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 catalogue 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.
Read more about this topic: Asterism (astronomy)
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