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SIRIUS (Alpha Canis Majoris). FromOrion, look south and to the east to find brilliantSirius, as if one really needs directions to find the brighteststar in the sky. Its name comes from the Greek word for "searing"or "scorching," certainly appropriate for a star that shines at thebright end of the "minus-first" (-1.47) magnitude. Sirius is theluminary of the constellationCanis Major,the Greater Dog, which represents Orion's larger hunting dog, andas such is commonly referred to as the "Dog Star." So great is itsprominence that it has two "announcer stars" that from the mid-northern hemisphere rise before it, Procyon and Mirzam. Famed fromtimes long past, the first glimpse of Sirius in dawn announced therising of the Nile in ancient Egypt. (It no longer does because ofprecession, the 26,000-year wobble ofthe Earth's axis.) Sirius is also part of a large asterism, theWinter Triangle, the other two stars ofwhich areBetelgeuse in Orion andProcyon in the smaller dog,Canis Minor. Because of its brilliance, Sirius is thechampion of all twinklers, the effect caused by variable refractionin the Earth's atmosphere. The star, a white class A (A1)hydrogen-fusing dwarf with a temperature of 9880 Kelvin, is brightin part because it is indeed rather luminous, 26 times more so thantheSun, but mostly because it is nearby, amere 8.6 light years away, just double that of the closest star tothe Earth (Alpha Centauri) and thefifth closest star system. Sirius is "metal rich," its ironcontent perhaps double that of the Sun, most likely from some sort ofelemental diffusion. With a radius of 1.75 solar (in agreementwith the measured angular diameter) and a minimum equatorialrotation speed of 16 kilometers per second, Sirius rotates in under5.5 days. The star's greatest claim to fame may be its dim eighthmagnitude (8.44) companion, Sirius B, which is visually nearly10,000 times fainter than the bright star, Sirius A. Sirius B,however, is actually the hotter of the two, a blue-white 24,800Kelvin. Though typically separated from each other by a fewseconds of arc, Sirius B is terribly difficult to see in the glareof Sirius A. The only way the companion star can be both hot anddim 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 orbiteach other with a 50.1 year period at an average distance of 19.8Astronomical Units, about Uranus's distance from the Sun, a largeorbital eccentricity carrying them from 31.5 AU apart to 8.1 AU andback again. They were closest in 1994 and will be again in 2044,while they will be farthest apart in 2019. From the orbit (andspectroscopic data), we find that Sirius A and B have respectivemasses of 2.12 and 1.03 times that of the Sun. Sirius B is thechief member of a trio of classicwhite dwarfs, the othersProcyon B and40Eridani B. Its high mass and tiny radius lead to an amazingaverage density of 1.7 metric tons per cubic centimeter, roughly asugar cube. White dwarfs are the end products of ordinary starslike the Sun, tiny remnants that were once nuclear-fusing coresthat have run out of fuel. Most are balls of carbon and oxygenwhose fates are merely to cool forever. To have evolved first,Sirius B must once have been more massive and luminous than SiriusA. That its mass is now lower is proof that stars loseconsiderable mass as they die. Given the mass of the whitedwarf and the 250 million year age ofthe system, Sirius B may once have been a hot class B3-B5 star thatcould have contained as much as 5 to 7 solar masses, the star perhapslosing over 80 percent of itself back into interstellar spacethrough earlier winds. (Thanks to Steve Ash for prompting arewrite.) Written byJim Kaler 2/06/98; lastupdate 9/26/09. Return toSTARS.
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