Galaxy

This article is about celestial bodies. For alternate meanings, see galaxy (disambiguation).

A galaxy is a vast 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 10¹¹ galaxies in the visible universe.

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 stars and has a total mass of about 6×10¹¹ 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).

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 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.

File:Pr3 400galaxypix.jpg

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

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 such number varies, 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.

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

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.They are fuckers!

References

James Binney: Galactic Astronomy, Princeton University Press, 1998
Terence Dickinson: The Universe and Beyond (Fourth Edition), Firefly Books Ltd. 2004, 2004

External links

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