A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and possibly a hypothetical substance known as dark matter.[1][2] The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million[3] (107) stars up to giants with one trillion[4] (1012) stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy,[5] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.[6]
There are probably more than 100 billion (1011) galaxies in the observable universe.[7] Most galaxies are 1,000 to 100,000[4] parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).[8] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.[9]
Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleusEtymology
The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας), or kyklos galaktikos, meaning "milky circle" for its appearance in the sky. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.[11]
In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our (Milky Way) galaxy, to distinguish it from the billions of other galaxies.
The term Milky Way first appeared in the English language in a poem by Chaucer.
"See yonder, lo, the Galaxyë
Which men clepeth the Milky Wey,
For hit is whyt."—Geoffrey Chaucer, Geoffrey Chaucer The House of Fame, c. 1380.[12]
When William Herschel constructed his catalog of deep sky objects, he used the name spiral nebula for certain objects such as M31. These would later be recognized as immense conglomerations of stars, when the true distance to these objects began to be appreciated, and they would be termed island universes. However, the word universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.[13]
[edit] Observation history
The realization that we live in a galaxy, and that there were, in fact, many other galaxies, parallels discoveries that were made about the Milky Way and other nebulae in the night sky.
[edit] The Milky Way
The Greek philosopher Democritus (450–370 B.C.) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[14] Aristotle (384-322 B.C.), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."[15] The Arabian astronomer, Alhazen (965-1037 A.D.), refuted this by making the first attempt at observing and measuring the Milky Way's parallax,[16] and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere."[17]
The Persian astronomer, Abū Rayhān al-Bīrūnī (973-1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars.[18] Ibn Bajjah ("Avempace", d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth's atmosphere.[15] Ibn Qayyim Al-Jawziyya (1292-1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.[19]
Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.[20] In 1750 Thomas Wright, in his An original theory or new hypothesis of the universe, 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 a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.[21] In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.
The first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center.[22][23] Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center.[21] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.[24]
[edit] Distinction from other nebulae
In the 10th century, the Persian astronomer, Abd al-Rahman al-Sufi (known in the West as Azophi), made the earliest recorded observation of the Andromeda Galaxy, describing it as a "small cloud".[25] Al-Sufi also identified the Large Magellanic Cloud, which is visible from Yemen, though not from Isfahan; it was not seen by Europeans until Magellan's voyage in the 16th century.[26][27] These were the first galaxies other than the Milky Way to be observed from Earth. Al-Sufi published his findings in his Book of Fixed Stars in 964.
In 1054, the creation of the Crab Nebula resulting from the SN 1054 supernova was observed by Chinese and Arab/Persian astronomers. The Crab Nebula itself was observed centuries later by John Bevis in 1731, followed by Charles Messier in 1758 and then by the Earl of Rosse in the 1840s.[28]
In 1750 Thomas Wright, in his An original theory or new hypothesis of the universe, speculated (correctly) that Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[21][29] In 1755 Immanuel Kant introduced the term "island universe" for these distant nebulae.
Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel.[21] 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.[30]
In 1917, Heber Curtis had observed the nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.[31]
In 1920 the so-called Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[32]
The matter was conclusively 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.[33] In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.[34]
[edit] Modern research
In 1944 Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm resulting from interstellar atomic hydrogen gas;[35] this radiation was observed in 1951. The radiation allowed for much improved study of the Milky Way 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.[36] 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 (from the stars and gas) 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.[37]
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.[38] The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion galaxies in the universe.[39] Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble. Particularly, galaxy surveys in the zone of avoidance (the region of the sky blocked by the Milky Way) have revealed a number of new galaxies.[40]
[edit] Types and morphology
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 (in starburst galaxies) and activity in the core (in active galaxies).[6]
[edit] Ellipticals
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead the galaxy is dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller globular clusters.[41]
The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters.[42] Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.
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