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Brocchi\'s Cluster, also known as the Coathanger: a conspicuous asterism easily seen with binoculars in the constellation Vulpecula.
In astronomy, an asterism is a pattern of stars seen in Earth\'s sky which is not an official constellation. Like constellations, they are composed of stars which, while they are in the same general direction, are not physically related, often being at significantly different distances from Earth. An asterism may be composed of stars from one or more constellations. They are mostly simple shapes and few stars make these patterns easy to identify and thus particularly useful to those just learning to orient themselves when viewing the night sky.
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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 Sumerians.
Our current list is based on that of the Greco-Roman astronomer, Claudius Ptolemy of Alexandria (c 85–c 165). His list of 48 constellations was accepted as the standard for 1800 years. As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use the left over, non-figure ("amorphic") stars to create and squeeze in a new grouping among the established constellations. Two astronomers particularly known for expanding Ptolemy\'s catalogue were Johann Bayer (1572–1625) and Nicolas Louis de Lacaille (1713–1762). Bayer listed 12 new constellations to fill the area near the South Celestial Pole, unobserved by the ancients; Lacaille created 14 new constellations in this area and around. 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.
Points to bear in mind —
By happenstance, in each of the four seasons there is a large asterism overhead near midnight. Their component stars are bright and mark out simple geometric shapes.
Undoubtedly, the best-known asterism in the Northern Hemisphere is the Big Dipper or Plough. Composed of the seven brightest stars in Ursa Major, where they delineate the Bear\'s hindquarters and exaggerated tail, the Dipper is often the first grouping of stars shown to beginners in the Northern Hemisphere. With its even more ridiculously lengthened tail, Ursa Minor hardly appears bearlike at all. It does much better under its pseudonym of the Little Dipper.
Ursa Minor is not the only constellation that does not look very much like what it represents. Very few do. This has led to nicknames for some of the constellations. These nicknames are another variety of asterism. A glance at the stick-figures shown in the charts under the constellation names will easily explain the origin of these asterisms.
Other asterisms are also composed of stars from one constellation, but do not refer to the traditional figures.
Like the Seasonal asterisms, there are others that are formed from stars in more than one constellation.
Asterisms range from the large and obvious to the small, and even telescopic.
Argo is a special case. Argo Navis, ("the ship Argo"), was, by far, the largest of Ptolemy\'s constellations. Starting with Lacaille in his Coelum Australe Stelliferum (1763), it became common to refer to its various parts as the Keel, the Poop, and the Sails. In the 1930 IAU arrangement, Argo was deemed too large, and these old sectional asterisms were recognized as official constellations (Carina, Puppis, and Vela), thereby turning Argo, as a whole, into an asterism.
The Southern Cross is not an asterism, but merely a variation on the meaning of Crux. Crux was an asterism when Bayer created it in Uranometria (1603) from stars in the hind legs of Centaurus. It was given constellation status in 1930, thereby mutilating the Centaur.
In its original figuration, Leo included a spray of faint stars pictured as the tuft in the Lion\'s Tail which stretched straight out from its body. Antedating even Ptolemy by centuries, Conon of Alexandria created the asterism "Berenice\'s Hair" commemorating his queen in 243 BC. Following Tycho\'s acceptance of Coma, Bayer recorded it and refigured the Lion. The IAU confirmed Coma\'s status as a constellation.
Even so venerable a constellation as Libra was once merely an asterism. Until the middle of the first millennium BC, the Zodiac consisted of only eleven constellations. The biblical reference to "the eleven stars" (Genesis 37:9) is more accurately "the eleven asterisms/constellations (of the Zodiac)." At the time, Scorpius\' claws were pictured as extending to Zubenelgenubi, "the southern claw" and Zubeneschamali, "the northern claw" (Alpha and Beta Librae). Later, when Virgo was reimagined as Astraea, the goddess of justice, the Claws became a set of scales held in her hand. By Ptolemy\'s day, Libra had become an independent constellation, unconnected with either of its neighbors. Still, the names of its stars reflect the time when it was the asterism of "The Claws" and its figuration is that of the old sectional asterism within Virgo. Also, compare Arabic zubānā = "scorpion\'s claws", Babylonian zibānītu = "weighing scales"; old-type weighing scales were often held up for use by a string tied to the middle of the cross-arm, with a fancied resemblance to holding a scorpion by the end of its tail, instead of being on a stand; this may have led to a word for "scorpion\'s claws" coming also to mean "weighing scales", and thus to the constellation\'s name being reinterpreted.
Lupus was also an asterism, regarded as part of Centaurus until the 200s BCE.
Open clusters are groups of stars that are physically related — gravitationally bound together and moving through the galaxy in the same direction and speed. As these groupings are not human constructs, but real phenomena, they do not count as asterisms. Among the best-known and closest are the Pleiades (M45) and the Hyades in Taurus and the Beehive (M44) in Cancer (in fact Aldebaran, in the same line of sight as the Hyades open cluster, completes a V-shaped asterism with it).
The fine point of what constitutes an asterism may be seen in two examples. Theta Orionis (θ Ori) is embedded in, and illuminates, the Orion Nebula (M42). Looked at telescopically, it resolved into four stars arranged in a trapezoid, and they were nicknamed the Trapezium. The asterism retained this name even when it was discovered that there were yet more stars in the group. However, it has since been determined that the Orion Nebula is a stellar nursery and that the Trapezium is actually an Open Cluster. Thus it is no longer an asterism. On the other hand, M73 in Aquarius, which was thought to be an Open Cluster, turns out to be composed of unrelated stars, and may now be considered to be an asterism.
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