Encyclopedia
A
galaxy is a huge
gravitationally bound system of
stars,
interstellar gas and dust,
plasma, and unseen
dark matter. Typical galaxies contain ten million to one trillion 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 galaxies in our observable universe.
Etymology
The word
galaxy derives from the Greek term for our own galaxy,
galaxias 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 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.
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 ellipsoid galaxy with Sol close to the center. A different method by
Harlow Shapley based on the cataloging of
globular clusters lead to a radically different picture: a flat disk with diameter ~70 kiloparsecs and Sol far from the center. 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 center 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 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 , allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the zone of avoidance have revealed a number of new galaxies.
Types of galaxies
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 or activity in the core .
Our own galaxy, the
Milky Way, sometimes simply called
the Galaxy , 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 stars and has a total mass of about 6×10
11 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 center, 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 harbor 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 .
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 .
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 model, protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. In bottom-up theories such as the Searle-Zinn 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 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 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" 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 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. Since then, other very old stars 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, especially in dense
galaxy clusters. 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 , 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
...
References
- Terence Dickinson: The Universe and Beyond , Firefly Books Ltd. 2004, 2004
External links
