Asterism

Asterisms are patterns of stars seen in Earth's sky which are not official constellations. Examples include:

• the Big Dipper in Ursa Major;
• the Little Dipper in Ursa Minor;
• the Teapot in Sagittarius,
• Orion's belt;
• the Summer Triangle (Deneb, Altair, and Vega, or α Cygni, Aquilae, and Lyrae);
• the Great Square of Pegasus (which includes α Andromedae);
• the Urn in Aquarius;
• the Sickle in Leo.

One ancient asterism, Coma Berenices, is now officially a constellation.

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A second meaning of asterism is the optical phenomenon displayed by some rubies, sapphires, and other gems of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone. This is caused by small needles of rutile (titanium oxide) in the gem. The stars may have six rays or twelve. The rays are reflections from the the rutile crystal. Rutile can also cause a "cat's eye" effect in gemstones.