Sirius, the brightest star in the night sky. It is called the Dog Star, being the chief star of the constellation Canis Major (great dog). Sirius can be seen in the Northern Hemisphere during the evening in winter and early spring. After early spring it cannot be seen until around midsummer, when it becomes visible at dawn. In ancient Egypt, the reappearance of Sirius heralded the annual flooding of the Nile and marked the beginning of the new year.

Sirius is about 8.6 light-years from earth. It is actually a double star, but only the brighter of the pair of stars is visible to the unaided eye. The brighter star has an apparent magnitude of 1.5; the fainter star an apparent magnitude of 8.7, (The larger the magnitude number, the fainter the star.) The faint companion of Sirius, a white dwarf star, is from one-half to twice the size of the earth and has a mass approximately equal to that of the sun. The two stars revolve around each other with a period of 50.1 years.

That Sirius is a double star was detected first by irregularities in its motion. The existence of the faint companion was proposed in 1844; the companion was first seen by Alvan Clark in 1862.