The huge star family to which our Sun belongs, seen as the Milky Way. In shape it is basically a bulging flat disc, diameter 35 kiloparsec, thickness 3000 parsec at the centre, and 300 parsec elsewhere. The Sun is 8000 parsec from the nucleus. Within this disc there are star clusters and interstellar matter. A pair of spiral arms merges from the nucleus and is superimposed on the general distribution of stars. The Galaxy as a whole rotates, faster at the centre than further out. The Sun takes 250 million years for one circuit. There are over 100 000 000 000 stars in all, and the Galaxy is 1015 thousand million years old.
A galaxy is a massive gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain ten million to one trillion (10) 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.
Observation history
This account of the history of the investigation of our own and other galaxies is largely taken from James Binney and Michael Merrifield: Galactic astronomy. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Galaxy might be a rotating body of a huge number of stars, held together by gravitational forces akin to the solar system but on much larger scales. However, the nebulae were not unanimously accepted as distant separate galaxies until the matter was settled by Edwin Hubble in the early 1920s using a new telescope. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter ~15 kiloparsecs) ellipsoid galaxy with Sol close to the center. This radiation allowed for much improved study of the Galaxy, since it is not affected by dust absorption and its doppler shift can be used to map the motion of the gas in the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was discovered in Vera Rubin's study of the rotation speed of gas in galaxies that the total visible mass (from stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about one hundred and seventy-five billion galaxies in the universe. Improved technology in detecting the spectra invisible to humans (radio telescopes, infra-red cameras, x-ray telescopes), allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the zone of avoidance (the region of the sky blocked by the Milky Way) have revealed a number of new galaxies.
Types of galaxies
Galaxies come in three main types: ellipticals, spirals, and irregulars. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) or activity in the core (in active galaxies).
Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kiloparsecs or a hundred light millennia in diameter and three light millennia in thickness.
In spiral galaxies, the spiral arms have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundred times smaller than the Milky Way, containing only a few billion stars. Many dwarf galaxies may orbit a single larger galaxy; Dwarf galaxies may also be classified as elliptical, spiral or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.
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.
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; 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;
Our galaxy is a member of the Local Group, which it dominates together with the Andromeda Galaxy;
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. 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. The galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life;
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. 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.
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. 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.
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 (10 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.
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