Sirius — Sirius (α Canis Major, also known as the Dog Star) is the brightest star (-1.46m) in the night sky and can be seen from every inhabited region of the Earth’s surface.

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. 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, it is not clear why ancient observations of Sirius describe it as a red star, when today Sirius A is bluish white. It is not believed likely that changes in either Sirius A or Sirius B could have occurred in such as short time.

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.

Image Credit: NASA, ESA, H. Bond (STScI) and M. Barstow (University of Leicester)