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A galaxy is a huge gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain ten million to one trillion (107 to 1012) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years. Although theoretical dark matter appears to account for around 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. Intergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than a hundred billion (1011) galaxies in our observable universe.
Etymology The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας) or kyklos galaktikos meaning "milky circle" for the system’s appearance in the sky. In Greek mythology, Zeus placed his son by a mortal woman, the infant Hercules, on Hera's breast as she was asleep, so that the baby would drink her divine milk and thus become immortal. Hera woke up while breastfeeding, and realized that she was nursing an unknown baby: she pushed the baby away and a jet of her milk sprayed the night sky. When astronomers speculated that certain objects previously classified as spiral nebulae were actually vast congeries of stars, this was called the "island universe theory"; but this was an obvious misnomer, since universe means everything there is. Consequently, this term fell into disuse, replaced by applying the term galaxy generically to all such bodies. Observation history
Types of galaxies
Active galaxies A portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energy output from the galaxy is emitted by a source other than the stars, dust and interstellar medium. The standard model for such active galactic nucleus is based upon energy generation from matter falling into a supermassive black hole at the core region. Galaxies that emit high-energy radiation in the form of x-rays are classified as Seyfert galaxies, quasars and blazars. Active galaxies that emit radio frequencies from relativistic jets erupting from the core are classified as Radio galaxies. A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer. Larger scale structures Very few galaxies exist by themselves; these are known as field galaxies. Most galaxies are gravitationally bound to a number of other galaxies. Structures containing up to about 50 galaxies are called groups of galaxies, and larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. Clusters of galaxies are often dominated by a single giant elliptical galaxy, which over time tidally destroys its satellite galaxies and adds their mass to its own. Superclusters are giant collections containing tens of thousands of galaxies, found in clusters, groups and sometimes individually; at the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids. Above this scale, the universe appears to be isotropic and . Our galaxy is a member of the Local Group, which it dominates together with the Andromeda Galaxy; overall the Local Group contains about thirty galaxies in a space about one megaparsec across. The Local Group is part of the Virgo Supercluster, which is dominated by the Virgo Cluster (of which our Galaxy is not a member). Galaxy formation and evolution The study of galactic formation and evolution attempts to answer questions regarding how galaxies formed and their evolutionary path over the history of the universe. Some theories on this field have now become widely accepted, but it is still an active area of study in astrophysics. Formation The method of galactic formation is a major open question in astronomy. Theories may be divided into two categories: top-down and bottom-up. In top-down theories such as the Eggen–Lynden-Bell–Sandage (ELS) model, protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. In bottom-up theories such as the Searle-Zinn (SZ) model, globular clusters form first, and then a number of such bodies accrete to form a larger galaxy. Modern theories must be modified to account for the probable presence of large dark matter halos. A sketch of a galactic formation model follows. Shortly after recombination, baryonic matter begins to condense around cold dark matter halos. Zero-metal high-velocity halo stars (called Population III stars) are the first to develop around a protogalaxy as it starts to contract. These huge stars quickly supernova, releasing heavy elements into the interstellar medium. Within the next billion years, globular clusters, the central supermassive black hole and galactic bulge of metal-poor Population II stars form. Within two billion years, the remaining material settles into a galactic disk. The galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life; the cycle of stellar birth and death will increase the abundance of heavy elements, eventually allowing the formation of planets. Probably the oldest galaxy yet found, IOK-1, was discovered in September 2006 by Masanori Iye at National Astronomical Observatory of Japan using the Subaru Telescope in Hawaii. Its emission of Lyman alpha radiation has a redshift of 6.96, making it thirteen billion years old. While some scientists have claimed other objects (such as Abell 1835 IR1916) to be even older, the IOK-1's age and composition have been more reliably established. The existence of such old protogalaxies suggests that they must have grown in the so-called "Dark Ages" (before the first generation of stars) from anisotropic irregularities present during the era of recombination, some three hundred thousand years after the Big Bang. Such irregularities of the right scale were observed using the Wilkinson Microwave Anisotropy Probe (WMAP) in 2003. More evidence for this model of galactic formation comes from detection of ancient Population III stars. The giant star, HE0107-5240, discovered in 2002 by researchers at the University of Hamburg, is believed to be the oldest yet discovered star in the Milky Way, since unlike younger stars, it is virtually metal-free. (See *.) Since then, other very old stars (like HE 1327) have also been found. Evolution Studies show that the Milky Way Galaxy is moving towards the nearby Andromeda Galaxy at about 130 km/s, and depending upon the lateral movements, the two may collide in about five to six billion years. Such galaxy collisions are fairly common. Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms will produce a long train of stars, similar to that seen in NGC 250 or the Antennae Galaxies. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing. Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they continue to have dense molecular clouds of interstellar hydrogen in their spiral arms. Elliptical galaxies are already largely devoid of this gas and so form no new stars. However, the supply of star-forming material is finite; as stars convert hydrogen into heavier elements, fewer stars will form. After the end of stellar formation in under one hundred billion years, the "stellar age" will come to an end after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest longest-lived stars in our astrosphere, tiny red dwarfs begin to fade. At the end of the stellar age galaxies will comprise compact objects: brown dwarfs, black dwarfs, cooling white dwarfs, neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into the central supermassive black hole of the galaxies, or be flung into the depths of intergalactic space as a result of collisions. Galactic biology Biology as we know it is currently assumed to exist only around single, third-generation '''G'''-type stars in the middle regions of the spiral arms of spiral galaxies, like the sun. Elliptical galaxies, produced as a result of many galactic collisions, quickly lose their clouds of interstellar hydrogen gas, and cannot make new generations of stars. Irregular galaxies have few elderly stars and thus seem to have low concentrations of the heavier elements on which Earth-like biology depends. Even within spiral galaxies biology as we know it would appear to be limited to the middle reaches of the spiral arm, as in the galactic halo or outer spiral arms heavier elements are in short supply, whilst in the gas clouds around the galactic centre heavier elements are in concentrations too high, and interstellar interactions are too frequent to allow earth-sized planets to form in stable circular orbits around their stars. See also | ||||||||||||
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