A galaxy is a huge gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain 10 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 so-called dark matter and dark energy appear to account for well over 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 the visible universe.

Etymology

The word galaxy was derived from the Greek term for our own galaxy, kyklos galaktikos meaning "milky circle" for the system’s appearance in the 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.

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. While the Hubble sequence does encompass all galaxies, it is entirely based upon visual morphological type. Hence, it may miss certain important characteristics of galaxies such as star formation rate.

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 100,000 light years in diameter and 3,000 light years in thickness. It contains about 3×10¹¹ (three hundred billion) times the amount of stars and has a total mass of about 6×10¹¹ (six hundred billion) times the mass of the Sun.

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.

A new set of galaxies, classified as Ultra Compact Dwarf Galaxies, were discovered in 2003 by Michael Drinkwater of the University of Queensland.

Larger scale structures

Only a 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 30 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).

Evolution of Galaxies

Main article: Galaxy formation and evolution

The oldest galaxy yet found was discovered in 2004 by scientists at CalTech using the Hubble Space Telescope and the Keck telescopes on Mauna Kea in Hawaii. The light from the Galaxy which has now reached Earth left its source just 750 million years after the beginning of our universe itself.

The Protogalaxy of about 1 million stars was identified as a result of the gravitational lens effect of the Abell 2218 cluster which lies in front of it and which is so massive that it bends and amplifies the light passing through it, acting as a natural lens in space.

For galaxies to be so old, suggests that they must have grown in the so-called "Dark Ages" from anisotropic irregularities present in the Big Bang itself before the decoupling of matter and energy, some 390,000 years after the Big Bang. Such irregularities of the right scale were observed using the Wilkinson Microwave Anisotropy Probe WMAP) in 2003. The "Dark Ages" refers to the time in the history of the universe after decoupling had allowed hydrogen atoms to first form but stars had not been formed. The age of the small galaxy, (it is only 2,000 lightyears in radius, compared to the 70,000 lightyear radius of the Milky Way) has been estimated as a result of the recessional velocity's Doppler Shift moving wavelengths from their normal ultraviolet position to the infrared. The galaxy seems to lack the bright hydrogen emission line formed by stars in normal galaxies, while its ultraviolet light is much stronger than that seen in star-forming galaxies closer by. [[1]]

Galaxy Abell 1835 IR1916 is seen as a tiny dot in this photo of distant galaxies. Image courtesy of ESO.

In 2004, this galaxy was displaced by galaxy Abell 1835 IR1916, which became the most distant galaxy ever seen by humans.

The Eggen Lynden-Bell Sandage model for the formation of galaxies suggests that low-metal high-velocity halo stars may be the first to develop around a Protogalaxy as it starts to contract. With almost zero angular momentum, more recently it has been suggested that these halo objects may be the remains of ancient Dwarf Galaxies cannibalised by being ripped apart by near passage with larger ones. Models of star formation suggest that after the Big Bang, the Universe consisted of mostly hydrogen, some helium and a little lithium, so the first stars to form, large short-lived stars would have contained only these elements. This first generation of stars converted these lighter elements into heavier (such as carbon, phosphorous, iron and lead - collectively known to astronomers as) metals. Upon becoming supernova, when these stars exploded they seeded these heavier elements throughout the gas of the Protogalaxy, gas which went into form later generations of stars - like our Sun. The giant star, HE0107-5240, discovered in 2002 by Norbert Christlieb, at the University of Hamburg in Germany, is believed to be the oldest yet discovered star of the Milky Way galaxy because unlike younger stars this rare body is virtually metal-free. [[2]] Since its discovery other very old stars, like HE 1327, first seen by researchers in Chile with data also collected using Japan's telescope in Hawaii, have also been found.

Many of these supermassive stars would have become rapidly spinning black holes, acting as the centre for future globular clusters of stars. The rapid depletion of gas in such globular clusters would lead to the end of star formation, with the result that today are comprised largely of ancient red and white dwarf stars. The Searle Zinn model for the formation of galaxies suggest that such objects gradualy collapsed gravitationally to form the galactic nucleus which was formed from accretion of a number of bodies to form a supermassive black hole.

History

This account of the history of the investigation of our own and other galaxies is largely taken from [1].

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.

Towards the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae, later followed by a larger catalog of 5000 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 universally 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 the Sun 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 the Sun 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 the Sun 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 realized that the total visible mass of galaxies (from stars and gas) does not properly account for the speed of the rotating gas, thus leading to the postulation of 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. It photographed the Hubble Deep Field, providing evidence for hundreds of billions of galaxies in existence in the visible universe alone. Many scientists have tried to obtain a good estimate for the number of galaxies in the universe formally. The methods used to achieve this number vary, and therefore, the results are varying too. Also, as new and improved technology becomes available, astronomers can detect fainter objects that were not seen before. These objects that have come into view will in turn change the estimated number of galaxies. In 1999 the Hubble Space Telescope estimated that there were 125 billion galaxies in the universe, and recently with the new camera HST has observed 3000 visible galaxies, which is twice as much as they observed before with the old camera. The term "visible" is emphasized because observations with radio telescopes, infrared cameras, x-ray cameras, etc. would detect other galaxies that are not detected by Hubble. As observations keep on going and astronomers explore more of our universe, the number of galaxies detected will increase.

Life in Galaxies

Life as we know it would seem to be a phenomenon found 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 inter-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 life depends. Even within spiral galaxies life 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 inter-stellar movements are too frequent to allow earthsized planets to form in stable circular orbits around their stars.

Lee Smolin as a part of his fecund universes theory has shown how within these spiral arms, an ecology involving star formation, supernova explosions, circulation and compaction of opaque molecular interstellar clouds, and continuous infall of intergalactic hydrogen, eventually forms some kind of far from equilibrium, evolutionary self-organizing homeostatic system, which resembles the autopoiesis of life as a whole.

More speculatively, the Drake equation shows the hypothetical frequency of life in our galaxy, and of the technological civilisations like ours that evolve within such conditions. But life as we know it is currently limited to the planetary surface of Earth, in one solar system, our own. If life is to survive in the medium to long term, it will have to leave this surface within 500 million years, as the depletion of hydrogen fuel in the core of the sun, the accumulation of helium "ash" and the resultant fusion "shell" will propell the sun towards the condition of a red giant star, making conditions on Earth too hot for liquid water, and therefore, life itself, to exist. The fact that, as the Fermi Paradox shows, there has been no confirmed discovery of extraterrestrial life, suggests that conditions like our own in the Milky Way galaxy are probably very rare.

Freeman Dyson showed how for living systems that escape the limitations of a planetary surface, a Dyson sphere, in which life attempts to capture all of the useful light from a central star, is probably theoretically possible. Such a system would probably be between a Kardashev Type I (using all the available energy on a planetary surface) and a Kardashev Type II (using all available energy from a single star). The recently proposed Matrioshka Brain would be the ultimate condition in such a development. Whether a Kardashev Type III - A system that is able to harness all of the power available from a single galaxy, approximately 1036 W - can exist is impossible to know at present, and will be a concern only for the far distant future.

The Future of Galaxies

Studies show that our Milky Way Galaxy is moving towards the nearby Andromeda Galaxy at about 250,000 miles per hour, and depending upon the lateral movements, we may collide in about 5-6 billion years.

NGC 250 is a result of two spiral galaxies colliding, in a fashion similar to that expected for the Andromeda and Milky Way. Image courtesy of Gemini.

Astronomers have been studying such galactic collisions and believe, given the distances between the stars, that despite this collision, the great majority of star systems will be unaffected. However, gravitational stripping may occur of the interstellar gas and dust that makes up the spiral arms, producing a long train of new stars, similar to that seen in NGC 250, where two spiral galaxies of a similar type to our own, are colliding.

Although such collisions are generally rare, evidence of past collisions of the Milky Way with other smaller galaxies is increasing, and within Galactic clusters and superclusters, as galaxies become increasingly gravitationally bound together, such collisions will take place. Within the enormous future time-spans remaining for our universe (currently only 13.7 x 10⁹ years old), as a result of this gravitational relaxation, eventually 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 these collisions.

Spiral galaxies, like our own, only produce new generations of stars as long as they continue to have infalling clouds of interstellar hydrogen in their spiral arms. Elliptical galaxies are already largely devoid of this gas and so form no new stars. However, even for the largest spirals, interstellar clouds are not infinite. As stars convert hydrogen into heavier elements, so fewer stars will form form. The "stellar age" will come to an end after about 10 trillion to 100 trillion years (10¹³ - 10¹⁴ years), and galaxies in the cosmos then will then gradually fade. The smallest longest-lived stars in the universe, tiny red dwarfs that use their nuclear fuel sparingly, will also only continue to shine for that long. At the end of the "stellar age" galaxies will comprise ultracompact objects: cooling white dwarfs, neutron stars, and black holes. Collisions between these bodies may release temporary bursts of light, but by and large, galaxies will slowly become invisible. As stars slowly fade to become black dwarfs, so our galaxy's stars will be either consumed by black holes or will be comprised of degenerate matter or neutrons.

But even the "degenerate matter age" of the Galaxies will eventually come to an end. Quantum tunnelling would convert all degenerate matter and the neutron stars ultimately into black holes of the 4th "black-hole age" of the cosmos. Then finally these too start to disappear. Evaporation of galaxy-sized Black Holes by Hawking radiation would eventually after 10¹⁰⁰ years, see the destruction of even the largest supermassive black hole, but even then, long after the disappearance of the largest stars, the universe of galaxies would, for a long while, continue.

According to the theory of proton decay, as the strong nuclear force that binds its three quarks together breaks down, two quarks spontaneously merge into a massive leptoquark. This unstable particle ejects a positron (antimatter electron)and an antiquark, which merges with the remaining quark from the proton to create yet another transient particle, a short-lived zero pion (or pi meson particle). These unstable particles decay into a flash of gamma-ray photons which could form pairs of electrons and positrons which would eventually destroy each other and release still more photons. In this way the galaxies are ultimately reduced to nothing but light when all the solid matter as we know it vanishes from the cosmos. Time and space as we know it can then be said to have brought the galaxies and the universe as a whole to an end.