Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular. Besides simple stars, they typically contain various types of star clusters and nebulae.
Spiral galaxies usually consist of two major components: A flat, large disk which often contains a lot of interstellar matter (visible sometimes as reddishdiffuse emission nebulae, or as dark dust clouds) and young (open) star clusters and associations, which have emerged from them (recognizable from the blueish light of their hottest, short-living, most massive stars), often arranged in conspicuous and striking spiral patterns and/or bar structures, and an ellipsoidally formed bulge component, consisting of an old stellar population without interstellar matter, and often associated with globular clusters. The young stars in the disk are classified as stellar population I, the old bulge stars as population II. The luminosity and mass relation of these components seem to vary in a wide range, giving rise to a classification scheme. The pattern structures in the disk are most probably transient phenomena only, caused by gravitational interaction with neighboring galaxies.
Our sun is one of several 100 billion stars in a spiral galaxy, the Milky Way.
These are, in short, "spiral galaxies without spiral structure", i.e. smooth disk galaxies, where stellar formation has stopped long ago, because the interstellar matter was used up. Therefore, they consist of old population II stars only, or at least chiefly. From their appearance and stellar contents, they can often hardly be distinguished from ellipticals observationally.
Elliptical galaxies are actually of ellipsoidal shape, and it is now quite safe from observation that they are usually triaxial (cosmic footballs, as Paul Murdin, David Allen, and David Malin put it). They have little or no global angular momentum, i.e. do not rotate as a whole (of course, the stars still orbit the centers of these galaxies, but the orbits are statistically oriented so that only little net orbital angular momentum sums up). Normally, elliptical galaxies contain very little or no interstellar matter, and consist of old population II stars only: They appear like luminous bulges of spirals, without a disk component.
However, for some ellipticals, small disk components have been discovered, so that they may be representatives of one end of a common scheme of galaxy forms which includes the disk galaxies.
Often due to distortion by the gravitation of their intergalactic neighbors, these galaxies do not fit well into the scheme of disks and ellipsoids, but exhibit peculiar shapes. A subclass of distorted disks is however frequently occuring.
The first known galaxies were longly known before their nature as "island universes" came to light - this fact was finally proven only in 1923 by Edwin Powell Hubble, when he found Cepheid variable stars in the Andromeda Galaxy M31. Ancient observers have known the Milky Way and - on the Southern Hemisphere - the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) since prehistoric times. There are speculations that also the Andromeda Galaxy M31 may have been observed and recorded as a nebulous patch by anonymous Babylonian observers around 1,300 B.C.. This object was certainly known to medevial Persian astronomers before 905 A.D., and cataloged and described by Persian astronomer Al Sufi in 964 A.D, who also describes the LMC. Both LMC and SMC have become known by the reports of Vespucci and Magellan in the early 16th century. All other galaxies have been discovered only after the invention of the telescope: The Triangulum Galaxy M33 was first seen by Italian Priest astronomer G.B. Hodierna before 1654. Next, French astronomer Legentil discovered M32, a companion of the Andromeda Galaxy, in 1749, and his compatriot Nicholas Louis de Lacaille found M83 in 1752, the first galaxy beyond the Local Group to be discovered. These six were all external galaxies to be known, before Charles Messier started to survey the sky for comets and "nebulae." His first original discovery of a galaxy, M49, a giant elliptical member of the Virgo Cluster, occurred in 1771. The Messier Catalog in his modern form contains 40 galaxies, all but the two Magellanic Clouds that have been found up to 1782. Starting in 1783, William Herschel found and cataloged over 2,500 star clusters and "nebulae" up to 1802, 2,143 of them actually galaxies. J.L.E. Dreyer's NGC catalog contains 6,029 (about 75.9%), and his IC catalog another 3,971 galaxies (about 73.7%).
Today's modern catalogs contain far larger numbers; millions of galaxies have been cataloged, and it was estimated that the observable part of the universe contains probably hundreds of billions (10^11) galaxies. For example, at the time of this writing (2009), the Sloan Digital Sky Survey project has scanned more than about 1/4 of the sky, and determined properties of more than one million of galaxies.
From their appearance, galaxies are classified in types as given above, as spiral, lenticular, elliptical, and irregular galaxies, where spirals may be further classified for the presence of a bar (S: spirals, SAB: Intermediate, SB: Barred spirals). More precisely, ellipticals are sub-classified for ellipticity from E7 (strongly elongated) to E0 (circular), and spirals for prominence of bulge versus spiral arms from Sa (or SABa, SBa) to Sc or Sd. This so-called Hubble Classification Scheme can well be illustrated by Messier's galaxies:
- Hubble Scheme with Messier galaxies for HTML browser supporting tables (e.g., Netscape, IBM Webexplorer)
- Hubble Scheme for HTML browsers not supporting tables (Lynx, Mosaic)
Historically, according to obsolete models of "nebula" evolution, the terms of "Early" and "Late" types of galaxies have been introduced and are still sometimes used; "Early" types include elliptical ("E") and lenticular ("S0") galaxies as well as Sa spirals, while "later" Sb spirals are called "intermediate," and "Sc" and "Sd" spirals as well as irregulars are called "late."
Galaxies of all types, though of a wide variety of shapes and appearances, have many basic common features. They are huge agglomerations of stars like our Sun, counting several millions to several trillions. Most of the stars are not lonely in space like our Sun, but occur in pairs (binaries) or multiple systems.
The most massive galaxies are giants which are a million times more massive than the lightest: Their mass range is from at most some million times that of our Sun in case of the smallest dwarfs, to several trillion solar masses in case of giants like M87 or M77. Accordingly, the number of stars in them varies in the same range.
The linear size of galaxies also scatters, ranging from small dwarfs of few thousands of light years diameter (like M32) to respectable several 100,000 light years. Among the biggest Messier galaxies are the Andromeda galaxy M31 and the bright active Seyfert II galaxy M77.
Our Milky Way Galaxy, a spiral galaxy, is among the massive and big galaxies with at least 250 billion solar masses (there are hints that the total mass may even be as large as 750 billion to 1 trillion times that of the Sun) and a disk diameter of 100,000 light years.
Besides very many individual stars, most galaxies contain the following typical objects:
- Globular star clusters, large but quite compact agglomerations of some 100,000 to several million stars. These large clusters have about the same mass as the smallest galaxies, and are among the oldest objects in galaxies. Often, they form conspicuous systems, and occur at galaxies of every type and size. The globular cluster systems vary in a wide range in richness between the individual galaxies.
- As the stars develop, many of them leave nebulous remnants (planetary nebulae or supernova remnants) which then populate the galaxies.
- While the older stars, including the globular clusters, tend to form an ellipsoidal bulge, the interstellar gas and dust tends to accumulate in clouds near an equatorial disk, which is often conspicuous (i.e., in spiral and lenticular galaxies).
- A rather dense galactic nucleus, which is somewhat similar to a "superlarge" globular cluster. In many cases, galactic nuclei contain supermassive central objects, which are often considered as Black Hole candidates. Some of the more massive and conspicuous globulars are suspected to be the remnants of former nuclei of small galaxies which have been disrupted and cannibalized by larger galaxies.
Galaxies normally emit light of every wavelength, from the long radio and microwave end over the IR, visual and UV light to the short, high-enregy X- and gamma rays. Interstellar matter is coolest and therefore best visible in radio and IR, while supernova remnants are most conspicuous in the high-energy part of the electromagnetic spectrum. Galaxies with high star formation activity, like M82, are brightest in the infrared; at wide ranges of infrared wavelengths, considerably small M82 is the brightest galaxy in the sky. At radio wavelengths, where M82 is also considerably bright, giant galaxies M87 and M77 are among the most conspicuous.
Some galactic nuclei are remarkably distinguished from the average: These so-called Active Galactic Nuclei (AGNs) are intensive sources of light of all wavelengths from radio to X-rays. The activities seen in the AGNs are caused by gaseous matter falling into, and interacting with, the supermassive central objects mentioned above, according to the current consensus of most researchers. See Peterson (1997) for a semi-recent review and textbook on AGN. Sometimes, the spectra of these nuclei indicate enormous gaseous masses in rapid motion; galaxies with such a nucleus are called Seyfert galaxies (for their discoverer, Karl Seyfert; see Seyfert 1943). M77 is the brightest Seyfert galaxy in the sky. Some of the AGNs are faint or quiet, others bright or loud in the radio light; the latter are called radio galaxies; a famous radio galaxy is M87. Few galaxies have even more exotic nuclei, which are extremely compact and extremely bright, outshining their whole parent galaxy; these are called quasars (an acronym for QUAsi-StellAR objects). From their properties, quasars resemble extremely active Seyfert galaxy nuclei. However, quasars are so rare and the nearest is so remote that the brightest of them, 3C273, about 2 billion lightyears away in the constellation Virgo, is only of magnitude 13.7, and none of them is in Messier's or even in the NGC or IC catalog.
Studies have shown that about 1/3 of all galaxies show low nuclear-luminosity activity in their nucleus; this type of AGN was discovered by Heckman (1980) is called Low-Ionization Nuclear Emission-line Region (LINER); examples include the Andromeda Galaxy (M31) and M65.
Occasionally, at irregular intervals given by chance, in any type of galaxies, a supernova occurs: This is a star suddenly brightning to a high luminosity which may well outshine the whole galaxy; the maximal absolute magnitude of a supernova may well reach -19 to -20 magnitudes. This remarkable phenomenon has attracted the attention of many astronomers (equally both professionals and amateurs), who observe galaxies regularly as they "hunt" supernovae. Supernovae have been observed in several Messier catalog galaxies.
The formation and evolution of galaxies is a major issue of current research. For a long time, two different types of models of galaxy formation were common: First, "top-down" theories according to which galaxies have formed during a comparatively short period, at about the same time, within the first billion years after the universe started to expand, from an initial hot state, such as the Eggen - Lynden-Bell - Sandage (ELS) model (Eggen et.al. 1962). According to the second type of models, "bottom-up" theories, smaller structures of perheps the size of globular clusters formed first, and later coalesc or accrete to form larger galaxies, e.g. the Searle-Zinn (SZ) model (Searle, Zinn 1978).
During the last couple of years, new deep observations, in particular with the Hubble Space Telescope, have revealed evolutionary effects of galaxies on cosmological timescales: During the last 3-4 billion years, galaxies seem to be of similar types as they are observed in our neighborhood, with disk galaxies showing expressed spiral and bar structures. For times further back than about 5 billion years, barred spirals get less frequent, and spiral arms appear less developed. Back 6 billion years in time, many more interacting galaxies and mergers are observed, and the percentage of irregular systems increases rapidly. These results indicate that in the early universe, about 10-15 billion years ago, small building blocks were formed first, when primordial clouds of gaseous matter (hydrogen and helium),were singled out and started to collapse by their own gravity to form proto-galaxies. Halos of dark matter and with massive central nuclei, as well as interaction with neighboring systems, seem to play an important role in the formation and evolution of galaxies to their present state.
Messier's galaxies are not distributed equally across the sky, but can be grouped into a large group of Northern Spring/Southern Fall, and a smaller Northern Fall/Southern Spring group:
- Northern Fall/Southern Spring galaxies (6):
- Northern Spring/Southern Fall galaxies (34):
In the regions between, there are RA ranges without any Messier galaxies (3-8 and 16-23h in RA); these include the regions of the Milky Way band of stars and interstellar matter, which obscures the background galaxies.
- Galaxies and the Universe - WWW Course Notes by Bill Keel
- Galaxy Catalogs List
- From An Atlas of the Universe:
- O.J. Eggen, D. Lynden-Bell, and A.R. Sandage, 1962. Evidence from the motion of old stars that the Galaxy collapsed. Astrophysical Journal, Vol. 136, p. 748. [ADS: 1962ApJ...136..748E]
- Timothy M. Heckman, 1980. An optical and radio survey of the nuclei of bright galaxies - Activity in normal galactic nuclei. Astronomy and Astrophysics, Vol. 87, pp. 152-164. [ADS: 1980A&A....87..152H]
- Bradley M. Peterson, 1997. An introduction to active galactic nuclei. Cambridge University Press. 238+xvi pp.
- L. Searle and R. Zinn, 1978. Composition of halo clusters and the formation of the galactic halo. Astrophysical Journal, Vol. 225, No. 1, pp. 357-379 [ADS: 1978ApJ...225..357S]
- Carl K. Seyfert, 1943. Nuclear Emission in Spiral Nebulae. Astrophysical Journal, Vol. 97, pp. 28-40 (01/1943) [ADS: 1943ApJ....97...28S]
Imagery and atlasses:
- Allan Sandage. The Hubble Atlas of Galaxies. Carnegie Institution of Washington, 1961. 185 superb black & white photographs of galaxies of all types, obtained by the Mt. Palomar and Mt. Wilson Observatory telescopes, with captions and data, and a technical and scientific introduction.
- James D. Wray. The Color Atlas of Galaxies. Cambridge University Press, 1988. 3-color (UBV) images of 616 galaxies (including all Messier galaxies but M89), taken with telescopes at the McDonald Observatory, Texas, and the Cerro Tololo Interamerican Observatory, Chile, with data and captions.
- Timothy Ferris. Galaxies. Sierra Club Books, San Francisco, 1980. Superb book (look to get the more expensive full-size edition) with color and b/w photographs of galaxies and some other objects, from various observatories.
Of course, fine galaxy photos can be found in many more general astronomy books also.
Special observing Guides:
- Kenneth Glyn Jones (editor). Webb Society Deep-Sky Observer's Handbook, Volume 4, Galaxies, 1981; Volume 6, Anonymous Galaxies, 1987. Enslow Publishers, Hillside, NJ.
Most general Deep Sky Observing Guides are good as well.
- Dimitri Mihalas and James Binney. Galactic Astronomy. W.H. Freeman, 1981 (probably out of print).
Now replaced by: James Binney and Michael Merrifield. Galactic Astronomy. Princeton University Press, 1998.
This is a good introduction and review especially for the observational properties of galaxies (as they were known at the time of publication).
- James Binney and Scott Tremaine. Galactic Dynamics. Princeton Series in Astrophysics, Princeton University Press, 1987.
In-depth treatment of the physics of galaxies. Some mathematical and physical background is required for this book.
- Paul W. Hodge. Galaxies. Harvard University Press, 1986.
- Richard Berendzen, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. Science History Publications, Neale Watson Academic Publications, New York 1976.
Some galaxies are isolated "island universes" which float lonely through an otherwise empty region of the universe. But usually, space is too densely crowded with them, so that they form groups of some galaxies (or some dozens of galaxies), or even large clusters of up to several thousands of galaxies. The galaxies of these groups are in mutual gravitational interaction which may have significant influence on their appearance.