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The Andromeda Galaxy (), also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth.〔 It is the nearest major galaxy to the Milky Way and was often referred to as the Great Andromeda Nebula in older texts. It received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. Being approximately 220,000 light years across, it is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller galaxies. The Andromeda Galaxy is the most massive galaxy in the Local Group as well. Despite earlier findings that suggested that the Milky Way contains more dark matter and could be the most massive in the grouping,〔 the 2006 observations by the Spitzer Space Telescope revealed that Andromeda contains one trillion (1012) stars:〔 at least twice the number of stars in the Milky Way, which is estimated to be 200–400 billion.〔 The Andromeda Galaxy is estimated to be 1.5 solar masses,〔 while the mass of the Milky Way is estimated to be 8.5 solar masses. In comparison, a 2009 study estimated that the Milky Way and M31 are about equal in mass,〔 while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The Milky Way and Andromeda are expected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy 〔 or perhaps a large disk galaxy. The apparent magnitude of the Andromeda Galaxy, at 3.4, is one of the brightest of any of the Messier objects,〔 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 binoculars or a small telescope, making it appear similar to a star. == Observation history == The Persian astronomer Abd al-Rahman al-Sufi wrote a line about the chained constellation in his ''Book of Fixed Stars'' around 964, describing the Andromeda Galaxy as a "small cloud".〔〔 Star charts of that period labeled as the ''Little Cloud''.〔 The first description of the Andromeda Galaxy based on telescopic observation was given by German astronomer Simon Marius on December 15, 1612.〔 Charles Messier catalogued Andromeda as object M31 in 1764 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of M31. He believed M31 to be the nearest of all the "great nebulae" and based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius.〔 William Huggins in 1864 observed the spectrum of M31 and noted that it differs from a gaseous nebula.〔 The spectra of M31 displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. M31's spectrum is very similar to the spectra of individual stars, and from this it was deduced that M31 has 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".〔 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.〔 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 M31 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 , moving in the direction of the Sun.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Andromeda Galaxy」の詳細全文を読む スポンサード リンク
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