Distinctive groupings of stars forming part of the recognizedconstellation outlines, or lying within their boundaries, are known as asterisms. Ranging in size from sprawling naked-eye figures to minute stellar settings, they are found in every quarter of the sky and at all seasons of the year.
The larger asterisms — ones like the Big Dipper in Ursa Major and the Great Square of Pegasus — are often better known than their host constellations. One of the most famous is in the northwest these frosty December evenings.
The Northern Cross
The brightest six stars of the constellation Cygnus (the Swan) — which was known simply as the “Bird” in ancient times — compose an asterism popularly called the Northern Cross. [Night Sky Observing Guide for December 2012 (Gallery)]
Bright Deneb decorates the top of the Cross. Albereo, at the foot of the Cross, is really a pair of stars of beautifully contrasting colors: a third-magnitude orange star and its fifth-magnitude blue companion are clearly visible in even a low-power telescope. (In astronomy, lower magnitudes signify relatively brighter objects. Venus' magnitude is around -5, for example, while the full moon's is about -13.)
While usually regarded as a summertime pattern, the Northern Cross is best oriented for viewing now. It appears to stand majestically upright on the northwest horizon at around 8:30 p.m. local time, forming an appropriate Christmas symbol. (And just before dawn on Easter morning, the cross lies on its side in the eastern sky.)