The 88 constellations are by definition asterisms as well, but in the sky we can identify much more. Asterisms can, for example, be subgroups within a constellation, such as the seven brightest stars of the Big Dipper. Or again, asterisms can be formed by bright stars belonging to different constellations, such as the Summer Triangle, whose vertices are formed by Vega, Lira, Altair, Aquila, and Deneb, Swan.
Asterisms can also be formed by small groups of faint stars located within a constellation. These often being bright and easily recognizable to the telescope are exploited by amateur astronomers as a starting point to find other celestial objects.
One of the most famous examples of this type of asterism is the Coat Rack, visible inside the Volpetta and composed of about ten stars of apparent magnitude between the fifth and the seventh.
The asterism consists of six stars that extend in a straight line in the sky to just over a degree and four other stars located below them. Known to man since before the year 1000, the Coat Rack was considered to be a cluster open until the end of the last century.
The accurate distance measurements obtained by Hipparcos have shown that all its stars are indeed at different distances and are not physically bound together. The Coat Rack has thus been revoked its status as an open cluster, but, due to its particular configuration very recognizable, remains an asterism much loved by amateur astronomers.
Credit: Petr Novák.
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