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In observational astronomy, an asterism is a popularly known pattern or group of stars that can be seen in the night sky. This colloquial definition[a] makes it appear quite similar to a constellation, but they differ mostly in that a constellation is an officially recognized area of the sky, while an asterism is a visually obvious collection of stars and the lines used to mentally connect them; as such, asterisms do not have officially determined boundaries and are therefore a more general concept which may refer to any identified pattern of stars. This distinction between terms remains somewhat inconsistent, varying among published sources. An asterism may be understood as an informal group of stars within the area of an official or defunct former constellation. Some include stars from more than one constellation.
Asterisms range from simple shapes of just few stars to more complex collections of many bright stars. They are useful for people who are familiarizing themselves with the night sky. For example, the asterisms known as The Plough (Charles' Wain, the Big Dipper, etc.) comprises the seven brightest stars in the International Astronomical Union (IAU) recognised constellation Ursa Major. Another is the asterism of the Southern Cross, whose recognised constellation is Crux.
In many early civilizations, it was already common to associate groups of stars in connect-the-dotsstick-figure patterns; some of the earliest records are those of the Babylonians. This process was essentially arbitrary, and different cultures have identified different constellations, although a few of the more obvious patterns tend to appear in the constellations of multiple cultures, such as those of Orion and Scorpius. As anyone could arrange and name a grouping of stars there was no distinct difference between a constellation and an asterism. e.g. Pliny the Elder (23-79 AD) in his book Naturalis Historia refers and mentions 72 asterisms.
A general list containing 48 constellations likely began to develop with the astronomer Hipparchus (c. 190 - c. 120 BC ), and was mostly accepted as standard in Europe 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 any leftover stars to create and squeeze in a new grouping among the established constellations.
Furthermore, exploration by Europeans to other parts of the globe exposed them to stars unknown to them. Two astronomers particularly known for greatly expanding the number of southern constellations were Johann Bayer (1572-1625) and Nicolas Louis de Lacaille (1713-1762). Bayer had listed twelve figures made out of stars that were too far south for Ptolemy to have seen; Lacaille created 14 new groups, mostly for the area surrounding South Celestial Pole. Many of these proposed constellations have been formally accepted, but the rest have historically remained as asterisms.
In 1928, the International Astronomical Union (IAU) precisely divided the sky into 88 official constellations following geometric boundaries encompassing all of the stars within them. Any additional new selected groupings of stars or former constellations are often considered as asterisms. However, depending on the particular literature source, any technical distinctions between the terms 'constellation' and 'asterism' often remain somewhat ambiguous. e.g. Both the open clustersThe Pleiades or Seven Sisters and The Hyades in Taurus are sometimes considered asterisms, but this depends on the source.
Large or bright asterisms
Component stars of asterisms are bright and mark out simple geometric shapes.
The Summer Triangle of Deneb, Altair, and Vega -- ? Cygni, ? Aquilae, and ? Lyrae -- is easily recognized in the northern hemisphere summer skies, as its three stars are all of the 1st magnitude. The stars of the Triangle are in the band of the Milky Way which marks the galactic equator, and are in the direction of the galactic center.
One-third of the 1st-magnitude stars visible in the sky (seven of twenty-one) are in the so-called Winter Hexagon with Capella, Aldebaran, Rigel, Sirius, Procyon, and Pollux with 2nd-magnitude Castor, on the periphery, and Betelgeuse off-center. Although somewhat flattened, and thus more elliptical than circular, the figure is so large that it cannot be taken in all at once, thus making the lack of true circularity less noticeable. (The projection in the chart exaggerates the stretching.) Some prefer to regard it as a Heavenly 'G'.
A familiar asterism is the Big Dipper, Plough or Charles's Wain, which is composed of the seven brightest stars in Ursa Major. These stars delineate the Bear's hindquarters and exaggerated tail, or alternatively, the "handle" forming the upper outline of the bear's head and neck. With its longer tail, Ursa Minor hardly appears bearlike at all, and is widely known by its pseudonym, the Little Dipper.
The Fish Hook is the traditional Hawaiian name for Scorpius. The image will be even more obvious if the chart's lines from Antares (? Sco) to Beta Scorpii (? Sco) and Pi Scorpii (? Sco) are replaced with a line from Beta through Delta Scorpii (? Sco) to Pi forming a large capped "J."
The Southern Cross is an asterism by name, but the whole area is now recognised as the constellation Crux. The main stars are Alpha, Beta, Gamma, Delta, and arguably also Epsilon Crucis. Earlier, Crux was deemed an asterism when Bayer created it in Uranometria (1603) from the stars in the hind legs of Centaurus, decreasing the size of Centaur. These same stars were probably identified by Pliny the Elder in his Naturalis Historia as the asterism 'Thronos Caesaris.'
Adding vertical lines to connect the limbs at the left and right in the main diagram of Hercules will complete the figure of the Butterfly.
Although hardly an ancient notion, it is easy to see why the Ice Cream Cone is sometimes applied to Boötes. It is even better known as the Kite.
The stars of Cassiopeia form a W which is often used as a nickname.
Some asterisms may also be part of a constellation referring to the traditional figuring of the whole outline. For example, there are:
Dubhe and Merak (Alpha and Beta Ursae Majoris), the two stars at the end of the bowl of the Big Dipper are habitually called The Pointers: a line from ? to ? and continued for a bit over five times the distance between them, arrives at the North Celestial Pole and the star Polaris (? UMi/Alpha Ursae Minoris), the North Star.
Rigil Kentaurus (? Centauri) and Hadar (? Centauri) are the Southern Pointers leading to the Southern Cross and thus helping to distinguish Crux from the False Cross.
Other asterisms that are formed from stars in more than one constellation.
There is another large asterism which, like the Diamond of Virgo, is composed of a pair of equilateral triangles. Sirius (? CMa), Procyon (? CMi), and Betelgeuse (? Ori) form one to the North (Winter Triangle) while Sirius, Naos (? Pup), and Phakt (? Col) form another to the South. Unlike the Diamond, however, these triangles meet, not base-to-base, but vertex-to-vertex, forming the Egyptian X. The name derives from both the shape and, because the stars straddle the Celestial Equator, it is more easily seen from south of the Mediterranean than in Europe.
The Lozenge is a small diamond formed from three stars - Eltanin, Grumium, and Rastaban (Gamma, Xi, and Beta Draconis) - in the head of Draco and one - Iota Herculis - in the foot of Hercules.
The diamond-shaped False Cross is composed of the four stars Alspehina (? Velorum) and Markeb (? Velorum) and Avior (? Carinae) and Aspidiske (? Carinae). Although its component stars are not quite as bright as those of the Southern Cross, it is somewhat larger and better shaped than the Southern Cross, for which it sometimes mistaken, causing errors in astronavigation. Like the Southern Cross, three of its main four stars are whitish and one orange.
From latitudes above 40 degrees north especially, a prominent upper-case Y is formed by Arcturus (? Boötis), Seginus (? Boötis) and Izar (? Boötis), and Alpha Coronae Borealis (Alphecca or Gemma). Alpha Coronae Borealis is far brighter than either Delta or Beta Bootis, distorting the "kite" or "ice-cream cone" shape of Bootes. From the United Kingdom in particular, where there is serious light pollution in many areas and also twilight all night for much of the time these constellations appear, this "Y" is often visible while ? and ? Bootis and the other stars in Corona Borealis are not.
Asterisms range from the large and obvious to the small, and even telescopic.
^ In usage, 'constellation' is sometimes a synonym of 'asterism', which reflects the fact that the respective words in Latin (constellatio) and Greek (?, asterismos), from which the English terms are derived, are synonymous. While a terminological distinction may be present in English, such a distinction does not necessarily occur in other languages.