What’s Sirius?

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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 daylight when the sky is very clear, the observer is at a high altitude, and the Sun is near the horizon. Sirius is the brightest star in the constellation Canis Major, the dog. As a result, he is sometimes referred to as a Dog Star. The Hubble Space Telescope’s images of Sirius are among the most beautiful of any star besides the Sun.

In addition to being one of the brightest stars, Sirius is also among the closest. Sirius is only 8.6 light years from Earth, making it the eighth closest star. Its proximity has sometimes led to Sirius being the subject of speculation about interstellar travel.

Although often 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 shell created when a larger star burns off 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 the Earth. It continues to glow due to residual heat from when it was a hydrogen fusing star (main sequence). 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 much larger than its companion at about 5 solar masses.

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

Because Sirius is more massive than the Sun, it melts hydrogen faster and therefore has a higher surface temperature, about 9.940 K compared to the Sun’s 5.780 K. Both stars are brilliant white.

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