An asterism is a popularly-known pattern or group of stars that can be seen in the night sky. This colloquial definition makes it appear quite similar to a constellation, but they differ in that: a constellation is a formally-named area of the sky and all the celestial objects within it, representing an object, person, or animal, often mythological; while an asterism is a visually obvious collection of stars and the lines used to mentally connect them. Component stars of asterisms are bright and mark out simple geometric shapes.
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, or crossing the boundaries of two or more constellations.
Large or bright asterisms:
The Great Diamond consisting of Arcturus, Spica, Denebola, and Cor Caroli. An east-west line from Arcturus to Denebola forms an equilateral triangle with Cor Caroli to the North, and another with Spica to the South. The Arcturus, Regulus, Spica triangle is sometimes given the name Spring Triangle. Together these two triangles form the Diamond.
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.
The Great Square of Pegasus is the quadrilateral formed by the stars Markab, Scheat, Algenib, and Alpheratz, representing the body of the winged horse. The asterism was recognized as the constellation ASH.IKU “The Field” on the MUL.APIN cuneiform tablets from about 1100 to 700 BC.
The Winter Hexagon includes one-third of the 1st-magnitude stars visible in the sky (seven of twenty-one) 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 Winter Triangle is visible in the northern sky’s winter and comprises the first magnitude stars Procyon, Betelgeuse and Sirius.
The Lightning Bolt, visible in the northern sky’s summer and autumn, is useful for orienting among constellations. From north to south, it consists of the stars Epsilon Pegasi, Alpha Aquarii, Beta Aquarii and Delta Capricorni.
Commonly recognised asterisms:
Four other stars (Beta, Upsilon, Theta, and Omega Carinae) form a well-shaped diamond — the Diamond Cross.
The Saucepan or Pot, being the same stars as the Belt and Sword of Orion. The end of the handle is at Orionis, with the far rim at Orionis.
The four central stars in Hercules, Epsilon , Zeta, Eta, and Pi, form the well-known Keystone.
The curve of stars at the front end of the Lion from Epsilon to Regulus, looking much like a mirror-image question mark, has long been known as the Sickle.