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
