Sirius, << SIHR ee uhs, >> also called the Dog Star, is the brightest star that can be seen from the earth at night. Sirius has a diameter more than twice as large as that of the sun, and it gives off nearly 30 times as much light. It is a star of the first magnitude (see Star (Brightness of stars)).
Sirius forms part of Canis Major, a constellation in the sky of the Southern Hemisphere. Sirius is one of the stars nearest the earth. It is about nine light-years away. A light-year, the distance that light travels in a year, equals about 5.88 trillion miles (9.46 trillion kilometers).
Astronomers classify Sirius as a binary star because it has a companion star, Sirius B (see Binary star). Sirius B is a white dwarf, an extremely dense star that can be as small as the earth. It consists of matter that is 4 million times as dense as water. If matter from Sirius B were brought to the earth’s surface, it would weigh approximately 120,000 tons per cubic foot (4,000,000 metric tons per cubic meter). Because of its tremendous density, Sirius B exerts a strong gravitational pull on Sirius. This powerful gravitational force causes Sirius to move in a wavy line as it travels through space. Sirius and Sirius B make a complete orbit around each other about every 50 years.
See also Calendar; Relativity.
