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The icon shows M51, the Whirlpool Galaxy.
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.
We live in a giant spiral galaxy, the Milky Way Galaxy, of 100,000 light years diameter and a mass of roughly a trillion solar masses; our Sun is one of several 100 billions of stars of the Milky Way. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant (and closer in case of some dwarfs which are currently merged with the Milky Way), while the nearest giant neighbor, the Andromeda Galaxy (M31), also a spiral, is about 2-3 million light years distant.
Our sun is one of several 100 billion stars in a spiral galaxy, the
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.
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:
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:
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:
Special observing Guides:
Last Modification: April 26, 2013