The Sirius

From Orion, look south and to the east to find brilliant Sirius, as if one really needs directions to find the brightest star in the sky. Its name comes from the Greek word for “searing” or “scorching,” certainly appropriate for a star that shines at the bright end of the minus-first (−1.47) magnitude. Sirius is the luminary of the constellation Canis Major, the Greater Dog, which represents Orion’s larger hunting dog, and as such is commonly referred to as the “Dog Star.”

So great is its prominence that it has two “announcer stars” that from the mid-northern hemisphere rise before it, Procyon and Mirzam. Famed from times long past, the first glimpse of Sirius in dawn announced the rising of the Nile in ancient Egypt. (It no longer does because of precession, the 26,000-year wobble of the Earth’s axis.) Sirius is also part of a large asterism, the Winter Triangle, the other two stars of which areBetelgeuse in Orion and Procyon in the smaller dog, Canis Minor.

Because of its brilliance, Sirius is the champion of all twinklers, the effect caused by variable refraction in the Earth’s atmosphere. The star, a white class A (A1) hydrogen-fusing dwarf with a temperature of 9880 Kelvin, is bright in part because it is indeed rather luminous, 26 times more so than the Sun, but mostly because it is nearby, a mere 8.6 light years away, just double that of the closest star to the Earth (Alpha Centauri) and the fifth closest star system. Sirius is “metal rich,” its iron content perhaps double that of the Sun, most likely from some sort of elemental diffusion. With a radius of 1.75 solar (in agreement with the measured angular diameter) and a minimum equatorial rotation speed of 16 kilometers per second, Sirius rotates in under 5.5 days.

The star’s greatest claim to fame may be its dim eighth magnitude (8.44) companion, Sirius B, which is visually nearly 10,000 times fainter than the bright star, Sirius A. Sirius B, however, is actually the hotter of the two, a blue-white 24,800 kelvins. Though typically separated from each other by a few seconds of arc, Sirius B is terribly difficult to see in the glare of Sirius A. The only way the companion star can be both hot and dim is to be small, only 0.92 the size of Earth, the total luminosity (including its ultraviolet light) just 2.4 percent that of the Sun.

The two orbit each other with a 50.1 year period at an average distance of 19.8 Astronomical Units, aboutUranus’s distance from the Sun, a large orbital eccentricity carrying them from 31.5 AU apart to 8.1 AU and back again. They were closest in 1994 and will be again in 2044, while they will be farthest apart in 2019. From the orbit (and spectroscopic data), we find that Sirius A and B have respective masses of 2.12 and 1.03 times that of the Sun.

Sirius B is the chief member of a trio of classic white dwarfs, the others Procyon B and 40 Eridani B. Its high mass and tiny radius lead to an amazing average density of 1.7 metric tons per cubic centimeter, roughly a sugar cube. White dwarfs are the end products of ordinary stars like the Sun, tiny remnants that were once nuclear-fusing cores that have run out of fuel. Most are balls of carbon and oxygen whose fates are merely to cool forever. To have evolved first, Sirius B must once have been more massive and luminous than Sirius A. That its mass is now lower is proof that stars lose considerable mass as they die. Given the mass of the white dwarf and the 250 million year age of the system, Sirius B may once have been a hot class B3–B5 star that could have contained as much as 5 to 7 solar masses, the star perhaps losing over 80 percent of itself back into interstellar space through earlier winds.