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SIRIUS

(Alpha Canis Majoris). 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-second" (-1.46) 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 are Betelgeuse 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 is bright in part because it is indeed rather luminous, 23 times more so than the Sun. Though a "main sequence" "dwarf star" that, like the Sun, shines by hydrogen fusion, it is twice as massive as our star, and as a result is hotter and brighter, its 9400 Kelvin temperature making it quite white. But it is also bright to us because it is nearby, a mere 8.6 light years away, just double that of the closest star to the Earth, Alpha Centauri. Sirius's greatest claim to fame may be its dim companion. Though at eighth magnitude (8.44), visually some 10,000 times fainter than the bright star we see (which is called "Sirius A"), Sirius B is actually the hotter of the two, a blue-white 27,000 Kelvin. 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, smaller than Earth. The two orbit each other with a 50-year period at an average distance of 20 Astronomical Units (the AU the average distance between the Earth and the Sun), the orbital eccentricity carrying it from 31 AU to 8 AU and back again. From the orbit, we find that the little one has about the mass of the Sun. Called a "white dwarf," on the average it packs a metric ton into a cubic centimeter, roughly a sugar cube. White dwarfs are the end products of ordinary stars like the Sun, tiny remnants that have run out of nuclear fuel. Most are balls of carbon and oxygen whose fates are merely to cool forever. Sirius B itself is the end product of a star that at one time was much more massive and brilliant than Sirius A is today. (2/6/98; 12/13/02)