Sirius (α Canis Major, also known as the Dog Star) is the brightest star (-1.46m) in the night sky. This star can be seen from every inhabited region of the Earth's surface and, in the northern hemisphere, is known as a vertex of the Winter Triangle. At a distance of 8.6 light years, Sirius is also one of the nearest stars to Earth. It is a main sequence star of spectral type A0 or A1 and has a mass about 2.4 times that of the Sun.
Sirius has a white dwarf companion called Sirius B which orbits it with a period close to 50 years. It was the first white dwarf star to be discovered. It is sometimes call the Pup. The visible star is now sometimes known as Sirius A.
Historically, many cultures have attached special significance to Sirius. Sirius was worshipped in the valley of the Nile long before Rome was founded, and many ancient Egyptian temples were constructed oriented so that light from the star could penetrate to their inner altars. The Egyptians based their calendar on the heliacal rising of Sirius, which occurred just before the annual flooding of the Nile and the Summer solstice. In Greek mythology, Orion's dog became Sirius. The Greeks also associated Sirius with the heat of summer: the name Sirius is derived from Seirios meaning "the scorcher." This also explains the phrase "dog days of summer".
There are a few unsolved mysteries regarding Sirius. Firstly, it has been suggested that there is a third very small companion star, but it appears that this has not yet been definitely confirmed. Secondly, ancient observations of Sirius describe it as a red star, when today Sirius A is bluish white. The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy is rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when it was observed rising left the viewer with the impression that it was red. A third mystery is a suggestion that the Dogon tribe of Africa knew about unseen companion star(s) before they were discovered in the 19th century. This is a source of speculation for UFO enthusiasts and was the subject of the book The Sirius Mystery by Robert Temple.