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Galaxy

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   NGC 4414, a typical spiral galaxy in the constellation Coma Berenices,
   is about 56,000 light-years in diameter and approximately 60 million
   light-years distant.
   Enlarge
   NGC 4414, a typical spiral galaxy in the constellation Coma Berenices,
   is about 56,000 light-years in diameter and approximately 60 million
   light-years distant.

   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 (10^7 to 10^12)
   stars, all orbiting a common centre 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 centre 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 (10^11) 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

   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 1610, Galileo Galilei used a telescope to study the bright band on
   the night sky known as the Milky Way and discovered that it was
   composed of a huge number of faint stars. 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. The resulting disk of stars would be
   seen as a band on the sky from our perspective inside the disk. Kant
   also conjectured that some of the nebulae visible in the night sky
   might be separate galaxies.
   Sketch of the Whirlpool Galaxy by Lord Rosse in 1845
   Enlarge
   Sketch of the Whirlpool Galaxy by Lord Rosse in 1845

   Towards the end of the 18th century, Charles Messier compiled a catalog
   containing the 109 brightest nebulae, later followed by a larger
   catalog of five thousand nebulae assembled by William Herschel. In
   1845, Lord Rosse constructed a new telescope and was able to
   distinguish between elliptical and spiral nebulae. He also managed to
   make out individual point sources in some of these nebulae, lending
   credence to Kant's earlier conjecture. 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. He
   was able to resolve the outer parts of some spiral nebulae as
   collections of individual stars and identified some Cepheid variables,
   thus allowing him to estimate the distance to the nebulae: they were
   far too distant to be part of the Milky Way. In 1936, Hubble produced a
   classification system for galaxies that is used to this day, the Hubble
   sequence.

   The first attempt to describe the shape of the Milky Way and the
   position of Sol within it was carried out by William Herschel in 1785
   by carefully counting the number of stars in different regions of the
   sky. Using a refined approach, Kapteyn in 1920 arrived at the picture
   of a small (diameter ~15 kiloparsecs) ellipsoid galaxy with Sol close
   to the centre. A different method by Harlow Shapley based on the
   cataloging of globular clusters led to a radically different picture: a
   flat disk with diameter ~70 kiloparsecs and Sol far from the centre.
   Both analyses failed to take into account the absorption of light by
   interstellar dust present in the galactic plane; once Robert Julius
   Trumpler had quantified this effect in 1930 by studying open clusters,
   the present picture of our galaxy as described above emerged.

   In 1944, Hendrik van de Hulst predicted microwave radiation at a
   wavelength of 21 cm, resulting from interstellar atomic hydrogen gas;
   this radiation was observed in 1951. 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. These observations led to the postulation of a
   rotating bar structure in the centre of 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.

   Beginning in the 1990s, the Hubble Space Telescope yielded improved
   observations. 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

   Types of galaxies according to the Hubble classification scheme. An E
   indicates a type of elliptical galaxy; an S is a spiral, and SB is a
   barred-spiral galaxy.
   Enlarge
   Types of galaxies according to the Hubble classification scheme. An E
   indicates a type of elliptical galaxy; an S is a spiral, and SB is a
   barred-spiral galaxy.

   Galaxies come in three main types: ellipticals, spirals, and
   irregulars. A slightly more extensive description of galaxy types based
   on their appearance is given by the Hubble sequence. 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. It contains about 3×10^11 (three hundred
   billion) stars and has a total mass of about 6×10^11 (six hundred
   billion) times the mass of Sol.

   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. Like
   the stars, the spiral arms also rotate around the centre, but they do
   so with constant angular velocity. That means that stars pass in and
   out of spiral arms. The spiral arms are thought to be areas of high
   density or density waves. As stars move into an arm, they slow down,
   thus creating a higher density; this is akin to a "wave" of slowdowns
   moving along a highway full of moving cars. The arms are visible
   because the high density facilitates star formation and they therefore
   harbour many bright and young 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; the Milky Way has at least a dozen such
   satellites. 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.
   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
   homogeneous.

   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 (10^13–10^14 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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