A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also
NGC 40 is a planetary nebula discovered by William Herschel on November 25, 1788, is composed of hot gas around a dying star. The star has ejected its outer layer which has left behind a smaller, hot star with a temperature on the surface of about 50,000 degrees Celsius. Radiation from the star causes the shed outer layer to heat to about 10,000 degrees Celsius, is about one light-year across. About 30,000 years from now, scientists theorize that NGC 40 will fade away, leaving only a white dwarf star the size of Earth. NGC 40 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times. Earth's axis of rotation is tilted with respect to its orbital plane; the gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, slows its rotation. Earth is the largest of the four terrestrial planets. Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water by oceans; the remaining 29% is land consisting of continents and islands that together have many lakes and other sources of water that contribute to the hydrosphere.
The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, a convecting mantle that drives plate tectonics. Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms; some geological evidence indicates. Since the combination of Earth's distance from the Sun, physical properties, geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival.
Humans have developed diverse cultures. The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most spelled eorðe, it has cognates in every Germanic language, their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was being used to translate the many senses of Latin terra and Greek γῆ: the ground, its soil, dry land, the human world, the surface of the world, the globe itself; as with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus, Norse mythology included Jörð, a giantess given as the mother of Thor. Earth was written in lowercase, from early Middle English, its definite sense as "the globe" was expressed as the earth. By Early Modern English, many nouns were capitalized, the earth became the Earth when referenced along with other heavenly bodies. More the name is sometimes given as Earth, by analogy with the names of the other planets.
House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name but writes it in lowercase when preceded by the, it always appears in lowercase in colloquial expressions such as "what on earth are you doing?" The oldest material found in the Solar System is dated to 4.5672±0.0006 billion years ago. By 4.54±0.04 Bya the primordial Earth had formed. The bodies in the Solar System evolved with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, the planets grow out of that disk with the Sun. A nebula contains gas, ice grains, dust. According to nebular theory, planetesimals formed by accretion, with the primordial Earth taking 10–20 million years to form. A subject of research is the formation of some 4.53 Bya. A leading hypothesis is that it was formed by accretion from material loosed from Earth after a Mars-sized object, named Theia, hit Earth.
In this view, the mass of Theia was 10 percent of Earth, it hit Earth with a glancing blow and some of its mass merged with Earth. Between 4.1 and 3.8 Bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon and, by inference, to that of Earth. Earth's atmosphere and oceans were formed by volcanic outgassing. Water vapor from these sources condensed into the oceans, augmented by water and ice from asteroids and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing when the newly forming Sun had only 70% of its current luminosity. By 3.5 Bya, Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. A crust formed; the two models that explain land mass propose either a steady growth to the present-day forms or, more a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics
Spiral galaxies form a class of galaxy described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars and dust, a central concentration of stars known as the bulge; these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc; the spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin; the proportion of barred spirals relative to their barless cousins has changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe have bars.
Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope. Together with irregular galaxies, spiral galaxies make up 60% of galaxies in today's universe, they are found in low-density regions and are rare in the centers of galaxy clusters. Spiral galaxies may consist of several distinct components: A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components A central stellar bulge of older stars, which resembles an elliptical galaxy A bar-shaped distribution of stars A near-spherical halo of stars, including many in globular clusters A supermassive black hole at the center of the central bulge A near-spherical dark matter haloThe relative importance, in terms of mass and size, of the different components varies from galaxy to galaxy.
Spiral arms are regions of stars that barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have "loose" arms, whereas Sa and SBa galaxies have wrapped arms. Either way, spiral arms contain many blue stars, which make the arms so bright. A bulge is a large packed group of stars; the term refers to the central group of stars found in most spiral galaxies defined as the excess of stellar light above the inward extrapolation of the outer disk light. Using the Hubble classification, the bulge of Sa galaxies is composed of Population II stars, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars; some bulges have similar properties to those of elliptical galaxies. Many bulges are thought to host a supermassive black hole at their centers.
As of April 10th, 2019 the existence of these supermassive black holes was confirmed when scientists released the first image of a black hole in the center of the Messier 87 galaxy. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. Bar-shaped elongations of stars are observed in two-thirds of all spiral galaxies, their presence may be either weak. In edge-on spiral galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or -shaped structures which have a maximum visibility at half the length of the in-plane bar; the bulk of the stars in a spiral galaxy are located either close to a single plane in more or less conventional circular orbits around the center of the galaxy, or in a spheroidal galactic bulge around the galactic core. However, some stars inhabit a type of galactic halo; the orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or inclined orbits, or not move in regular orbits at all.
Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it. Unlike the galactic disc, the halo seems to be free of dust, in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc; the galactic halo contains many globular clusters. The motion of halo stars does bring them through the disc on occasion, a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy, these stars display unusually high proper motion; the oldest spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery.
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in two thirds of all spiral galaxies. Bars affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well; the Milky Way Galaxy, where our own Solar System is located, is classified as a barred spiral galaxy. Edwin Hubble classified spiral galaxies of this type as "SB" in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy. A new type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Clouds, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy.
Barred galaxies are predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers; the bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy; the creation of the bar is thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure. Bars are thought to be temporary phenomena in the lives of spiral galaxies. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have stubby bars.
Since so many spiral galaxies have bar structures, it is that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years. Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts; the general classification is "SB". The sub-categories are based on how tight the arms of the spiral are. SBa types feature bound arms. SBc types are at the other extreme. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Firehose instability Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity."
SPACE.com 16 August 2005. An article about the Spitzer Space Telescope's Milky Way discovery Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." 16 August 2005. The original press release regarding the article above, from the Univ. of Wisconsin'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March 2001. Hastings and Jane Hastings. Classifying Galaxies: Barred Spirals, 1995. "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15, 2000. A press release concerning NGC 1326 Barred spirals come and go Sky & Telescope April 2002. "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." November 29, 2001. A press release from the European Southern Observatory. Horton, Adam. "Spitzer NGC 1291 barred spiral galaxy seen in infrared." 22 October 2014
Messier 81 is a spiral galaxy about 12 million light-years away, with a diameter of 90,000 light years, about half the size of the Milky Way, in the constellation Ursa Major. Due to its proximity to Earth, large size, active galactic nucleus, Messier 81 has been studied extensively by professional astronomers; the galaxy's large size and high brightness makes it a popular target for amateur astronomers. Messier 81 was first discovered by Johann Elert Bode on December 31, 1774; the galaxy is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, subsequently listed in the Messier Catalogue. Messier 81 is located 10° northwest of Alpha Ursae Majoris along with several other galaxies in the Messier 81 Group. Messier 81 and Messier 82 can both be viewed using binoculars and small telescopes; the two objects are not observable to the unaided eye, although experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a dark sky.
Telescopes with apertures of 8 inches or larger are needed to distinguish structures in the galaxy. Its far northern declination makes it visible for observers in the northern hemisphere, it is not visible to most observers in the southern hemisphere, except those in a narrow latitude range south of the equator. Most of the emission at infrared wavelengths originates from interstellar dust; this interstellar dust is found within the galaxy's spiral arms, it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are effective at heating the dust and thus enhancing the infrared dust emission from these regions. Only one supernova has been detected in Messier 81; the supernova, named SN 1993J, was discovered on 28 March 1993 by F. García in Spain. At the time, it was the second brightest supernova observed in the 20th century; the spectral characteristics of the supernova changed over time.
It looked more like a type II supernova with strong hydrogen spectral line emission, but the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib. Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernova but did resemble the variations observed in type Ib supernovae. Hence, the supernova has been classified as a type IIb, a transitory class between type II and type Ib; the scientific results from this supernova suggested that type Ib and Ic supernovae were formed through the explosions of giant stars through processes similar to those taking place in type II supernovae. The supernova was used to estimate a distance of 8.5 ± 1.3 Mly to Messier 81. As a local galaxy, the Central Bureau for Astronomical Telegrams tracks novae in M81 along with M31 and M33. Messier 81 is the largest galaxy in the M81 Group, a group of 34 galaxies located in the constellation Ursa Major. At 11.7 Mly from the Earth, it makes this group and the Local Group, containing the Milky Way, relative neighbors in the Virgo Supercluster.
Gravitational interactions of M81 with M82 and NGC 3077 have stripped hydrogen gas away from all three galaxies, forming gaseous filamentary structures in the group. Moreover, these interactions have allowed interstellar gas to fall into the centers of M82 and NGC 3077, leading to vigorous star formation or starburst activity there. List of galaxies List of Messier objects M81 in fiction Messier object New General Catalogue StarDate: M81 Fact Sheet M81, SEDS Messier pages SST: Messier 81 NASA Astronomy Picture of the Day: Bright Galaxy M81 NightSkyInfo.com - M81, Bode's Galaxy Messier 81 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images ESA/Hubble images of M81 Galaxy Messier 81 Spitzer Image Gallery Helkit Observatory Deep image of the M81 Area Bode's Galaxy at Constellation Guide
IC 342 is an intermediate spiral galaxy in the constellation Camelopardalis, located close to the Milky Way. Despite its size and actual brightness, its location in dusty areas near the galactic equator makes it difficult to observe, leading to the nickname "The Hidden Galaxy", though it can be detected with binoculars; the dust makes it difficult to determine its precise distance. The galaxy was discovered by William Frederick Denning in 1892, it is one of the brightest in the IC 342/Maffei Group, one of the closest galaxy groups to the Local Group. Edwin Hubble first thought it to be in the Local Group, but it was determined not to be a member. In 1935, Harlow Shapley found that it was wider than the full moon, by angular size the third-largest spiral galaxy known, smaller only than the Andromeda Galaxy and the Triangulum Galaxy.. It has an H II nucleus. NASA Astronomy Picture of the Day – 22 December 2010 IC 342 IC 342 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images