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In astronomy, an asterism is a pattern of stars recognized in the Earth's night sky. It may be part of an official constellation or it may be composed of stars from more than one constellation. Colloquial usage does not draw a sharp distinction between "constellation" in the sense of an asterism (pattern of stars) and "constellation" in the sense of an area of the sky surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept. For example, the asterism known as the Big Dipper comprises the seven brightest stars in the IAU constellation that is called Ursa Major. Like constellations, asterisms are in most cases composed of stars which, although visible in the same general area, are often located at very different distances from Earth. Simple shapes composed of a few stars make asterisms easy to identify. Thus they are particularly useful to people who are familiarizing themselves with the night sky. == Background == Even before the dawn of civilization, it became common to clump various stars together in connect-the-dots stick-figure patterns. The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, ''e.g.'', Orion and Scorpius. Historically, without an "official" list, there was really no difference between a constellation and an asterism. Anyone could arrange and name a grouping which might or might not be generally accepted. Still, some of our own constellations go back at least as far as the Babylonians. Our current list is based on that of the Greco-Roman astronomer, Claudius Ptolemy of Alexandria (c 90–c 168). His list of 48 constellations was accepted as ''the'' standard for 1,800 years. As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use the leftover, non-figure ("amorphic") stars to create and squeeze in a new grouping among the established constellations. Two astronomers particularly known for attempting to expand Ptolemy's catalog were Johann Bayer (1572–1625) and Nicolas Louis de Lacaille (1713–1762). Bayer listed a dozen figures that had been suggested since Ptolemy's day; Lacaille created new groups, mostly for the area near the South Celestial Pole, unobserved by the ancients. Many of their proposed constellations have been accepted, the rest remaining asterisms, mostly obsolete. Clarification was necessary to determine which groupings ''are'' constellations and which stars belonged to them. The situation was finally regularized in 1930 when the International Astronomical Union (IAU) divided the sky into 88 official constellations with precise boundaries. Any other grouping is an asterism. *The seasons indicated here are for the Northern Hemisphere. For the Southern Hemisphere, substitute the opposite season. *The smaller the number of a star's magnitude, the brighter it is. Thus those of the 1st magnitude are brighter than those of the 2nd. Even negative magnitudes are possible, and the few so rated, though still called "first" magnitude, are the very brightest. *A true star cluster (see below), whose stars ''are'' gravitationally related, is not an asterism. 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「Asterism (astronomy)」の詳細全文を読む スポンサード リンク
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