在希臘神話中，安德洛墨達（Andromeda，古希臘語：Ἀνδρομέδα）是衣索比亞（Aethiopia原指尼羅河上游地區，即在現今的埃塞俄比亞境內，Ethipopia就是由Aethiopia演化而來）國王塞菲俄斯（Aethiopian King Cepheus 現今的英文含義是仙王座）和他的妻子卡西俄珀亞（Cassiopeia現含義仙后座）的女兒，即衣索比亞公主。話說衣索比亞公主長得非常美麗，她的母親傲慢地說她比所有的海中神女都美，因而觸怒眾海中神女。與此同時海神波塞冬（Poseidon，眾神之王Zeus宙斯的兄弟）受到冥王哈迪斯（Hades，宙斯的另一個兄弟）讒言的影響，決定派遣海怪希徳斯（Cetus現含義鯨魚座）蹂躪整個衣索比亞地區作為神的懲罰。為此，Andromeda被剝光衣服赤裸裸的被用鎖鏈拴在海岸邊一塊岩石上作為祭品獻給海怪，但碰巧珀耳修斯Perseus（英仙座）經過此地，將她救出，之後兩人成為夫妻。仙女座的傳說由此而來。再加多一句，漫畫《聖鬥士星矢》的作者車田正美對希臘神話相當的有研究，因此仙女座瞬的聖衣才會被設計成使用鎖鏈，贊一個。
言歸正傳，仙女星系Andromeda是距離我們銀河系最近的星系，也被稱為梅西耶31，M31，或NGC 224，是一個螺旋星系，距離地球約780 kiloparsecs（250萬光年）。這是距離銀河系最近的主要星系，用古希臘神話傳說中的仙女Andromeda命名，古文獻中被稱為仙女座大星雲。仙女座星系被相信是本星系群中最大的星系，直徑約20萬光年，外表頗似銀河系。本星系群的成員有仙女星系、銀河系、三角座星系，還有大約44個小星系。
仙女座星系是本地星系團中質量最大的星系。儘管早期的研究結果認為銀河系中含有較多的暗物質所以可能是本地星系團中最龐大的星系，但在2006年斯皮策太空望遠鏡發現仙女座包含一個萬億顆恆星（10的12次方）：至少是銀河系所含恆星數量（估計為2000-4000億）的兩倍。
仙女座星系估計為1.5×10^12個太陽質量，而銀河系的質量估計為8.5×10^11個太陽質量。相比之下，2009年的一項研究估計，銀河系和M31是大致相等的質量，而2006年的研究認為銀河系的質量約為仙女座質量的80％。銀河系和仙女座預計在37.5億年後碰撞，最終合併，形成一個巨大的橢圓星系或者一個大磁盤狀星系。
仙女座星系的可視星等值是3.4，是夜空中最亮的天體之一，在沒有月亮的夜晚，即便該地區有中度光汙染也可用裸眼看見。雖然用較大的望遠鏡拍攝到的仙女座寬度超過六倍寬的滿月，但在用肉眼、雙筒望遠鏡或小型望遠鏡觀測時只有中央區域明亮可見。
本期內容獻給所有愛好star gazing的小夥伴們，我們都是星辰的後裔，仰望星空就是在仰望我們的祖先。
In Greek mythology, Andromeda is the daughter of the Aethiopian king Cepheus and his wife Cassiopeia. When Cassiopeia's hubris leads her to boast that Andromeda is more beautiful than the Nereids, Poseidon, influenced by Hades, sends a sea monster, Cetus, to ravage Aethiopia as divine punishment. Andromeda is stripped and chained naked to a rock as a sacrifice to sate the monster, but is saved from death by Perseus.
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 (10^12) 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 1.5×10^12 solar masses, while the mass of the Milky Way is estimated to be 8.5×10^11 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.
At 3.4, the apparent magnitude of the Andromeda Galaxy is one of the brightest of any 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.
Today's article is for all the star gazing fans, every time we look at the stars, we are looking right at our ancestor.
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