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 due to its being the seventh closest star to the
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. The
white dwarf has 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 to white dwarf status 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.