What is Sirius?

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  • Written By: Michael Anissimov
  • Edited By: Bronwyn Harris
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  • Last Modified Date: 02 October 2019
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Sirius is the brightest star in the sky, besides our Sun of course. It is so bright that it can be seen in the daylight when the sky is very clear, the observer is at high altitude, and the Sun is close to the horizon. Sirius is the brightest star in the constellation Canis Majoris, the dog. Accordingly, it is sometimes called the Dog Star. The Hubble Space Telescope images of Sirius are among the finest of any star besides the Sun.

Besides being one of the brightest stars, Sirius is also among the closest. Sirius is only 8.6 light years away from the Earth, making it the eighth nearest star. Its proximity has sometimes led Sirius to be featured in speculation about interstellar travel.

Although frequently referred to as a single star, Sirius is actually a binary system including Sirius A and Sirius B. Sirius B is a white dwarf star, a husk created when a larger star burns most of its nuclear fuel. Sirius B has a mass similar to the Sun, but like other white dwarfs, a volume only around that of Earth. It continues glowing due to heat left over from when it was a hydrogen-fusing (main sequence) star. It is one of the most massive white dwarfs none, as the typical mass of a white dwarf is .5 - .6 solar. When Sirius B was a main sequence star, it was quite larger than its companion at ~5 solar masses.


Sirius B, the more prominent star in the binary system, has a mass about twice that of the Sun, and an absolute luminosity about 25 times greater. In astrophysics, star luminosity tends to increase exponentially with linear increases in mass, because the fusion reactions are sensitive to increases in temperature and pressure found in the cores of more massive stars.

Because Sirius is more massive than the Sun, it fuses hydrogen faster, and therefore has a higher surface temperature — about 9,940 K in comparison to the Sun's 5,780 K. Both stars are bright white.


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