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Sirius

The Dog Star, Canicula, Aschere,
Alpha Canis Majoris, 9 Canis Majoris

White main sequence star in Canis Major

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Proper Names	Aschere, Canicula, The Dog Star, Sirius
Bayer Designation	Alpha Canis Majoris
Flamsteed Number	9 Canis Majoris
HR (BSC)	2491
HD	48915
Constellation	Canis Major
Right Ascension	6h 45m 9s
Declination	-16° 42' 58"
Distance	8.6 light years 2.6 parsecs
Magnitude	Apparent: -1.44 Absolute: +1.46
Spectral Class	A1Vm white main sequence star
Optimum Visibility	January
Notes	The main white star (Sirius A) has a companion, Sirius B, a white dwarf star that orbits the primary in just over fifty years.

Sirius B is a white dwarf - the remnant of a once-giant star packed into a body smaller than the Earth. Though its nuclear fuel is now exhausted, it retains enough energy to generate a dim glow. Its great density also gives it tremendous heat: the surface of Sirius B is some six times hotter than that of the Sun.

The Galactic position and direction of Sirius relative to Earth's Sun. Note that, at this extreme scale, the two stars are effectively in the same place.

The brightest star in the sky is also one of the nearest to Earth: Sirius is just under nine light years away. This is a binary system: the primary star, Sirius A, is orbited by a white dwarf known as Sirius B.

Brilliant Sirius, just nine light years from Earth, is in the north of Canis Major, the constellation of the Greater Dog. From its home constellation it takes its common name, the 'Dog Star'.

The name Sirius comes from its use in the ancient Greek calendar. The time of year when this star rose and set with the Sun was the hottest part of the summer, and was given the name Seirios, 'scorching', a name that was acquired by the star itself. It played an important part in the Egyptian calendar, too, because its rising coincided with the annual flooding the Nile, and so with Egyptians' entire agricultural cycle. Indeed, the Egyptians went so far as to give it its own goddess, Sopdet (also known as Sothis).

The sky's brightest star, Sirius, as seen on a winter evening in the northern hemisphere. The three stars of Orion's Belt point the way to this brilliant near neighbour of the Sun.

Sirius is a luminous star in comparison to the Sun, and rather more massive (its mass is about double that of the Sun). In comparison with other stars, though, it is not notably luminous, and its particular brilliance in the sky is largely due to its being the seventh closest star to the Solar System.

Sirius is a binary system. As well as the bright white star we see in the sky, Sirius A, the system also contains a much fainter white dwarf star, Sirius B, with a mass very close to that of our own Sun. The two stars orbit one another over a period of fifty years.

The presence of a white dwarf in the Sirian system means that it did not always appear as it does now - at one time, Sirius B must have been a red giant star. Indeed, there is some slight evidence that it only completed its collapse into a white dwarf within the last few thousand years - ancient records seem to refer to Sirius having a reddish appearance, which may, just possibly, refer to the dying glow of Sirius B's red giant phase.

Imagery provided by Aladin sky atlas

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