Asterism (astronomy)
In observational astronomy, an asterism is a pattern or group of stars that can be seen in the night sky. Asterisms range from simple shapes of just a few stars to more complex collections of many stars covering large portions of the sky. The stars themselves may be bright naked-eye objects or fainter, even telescopic, but they are generally all of a similar brightness to each other. The larger brighter asterisms are useful for people who are familiarizing themselves with the night sky. For example, the asterism known as the Big Dipper comprises the seven brightest stars  in the constellation Ursa Major. Another is the asterism of the Southern Cross, within the constellation of Crux. The stars within an asterism may be physically associated. For example, the stars of Collinder 399, which resembles a coathanger, are members of an open cluster, the stars of Orion's Belt are all members of the Orion OB1 association, and five of the seven stars of The Plough are members of the Ursa Major Moving Group. In other cases, the stars are unrelated, such as in the Summer Triangle. The 88 constellations into which the sky is divided are based on  asterisms considered to represent an object, person, or animal, often mythological. However, they are formally defined regions of sky, and contain all the celestial objects within their boundaries. Asterisms do not have officially determined boundaries and are a more general concept which may refer to any identified pattern of stars. Background of asterisms and constellations. In many early civilizations, it was already common to associate groups of stars in connect-the-dots stick-figure patterns; some of the earliest records are those of ancient India in the Vedanga Jyotisha and 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. Large or bright asterisms. Component stars of some asterisms are bright and mark out simple geometric shapes. Constellation-based asterisms. Some  asterisms may also be part of a constellation referring to the traditional figuring of the whole outline, for example Orion's Belt, and the Y in Aquarius (historically called "the Urn"). Commonly recognised asterisms. Other asterisms are also composed of stars from one constellation, but do not refer to the traditional figures. Cross-border asterisms. Other asterisms that are formed from stars in more than one constellation. Telescopic asterisms. Asterisms range from the large and obvious to the small, and even telescopic.