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 9 light years away. This is a binary star: the primary
star, Sirius A, is orbited by a white dwarf
known as Sirius B.
The name Sirius comes from its use in the 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).
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.