once. He showed that this was expected in a universe model based on general relativity. Two years later, Hubble showed that the relation between the distances and velocities was a correlation and had a slope of about 500 km/s/Mpc. This correlation would come to be known as Hubbles law and would serve as the foundation for the expanding universe theories on which cosmology is still based. For example, the element Helium was first identified through its spectroscopic signature in the Sun before it was isolated as a gas on Earth, computing relative abundances was achieved through corresponding spectroscopic observations to measurements of the elemental composition of meteorites. A cosmic microwave background was predicted in 1948 by George Gamow and Ralph Alpher, moreover, Alpher and Herman were able to estimate the temperature, but their results were not widely discussed in the community. G. Doroshkevich and Igor Novikov, in the spring of 1964, in 1964, David Todd Wilkinson and Peter Roll, Dickes colleagues at Princeton University, began constructing a Dicke radiometer to measure the cosmic microwave background. Their instrument had an excess 3.5 K antenna temperature which they could not account for, after receiving a telephone call from Crawford Hill, Dicke famously quipped, Boys, weve been scooped. A meeting between the Princeton and Crawford Hill groups determined that the temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery, today, observational cosmology continues to test the predictions of theoretical cosmology and has led to the refinement of cosmological models. For example, the evidence for dark matter has heavily influenced theoretical modeling of structure. When trying to calibrate the Hubble diagram with accurate supernova standard candles and these observations have been incorporated into a six-parameter framework known as the Lambda-CDM model which explains the evolution of the universe in terms of its constituent material. This model has subsequently been verified by detailed observations of the microwave background
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Outline of astronomy
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Astronomy can be described as all the following, An academic discipline, one with academic departments, curricula and degrees, national and international societies, and specialized journals. A scientific field – widely recognized category of specialized expertise within science, a natural science – one that seeks to elucidate the rules that govern the natural world using empirical and scientific methods. A branch or field of space science A hobby or part-time pursuit for the satisfaction of personal curiosity or appreciation of beauty, astrobiology – studies the advent and evolution of biological systems in the universe. Astrophysics – branch of astronomy deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions. Physical cosmology – origin and evolution of the universe as a whole, the study of cosmology is theoretical astrophysics at its largest scale. Computational astrophysics – The study of astrophysics using computational methods and tools to develop computational models, galactic astronomy – deals with the structure and components of our galaxy and of other galaxies. High energy astrophysics – studies phenomena occurring at high energies including active galactic nuclei, supernovae, gamma-ray bursts, quasars, interstellar astrophysics – study of the interstellar medium, intergalactic medium and dust. Extragalactic astronomy – study of objects outside our galaxy, including Galaxy formation and evolution, stellar astronomy – concerned with Star formation, physical properties, main sequence life span, variability, stellar evolution and extinction. Plasma astrophysics – studies properties of plasma in outer space, relativistic astrophysics – studies effects of special relativity and general relativity in astrophysical contexts including gravitational waves, gravitational lensing and black holes. Solar physics – Sun and its interaction with the remainder of the Solar System, Planetary Science – study of planets, moons, and planetary systems. Atmospheric science – study of atmospheres and weather, exoplanetology – various planets outside of the Solar System Planetary formation – formation of planets and moons in the context of the formation and evolution of the Solar System. Astronomy divided by general technique used for research, Astrometry – study of the position of objects in the sky. Defines the system of coordinates used and the kinematics of objects in our galaxy, observational astronomy – practice of observing celestial objects by using telescopes and other astronomical apparatus. It is concerned with recording data. 7–350 µm Optical astronomy – 380–750 nm Ultraviolet astronomy – 10–320 nm X-ray astronomy –0. 01–10 nm Gamma-ray astronomy – Below 0, a total of 899 such planets have been identified as of 27 June 2013. Astronomy Net Resources, forums, articles on Astronomy. org - home of NASAs Hubble Space Telescope Astronomy - A History - G. Forbes -1909 Prof, sir Harry Kroto, NL, Astrophysical Chemistry Lecture Series. 8 Freeview Lectures provided by the Vega Science Trust, core books and core journals in Astronomy, from the Smithsonian/NASA Astrophysics Data System
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History of astronomy
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In some cultures, astronomical data was used for astrological prognostication. Early cultures identified celestial objects with gods and spirits and they related these objects to phenomena such as rain, drought, seasons, and tides. Ancient structures with possibly astronomical alignments probably fulfilled astronomical, religious, calendars of the world have often been set by observations of the Sun and Moon, and were important to agricultural societies, in which the harvest depended on planting at the correct time of year. Since 1990 our understanding of prehistoric Europeans has been changed by discoveries of ancient astronomical artifacts throughout Europe. The artifacts demonstrate that Neolithic and Bronze Age Europeans had a knowledge of mathematics. Among the discoveries are, Bone sticks from locations like Africa, the Warren Field calendar in the Dee River valley of Scotlands Aberdeenshire. The calendar takes the form of an early Mesolithic monument containing a series of 12 pits which appear to help the observer track lunar months by mimicking the phases of the moon. It also aligns to sunrise at the solstice, thus coordinating the solar year with the lunar cycles. Goseck circle is located in Germany and belongs to the pottery culture. First discovered in 1991, its significance was only clear after results from archaeological digs became available in 2004. The site is one of hundreds of similar circular enclosures built in a region encompassing Austria, Germany, the Nebra sky disc is a Bronze Age bronze disc that was buried in Germany, not far from the Goseck circle, around 1600 BC. It measures about 30 cm diameter with a mass of 2.2 kg, found by archeological thieves in 1999 and recovered in Switzerland in 2002, it was soon recognized as a spectacular discovery, among the most important of the 20th century. Investigations revealed that the object had been in use around 400 years before burial, the inlaid gold depicted the full moon, a crescent moon about 4 or 5 days old, and the Pleiades star cluster in a specific arrangement forming the earliest known depiction of celestial phenomena. Twelve lunar months pass in 354 days, requiring a calendar to insert a leap month every two or three years in order to keep synchronized with the years seasons. The earliest known descriptions of this coordination were recorded by the Babylonians in 6th or 7th centuries BC, a Bronze Age astronomical observatory was constructed there around 1900 BC and continuously served the nearby community that lived there until about 700 BC. The central space was used to observe the rising of the sun, three markings locate sunrise at the summer and winter solstices and at the two equinoxes. Four more give the minimum and maximum declinations of the moon, in summer. Two measure the lengths of lunar months, together, they reconcile solar and lunar cycles in marking the 235 lunations that occur during 19 solar years, regulating a lunar calendar
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Astronomer
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An astronomer is a scientist in the field of astronomy who concentrates their studies on a specific question or field outside of the scope of Earth. They look at stars, planets, moons, comets and galaxies, as well as other celestial objects — either in observational astronomy. Examples of topics or fields astronomers work on include, planetary science, solar astronomy, there are also related but distinct subjects like physical cosmology which studies the Universe as a whole. Astronomers usually fit into two types, Observational astronomers make direct observations of planets, stars and galaxies, and analyze the data, theoretical astronomers create and investigate models of things that cannot be observed. They use this data to create models or simulations to theorize how different celestial bodies work, there are further subcategories inside these two main branches of astronomy such as planetary astronomy, galactic astronomy or physical cosmology. Today, that distinction has disappeared and the terms astronomer. Professional astronomers are highly educated individuals who typically have a Ph. D. in physics or astronomy and are employed by research institutions or universities. They spend the majority of their time working on research, although quite often have other duties such as teaching, building instruments. The number of astronomers in the United States is actually quite small. The American Astronomical Society, which is the organization of professional astronomers in North America, has approximately 7,000 members. This number includes scientists from other such as physics, geology. The International Astronomical Union comprises almost 10,145 members from 70 different countries who are involved in research at the Ph. D. level. Before CCDs, photographic plates were a method of observation. Modern astronomers spend relatively little time at telescopes usually just a few weeks per year, analysis of observed phenomena, along with making predictions as to the causes of what they observe, takes the majority of observational astronomers time. Astronomers who serve as faculty spend much of their time teaching undergraduate and graduate classes, most universities also have outreach programs including public telescope time and sometimes planetariums as a public service to encourage interest in the field. Those who become astronomers usually have a background in maths, sciences. Taking courses that teach how to research, write and present papers are also invaluable, in college/university most astronomers get a Ph. D. in astronomy or physics. Keeping in mind how few there are it is understood that graduate schools in this field are very competitive
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Astronomical symbols
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Astronomical symbols are symbols used to represent astronomical objects, theoretical constructs and observational events in astronomy. The earliest forms of symbols appear in Greek papyri of late antiquity. The Byzantine codices in which the Greek papyri were preserved continued and extended the inventory of astronomical symbols, new symbols were further invented to represent many just-discovered planets and minor planets discovered in the 18th-20th centuries. All these symbols were commonly used by professional astronomers, amateur astronomers. Unicode has formally assigned codepoints to most symbols, mainly in Miscellaneous Symbols Block and Miscellaneous Symbols, the use of astronomical symbols for the Sun and Moon dates to antiquity. The forms of the symbols appear in the original papyri of Greek horoscopes are a circle with one ray for the Sun. The modern sun symbol, a circle with a dot, first appeared in Europe in the Renaissance and these symbols are also known to be used in alchemy texts, sun was representing gold, moon representing silver and so on. In modern academic usage, the sun symbol is used for astronomical constants relating to the sun, the luminosity, mass, and radius of stars are often represented using the corresponding solar constants as units of measurement. Symbols for the planets appear in the medieval Byzantine codices in which many ancient horoscopes were preserved. The written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri, the symbols for Jupiter and Saturn are identified as monograms of the corresponding Greek names, and the symbol for Mercury is a stylized caduceus. These cross-marks first appear around the 16th century, according to Maunder, the addition of crosses appears to be an attempt to give a savour of Christianity to the symbols of the old pagan gods. The symbols for Uranus were created shortly after its discovery, one symbol, invented by J. G. This symbol also combines the symbols of Mars and the Sun because in Greek Mythology, Uranus represented heaven, and represents the power of Mars spear. Another symbol, was suggested by Lalande in 1784, in a letter to Herschel, Lalande described it as un globe surmonté par la première lettre de votre nom. Several symbols were proposed for Neptune to accompany the suggested names for the planet. In October, he sought to name the planet Leverrier, after himself, and he had support in this from the observatory director, François Arago. However, this met with stiff resistance outside France. French almanacs quickly reintroduced the name Herschel for Uranus, after that planets discoverer Sir William Herschel, professor James Pillans of the University of Edinburgh defended the name Janus for the new planet, and proposed a key for its symbol
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Glossary of astronomy
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This page is a glossary of astronomy. This scientific study is concerned with objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy has a vocabulary and a significant amount of jargon. Absolute magnitude is a measure of a stars absolute brightness and it is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light years. Accretion disk is a circular mass of diffuse material in orbit around a central object. The material has acquired from a source external to the central object. Active galactic nucleus Albedo feature is an area on the surface of a reflecting object that shows a contrast in brightness or darkness with adjacent areas. Am star is a chemically peculiar star belonging to the general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals, apoapsis is the point of furthest excursion, or separation, between two orbiting objects. Apparent magnitude is a measure of the brightness of a body as seen by an observer on Earth. The brighter the object appears, the lower the value of its magnitude, appulse is the closest approach of one celestial object to another, as viewed from a third body. Asterism is a pattern of stars recognized on Earths night sky and it may form part of an official constellation, or be composed of stars from more than one. Astrometric binary is a type of system where evidence for an unseen orbiting companion is revealed by its periodic gravitational perturbation of the visible component. Astronomical unit, or AU, is the distance between the midpoints of the Earth and the Sun. Autumnal equinox is the point in the year when the Sun appears to cross the celestial equator and it represents the moment when the North Pole of the Earth begins to tilt away from the Sun. Azimuth is a measurement of an objects orientation along the horizon of the observer. When combined with the altitude above the horizon, it defines an objects current position in the coordinate system. Black hole is a concentration of mass so compact that it creates a region of space from which not even light can escape, the outer boundary of this region is called the event horizon
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Amateur astronomy
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Amateur astronomy is a hobby whose participants enjoy watching the sky, and the abundance of objects found in it with the unaided eye, binoculars, or telescopes. Many amateurs are beginners or hobbyists, while others have a degree of experience in astronomy and often assist. Amateur astronomers often look at the sky using nothing more than their eyes, people have studied the sky throughout history in an amateur framework, without any formal method of funding. It is only within about the past century, however, that amateur astronomy has become an activity clearly distinguished from professional astronomy, collectively, amateur astronomers observe a variety of celestial objects and phenomena. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep sky objects such as clusters, galaxies. Many amateurs like to specialise in observing particular objects, types of objects, one branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Most amateur astronomers work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum, non-visual amateur astronomy includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. Some amateur astronomers use home-made radio telescopes, while others use radio telescopes that were built for astronomy research but have since been made available for use by amateurs. The One-Mile Telescope is one such example, Amateur astronomers use a range of instruments to study the sky, depending on a combination of their interests and resources. Commercial telescopes are available, new and used, but it is common for amateur astronomers to build their own custom telescopes. Some people even focus on amateur telescope making as their primary interest within the hobby of amateur astronomy, although specialized and experienced amateur astronomers tend to acquire more specialized and more powerful equipment over time, relatively simple equipment is often preferred for certain tasks. Amateur astronomers also use star charts that, depending on experience and intentions, Amateur astronomers often like to keep records of their observations, which usually takes the form of an observing log. Observing logs typically record details about which objects were observed and when, sketching is sometimes used within logs, and photographic records of observations have also been used in recent times. The information gathered is used to help studies and interactions between amateur astronomers in yearly gatherings, although not professional information or credible, it is a way for the hobby lovers to share their new sightings and experiences. The Internet is a tool of amateur astronomers. The popularity of CCD imaging among amateurs has led to large numbers of web sites being written by individuals about their images, much of the social interaction of amateur astronomy occurs on mailing lists or discussion groups. Discussion group servers host numerous astronomy lists, a great deal of the commerce of amateur astronomy, the buying and selling of equipment, occurs online. Many amateurs use online tools to plan their observing sessions using tools such as the Clear Sky Chart
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Observational astronomy
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It is the practice of observing celestial objects by using telescopes and other astronomical apparatus. As a science, the study of astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible, however, this is partly compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined. This allows for data to be plotted on graphs. Nearby examples of phenomena, such as variable stars, can then be used to infer the behavior of more distant representatives. Those distant yardsticks can then be employed to measure other phenomena in that neighborhood, galileo Galilei turned a telescope to the heavens and recorded what he saw. Since that time, observational astronomy has made steady advances with each improvement in telescope technology, visible-light astronomy falls in the middle of this range. Infrared astronomy deals with the detection and analysis of infrared radiation, the most common tool is the reflecting telescope but with a detector sensitive to infrared wavelengths. Space telescopes are used at wavelengths where the atmosphere is opaque. Radio astronomy detects radiation of millimetre to decametre wavelength, the receivers are similar to those used in radio broadcast transmission but much more sensitive. High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, and extreme UV astronomy, in addition to using electromagnetic radiation, modern astrophysicists can also make observations using neutrinos, cosmic rays or gravitational waves. Observing a source using multiple methods is known as multi-messenger astronomy, optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is relatively transparent at the wavelengths being detected. Observatories are usually located at high altitudes so as to minimise the absorption and distortion caused by the Earths atmosphere, some wavelengths of infrared light are heavily absorbed by water vapor, so many infrared observatories are located in dry places at high altitude, or in space. Powerful gamma rays can, however be detected by the air showers they produce. For much of the history of astronomy, almost all observation was performed in the visual spectrum with optical telescopes. The seeing conditions depend on the turbulence and thermal variations in the air, locations that are frequently cloudy or suffer from atmospheric turbulence limit the resolution of observations. Likewise the presence of the full Moon can brighten up the sky with scattered light, for observation purposes, the optimal location for an optical telescope is undoubtedly in outer space. There the telescope can make observations without being affected by the atmosphere, however, at present it remains costly to lift telescopes into orbit. Thus the next best locations are certain mountain peaks that have a number of cloudless days
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Sidewalk astronomy
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Sidewalk or street corner astronomy refers to the activity of setting up a telescope in an urban setting on a for profit or non-profit basis as an entertainment and/or for public education. Examples of people setting up telescopes on urban streets for public astronomical viewing go back well into the 19th century, Sidewalk astronomers and their telescopes have been a common sight in most big cities. Some sidewalk pitch men charged the public to view objects through their telescope. In more recent years sidewalk astronomy has come to be associated with altruistic individuals or groups offering views of the nighttime sky as a free public educational service. During the day, sidewalk astronomy often includes use of a filter on the telescope to allow people to view the sun. Telescopes are often larger than the average available department store telescope and this usually involves individual or a groups of amateur astronomers with small-to-medium-sized telescopes. Dobson even promoted a design for large reflecting telescope that he called a Sidewalk Telescope
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Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
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Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
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Radio astronomy
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Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an object was in 1932. Subsequent observations have identified a number of different sources of radio emission and these include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy. Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources, in the 1860s, James Clerk Maxwells equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. These attempts were unable to detect any emission due to limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions, using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky originally suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis showed that the source was not following the 24-hour daily cycle of the Sun exactly and he concluded that since the Sun were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy. Jansky announced his discovery in 1933 and he wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of astronomy have been recognized by the naming of the fundamental unit of flux density. Grote Reber was inspired by Janskys work, and built a radio telescope 9m in diameter in his backyard in 1937. He began by repeating Janskys observations, and then conducted the first sky survey in the radio frequencies, on February 27,1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun. Later that year George Clark Southworth, at Bell Labs like Jansky, both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first. Several other people independently discovered solar radiowaves, including E. Schott in Denmark, at Cambridge University, where ionospheric research had taken place during World War II, J. A. This early research soon branched out into the observation of celestial radio sources. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis, the radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s
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Submillimetre astronomy
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Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and a millimetre and it is still common in submillimetre astronomy to quote wavelengths in microns, the old name for micrometre. Using submillimetre observations, astronomers examine molecular clouds and dark cloud cores with a goal of clarifying the process of formation from earliest collapse to stellar birth. Submillimetre observations of dark clouds can be used to determine chemical abundances. In addition, submillimetre observations give information on the mechanisms for the formation and evolution of galaxies, the most significant limitation to the detection of astronomical emission at submillimetre wavelengths with ground based observatories is atmospheric emission, noise and attenuation. Like the infrared, the atmosphere is dominated by numerous water vapour absorption bands. The ideal submillimetre observing site is dry, cool, has stable weather conditions and is away from population centres. There are only a handful of sites identified, they include Mauna Kea, the Llano de Chajnantor Observatory on the Atacama Plateau, the South Pole. Comparisons show that all four sites are excellent for submillimetre astronomy, there has been some recent interest in high-altitude Arctic sites, particularly Summit Station in Greenland where the PWV measure is always better than at Mauna Kea. The Submillimeter Array is another interferometer, located at Mauna Kea, the largest existing submillimetre telescope, the James Clerk Maxwell Telescope, is also located on Mauna Kea. With high-altitude balloons and aircraft, one can get even more of the atmosphere. The BLAST experiment and SOFIA are two examples, respectively, although SOFIA can also handle near infrared observations, space-based observations at the submillimetre wavelengths remove the ground-based limitations of atmospheric absorption. The Submillimeter Wave Astronomy Satellite was launched into low Earth orbit on December 5,1998 as one of NASAs Small Explorer Program missions, the mission of the spacecraft is to make targeted observations of giant molecular clouds and dark cloud cores. The focus of SWAS is five spectral lines, water, isotopic water, isotopic carbon monoxide, molecular oxygen, the SWAS satellite was repurposed in June,2005 to provide support for the NASA Deep Impact mission. SWAS provided water production data on the comet until the end of August 2005, the European Space Agency launched a space-based mission known as the Herschel Space Observatory in 2009. Herschel deploys the largest mirror ever launched into space and studies radiation in the far infrared, rather than an Earth orbit, Herschel entered into a Lissajous orbit around L2, the second Lagrangian point of the Earth-Sun system. L2 is located approximately 1.5 million km from Earth, herschels mission focuses primarily on the origins of galaxies and galactic formation
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Infrared astronomy
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Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers, Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves. Infrared astronomy began in the 1830s, a few decades after the discovery of infrared light by William Herschel in 1800. Early progress was limited, and it was not until the early 20th century that conclusive detections of astronomical objects other than the Sun, both fields also use solid state detectors, though the specific type of solid state detectors used are different. Infrared light is absorbed at many wavelengths by water vapor in the Earths atmosphere, so most infrared telescopes are at elevations in dry places. There are also infrared observatories in space, including the Spitzer Space Telescope, the discovery of infrared radiation is attributed to William Herschel, who performed an experiment where he placed a thermometer in sunlight of different colors after it passed through a prism. He noticed that the temperature increase induced by sunlight was highest outside the visible spectrum and he dubbed this radiation calorific rays, and went on to show that it could be reflected, transmitted, and absorbed just like visible light. Efforts were made starting in the 1830s and continuing through the 19th century to detect infrared radiation from astronomical sources. Radiation from the Moon was first detected in 1873 by William Parsons, ernest Fox Nichols used a modified Crookes radiometer in an attempt to detect infrared radiation from Arcturus and Vega, but Nichols deemed the results inconclusive. The field was mostly neglected by traditional astronomers though until the 1960s, Infrared radiation with wavelengths just longer than visible light, known as near-infrared, behaves in a very similar way to visible light, and can be detected using similar solid state devices. For this reason, the infrared region of the spectrum is commonly incorporated as part of the optical spectrum. Many optical telescopes, such as those at Keck Observatory, operate effectively in the infrared as well as at visible wavelengths. The far-infrared extends to submillimeter wavelengths, which are observed by such as the James Clerk Maxwell Telescope at Mauna Kea Observatory. Like all other forms of radiation, infrared is utilized by astronomers to study the universe. Indeed, infrared measurements taken by the 2MASS and WISE astronomical surveys have been effective at unveiling previously undiscovered star clusters. Examples of such embedded star clusters are FSR1424, FSR1432, Camargo 394, Camargo 399, Majaess 30, and Majaess 99. Infrared telescopes, which includes most major optical telescopes as well as a few dedicated infrared telescopes, need to be chilled with liquid nitrogen, the reason for this is that objects with temperatures of a few hundred Kelvin emit most of their thermal energy at infrared wavelengths. If infrared detectors were not kept cooled, the radiation from the detector itself would contribute noise that would dwarf the radiation from any celestial source and this is particularly important in the mid-infrared and far-infrared regions of the spectrum
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Visible-light astronomy
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Visible-light astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light. An example of spectroscopy is the study of lines to understand of what kind of matter light is going through. Visible astronomy also includes looking up at night and he later made improved versions with up to about 30x magnification. With a Galilean telescope the observer could see magnified, upright images on the earth—it was what is known as a terrestrial telescope or a spyglass. He could also use it to observe the sky, for a time he was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, he demonstrated one of his telescopes, with a magnification of about 8 or 9. His telescopes were also a sideline for Galileo selling them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a treatise entitled Sidereus Nuncius. The visibility of objects in the night sky is affected by light pollution. The presence of the Moon in the sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of light sources, however, light pollution has been a growing problem for viewing the night sky. List of largest optical reflecting telescopes
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Ultraviolet astronomy
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An ultraviolet light is not visible to the human eye. Light at these wavelengths is absorbed by the Earths atmosphere, so observations at these wavelengths must be performed from the atmosphere or from space. Ultraviolet line spectrum measurements are used to discern the chemical composition, densities, and temperatures of the medium. UV observations can provide essential information about the evolution of galaxies. The ultraviolet Universe looks quite different from the stars and galaxies seen in visible light. Most stars are relatively cool objects emitting much of their electromagnetic radiation in the visible or near-infrared part of the spectrum. Ultraviolet radiation is the signature of hotter objects, typically in the early, in the Earths sky seen in ultraviolet light, most stars would fade in prominence. Some very young stars and some very old stars and galaxies, growing hotter and producing higher-energy radiation near their birth or death. Clouds of gas and dust would block the vision in many directions along the Milky Way, charles Stuart Bowyer is generally given credit for starting this field
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X-ray astronomy
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X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earths atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy is the science related to a type of space telescope that can see farther than standard light-absorption telescopes, such as the Mauna Kea Observatories. X-ray emission is expected from astronomical objects that contain extremely hot gasses at temperatures from about a million kelvin to hundreds of millions of kelvin. Although X-rays have been observed emanating from the Sun since the 1940s and this source is called Scorpius X-1, the first X-ray source found in the constellation Scorpius. The X-ray emission of Scorpius X-1 is 10,000 times greater than its visual emission, in addition, the energy output in X-rays is 100,000 times greater than the total emission of the Sun in all wavelengths. Based on discoveries in this new field of X-ray astronomy, starting with Scorpius X-1 and it is now known that such X-ray sources as Sco X-1 are compact stars, such as neutron stars or black holes. Material falling into a hole may emit X-rays, but the black hole itself does not. The energy source for the X-ray emission is gravity, infalling gas and dust is heated by the strong gravitational fields of these and other celestial objects. Many thousands of X-ray sources are known, in addition, the space between galaxies in galaxy clusters is filled with a very hot, but very dilute gas at a temperature between 10 and 100 megakelvins. The total amount of hot gas is five to ten times the mass in the visible galaxies. The first sounding rocket flights for X-ray research were accomplished at the White Sands Missile Range in New Mexico with a V-2 rocket on January 28,1949. A detector was placed in the nose section and the rocket was launched in a suborbital flight to an altitude just above the atmosphere. X-rays from the Sun were detected by the U. S. Naval Research Laboratory Blossom experiment on board, an Aerobee 150 rocket was launched on June 12,1962 and it detected the first X-rays from other celestial sources. The largest drawback to rocket flights is their short duration. A rocket launched from the United States will not be able to see sources in the southern sky, in astronomy, the interstellar medium is the gas and cosmic dust that pervade interstellar space, the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the intergalactic medium. The interstellar medium consists of an extremely dilute mixture of ions, atoms, molecules, larger dust grains, cosmic rays, the energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field
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Gamma-ray astronomy
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Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy, the highest photon energies measured to date are in the TeV range, the record being held by the Crab Pulsar in 2004, yielding photons with as much as 80 TeV. Observation of gamma rays first became possible in the 1960s, very energetic gamma rays, with photon energies over ~30 GeV, can also be detected by ground based experiments. The extremely low photon fluxes at such high energies require detector effective areas that are large for current space-based instruments. The Imaging Atmospheric Cherenkov Telescope technique currently achieves the highest sensitivity, Gamma radiation in the TeV range emanating from the Crab Nebula was first detected in 1989 by the Whipple Observatory at Mt. Hopkins, in Arizona in the USA. Modern Cherenkov telescope experiments like H. E. S. S, VERITAS, MAGIC, and CANGAROO III can detect the Crab Nebula in a few minutes. The most energetic photons observed from an extragalactic object originate from the blazar and these measurements were done by the High-Energy-Gamma-Ray Astronomy air Cherenkov telescopes. Gamma-ray astronomy observations are limited by non-gamma-ray backgrounds at lower energies. Larger area detectors and better background suppression are essential for progress in the field, a discovery in 2012 may allow focusing gamma-ray telescopes. At photon energies greater than 700 keV, the index of refraction starts to increase again, long before experiments could detect gamma rays emitted by cosmic sources, scientists had known that the universe should be producing them. Work by Eugene Feenberg and Henry Primakoff in 1948, Sachio Hayakawa and these processes included cosmic ray interactions with interstellar gas, supernova explosions, and interactions of energetic electrons with magnetic fields. However, it was not until the 1960s that our ability to detect these emissions came to pass. The first gamma-ray telescope carried into orbit, on the Explorer 11 satellite in 1961 and they appeared to come from all directions in the Universe, implying some sort of uniform gamma-ray background. Such a background would be expected from the interaction of cosmic rays with interstellar gas, the first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. These first gamma-ray line observations were from OSO-3, OSO-7, and the Solar Maximum Mission, the solar observations inspired theoretical work by Reuven Ramaty and others. Significant gamma-ray emission from our galaxy was first detected in 1967 by the detector aboard the OSO-3 satellite and it detected 621 events attributable to cosmic gamma rays. However, the field of gamma-ray astronomy took great leaps forward with the SAS-2 and these two satellites provided an exciting view into the high-energy universe. They confirmed the findings of the gamma-ray background, produced the first detailed map of the sky at gamma-ray wavelengths
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Cosmic ray
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Cosmic rays are high-energy radiation, mainly originating outside the Solar System. Upon impact with the Earths atmosphere, cosmic rays can produce showers of particles that sometimes reach the surface. Composed primarily of protons and atomic nuclei, they are of mysterious origin. Data from the Fermi space telescope have been interpreted as evidence that a significant fraction of cosmic rays originate from the supernovae explosions of stars. Active galactic nuclei probably also produce cosmic rays, the term ray is a historical accident, as cosmic rays were at first, and wrongly, thought to be mostly electromagnetic radiation. In current usage, the cosmic ray almost exclusively refers to massive particles. Massive particles – those that have rest mass – can gain additional, kinetic, mass-energy when they are moving, through this process, some particles acquire tremendously high mass-energies. These are significantly higher than the energy of even the highest-energy photons detected to date. The energy of the massless photon depends solely on frequency, not speed, at the higher end of the energy spectrum, relativistic kinetic energy is the main source of the mass-energy of cosmic rays. Hence, the highest-energy detected fermionic cosmic ray was around 3×106 times more energetic than the highest-energy detected cosmic photons, of primary cosmic rays, which originate outside of Earths atmosphere, about 99% are the nuclei of well-known atoms, and about 1% are solitary electrons. Of the nuclei, about 90% are simple protons, i. e. hydrogen nuclei, 9% are alpha particles, identical to helium nuclei, a very small fraction are stable particles of antimatter, such as positrons or antiprotons. The precise nature of this fraction is an area of active research. An active search from Earth orbit for anti-alpha particles has failed to detect them, one can show that such enormous energies might be achieved by means of the Centrifugal mechanism of acceleration in Active galactic nuclei. At 50 J, the highest-energy ultra-high-energy cosmic rays have energies comparable to the energy of a 90-kilometre-per-hour baseball. As a result of discoveries, there has been interest in investigating cosmic rays of even greater energies. Most cosmic rays, however, do not have such extreme energies, however, his paper published in Physikalische Zeitschrift was not widely accepted. In 1911 Domenico Pacini observed simultaneous variations of the rate of ionization over a lake, over the sea, Pacini concluded from the decrease of radioactivity underwater that a certain part of the ionization must be due to sources other than the radioactivity of the Earth. In 1912, Victor Hess carried three enhanced-accuracy Wulf electrometers to an altitude of 5300 meters in a balloon flight
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Gravitational-wave astronomy
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Gravitational waves have a solid theoretical basis, founded upon the theory of relativity. They were first predicted by Einstein in 1916, although a specific consequence of general relativity, richard Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for this discovery. Subsequently, many other binary pulsars have been observed, all fitting gravitational-wave predictions, on 11 February 2016 it was announced that LIGO had directly observed the first gravitational waves in September 2015. The second observation of gravitational waves was made on 26 December 2015, ordinary gravitational waves frequencies are very low and much harder to detect, while higher frequencies occur in more dramatic events and thus have become the first to be observed. In 2015, the LIGO project was the first to directly observe gravitational waves using laser interferometers, the LIGO detectors observed gravitational waves from the merger of two stellar-mass black holes, matching predictions of general relativity. There are several current scientific collaborations for observing gravitational waves, LIGO and Virgo are currently being upgraded to their advanced configurations. Advanced LIGO began observations in 2015, detecting gravitational waves even though not having reached its design sensitivity yet, the more advanced KAGRA is scheduled for 2018. GEO600 is currently operational, but its sensitivity makes it unlikely to make an observation, an alternative means of observation is using pulsar timing arrays. Current bounds are approaching those expected for astrophysical sources, further in the future, there is the possibility of space-borne detectors. The European Space Agency has selected a mission for its L3 mission, due to launch 2034. Also in development is the Japanese Deci-hertz Interferometer Gravitational wave Observatory, astronomy has traditionally relied on electromagnetic radiation. Astronomy originated with visible-light astronomy and what could be seen with the naked eye, as technology advanced, it became possible to observe other parts of the electromagnetic spectrum, from radio to gamma rays. Each new frequency band gave a new perspective on the Universe, late in the 20th century, the detection of solar neutrinos founded the field of neutrino astronomy, giving an insight into previously invisible phenomena, such as the inner workings of the Sun. The observation of gravitational waves provides a means of making astrophysical observations. Gravitational waves provide complementary information to that provided by other means, by combining observations of a single event made using different means, it is possible to gain a more complete understanding of the sources properties. This is known as multi-messenger astronomy, Gravitational waves can also be used to observe systems that are invisible to measure by any other means, for example, they provide a unique method of measuring the properties of black holes. Gravitational waves can be emitted by many systems, but, to produce detectable signals, the main source is a binary of two compact objects. Example systems include, Compact binaries made up of two closely orbiting stellar-mass objects, such as dwarfs, neutron stars or black holes
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Multi-messenger astronomy
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Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets, while the phenomena include supernovae explosions, gamma ray bursts, more generally, all astronomical phenomena that originate outside Earths atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole, Astronomy is the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, during the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs can play an active role, especially in the discovery. Amateur astronomers have made and contributed to many important astronomical discoveries, Astronomy means law of the stars. Astronomy should not be confused with astrology, the system which claims that human affairs are correlated with the positions of celestial objects. Although the two share a common origin, they are now entirely distinct. Generally, either the term astronomy or astrophysics may be used to refer to this subject, however, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics, some titles of the leading scientific journals in this field includeThe Astronomical Journal, The Astrophysical Journal and Astronomy and Astrophysics. In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye, in some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye, most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the model of the Universe, or the Ptolemaic system. The Babylonians discovered that lunar eclipses recurred in a cycle known as a saros
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Australian Aboriginal astronomy
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One of the earliest records of indigenous astronomy was made by William Edward Stanbridge, an Englishman who emigrated to Australia in 1841 and befriended the local Boorong people. Some Aboriginal groups use the motions of bodies for calendar purposes. Many attribute religious or mythological meanings to celestial bodies and phenomena, there is a diversity of astronomical traditions in Australia, each with its own particular expression of cosmology. However, there appear to be common themes and systems between the groups, a constellation used in Aboriginal culture in Australia is the Emu in the sky, a constellation that is defined by dark nebulae that are visible against the Milky Way background, rather than by stars. The Emus head is the very dark Coalsack nebula, next to the Southern Cross, just North of Sydney, in the Kuringai National Park, are extensive rock engravings of the Guringai people who lived there, including representations of the creator-hero Daramulan and his emu-wife. An engraving near the Elvina Track shows an emu in the same pose, to the Wardaman, however, the Coalsack is the head of a lawman. The Yolngu people of northern Australia say that the constellation of Orion and they tell the story of three brothers who went fishing, and one of them ate a fish that was forbidden under their law. Seeing this, the Sun-woman, Walu, made a waterspout that carried him and this is an example of astronomical legends underpinning the ethical and social codes that people use on Earth. The Pleiades also figures in the Dreamings of several language groups, for example, in the central desert region, they are said to be seven sisters fleeing from the unwelcome attentions of a man represented by some of the stars in Orion. The close resemblance of this to Greek mythology is believed to be coincidental — there is no evidence of any cultural connection, however, stars were commonly used to measure time and the seasons and to regulate daily activities before written culture, and long after in some cultures. Aboriginal people came to Australia from Asia 50,000 years ago, the canoe is sent back to earth as a shooting star, letting their family on Earth know that they have arrived safely in the spirit-land. The Boorong people see in the Southern Cross a possum in a tree, many traditions have stories of a female Sun and a male Moon. The Yolngu say that Walu, the Sun-woman, lights a fire each morning. She paints herself with red ochre, some of which spills onto the clouds and she then lights a torch and carries it across the sky from east to west, creating daylight. At the end of her journey, as she descends from the sky, some of her ochre paints again rubs off onto the clouds and she then puts out her torch, and throughout the night travels underground back to her starting camp in the east. The Yolngu tell that Ngalindi, the Moon-man, was young and slim. His wives chopped bits off him with their axes, to them he climbed a tall tree towards the Sun. After remaining dead for three days, he rose again to repeat the cycle, and continues doing so till this day, the Kuwema people in the Northern Territory say that he grows fat at each full Moon by devouring the spirits of those who disobey the tribal laws
