Andromeda Galaxy

Andromeda Galaxy


The Andromeda Galaxy ( / æ n ˈ d r ɒ m <span class="IPA" style="font-family: 'Gentium Plus', Gentium, GentiumAlt, 'Charis SIL', 'Doulos SIL', 'DejaVu Sans', Code2000, 'TITUS Cyberbit Basic', 'Arial Unicode MS', 'Lucida Sans Unicode', 'Chrysanthi Unicode'; "><span style="border-bottom-width: 1px; border-bottom-style: dotted; border-bottom-color: initial; " title="schwa 'a' in 'about'">ə <span class="IPA" style="font-family: 'Gentium Plus', Gentium, GentiumAlt, 'Charis SIL', 'Doulos SIL', 'DejaVu Sans', Code2000, 'TITUS Cyberbit Basic', 'Arial Unicode MS', 'Lucida Sans Unicode', 'Chrysanthi Unicode'; "><span style="border-bottom-width: 1px; border-bottom-style: dotted; border-bottom-color: initial; " title="'d' in 'dye'">d <span class="IPA" style="font-family: 'Gentium Plus', Gentium, GentiumAlt, 'Charis SIL', 'Doulos SIL', 'DejaVu Sans', Code2000, 'TITUS Cyberbit Basic', 'Arial Unicode MS', 'Lucida Sans Unicode', 'Chrysanthi Unicode'; "><span style="border-bottom-width: 1px; border-bottom-style: dotted; border-bottom-color: initial; " title="schwa 'a' in 'about'">ə <span class="IPA" style="font-family: 'Gentium Plus', Gentium, GentiumAlt, 'Charis SIL', 'Doulos SIL', 'DejaVu Sans', Code2000, 'TITUS Cyberbit Basic', 'Arial Unicode MS', 'Lucida Sans Unicode', 'Chrysanthi Unicode'; " title="Representation in the International Phonetic Alphabet (IPA)">/ ) is a spiral galaxy approximately 2.5 million light-years from Earth<sup class="reference" id="cite_ref-Ribas2005_3-1" style="line-height: 1em; ">[4] in the constellation Andromeda. It is also known asMessier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the Milky Way, but not the closest galaxy overall. It gets its name from the area of the sky in which it appears, the Andromeda constellation, which was named after the mythological princess Andromeda. Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, Andromeda may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.<sup class="reference" id="cite_ref-DarkMatter_9-0" style="line-height: 1em; ">[10] The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (10<sup style="line-height: 1em; ">12 )stars,<sup class="reference" id="cite_ref-trillion-stars_6-1" style="line-height: 1em; ">[7] : at least twice more than the number of stars in our own galaxy, which is estimated to be c. 200–400 billion.<sup class="reference" id="cite_ref-10" style="line-height: 1em; ">[11]

<p style="margin-top: 0.4em; margin-bottom: 0.5em; line-height: 19px; color: rgb(0, 0, 0); font-family: sans-serif;">Andromeda is estimated to be 7.1 × 10<sup style="line-height: 1em; ">11 solar masses.<sup class="reference" id="cite_ref-Karachentsevetal2006_1-2" style="line-height: 1em; ">[2] In comparison a 2009 study estimated that the Milky Way and Andromeda are about equal in mass,<sup class="reference" id="cite_ref-CfA_11-0" style="line-height: 1em; ">[12] while a 2006 study put the mass of the Milky Way at ~80% of the mass of Andromeda. The Andromeda Galaxy and the Milky Way are expected to collide in perhaps 4.5 billion years.

<p style="margin-top: 0.4em; margin-bottom: 0.5em; line-height: 19px; color: rgb(0, 0, 0); font-family: sans-serif;">At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,<sup class="reference" id="cite_ref-12" style="line-height: 1em; ">[13] making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using a binoculars or a small telescope.

Observation History
The Persian astronomer Abd al-Rahman al-Sufi wrote a tantalizing line about the chained constellation in his Book of Fixed Stars around 964, describing it as a "small cloud".[14][15] Star charts of that period have it labeled as the Little Cloud.[15] The first description of the object based on telescopic observation was given by German astronomer Simon Marius[15] in 1612. Charles Messier catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi's earlier work. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the "great nebulae" and based on the colour and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius.[16] William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula.[17] The spectra of M31 displayed a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. The Andromeda nebula was very similar to the spectra of individual stars, and from this it was deduced that M31 had a stellar nature. In 1885, a supernova (known as "S Andromedae") was seen in M31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly "Nova 1885".[18] The first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.[19] However, at the time this object was still commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets. The radial velocity of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the Lowell Observatory, using spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (190 mi/s), moving in the direction of the Sun.[20]