From
http://www.astro.uiuc.edu/~kaler/sow/antares.html
ANTARES (Alpha Scorpii). A brilliant jewel set within the
Milky Way, Antares guides us to one of the great constellations of the sky, the
Zodiac's
Scorpius (or Scorpio), the celestial scorpion, one of the few constellations that actually looks like what it represents. Antares, a class M (M1.5) red
supergiant gleaming redly at the scorpion's heart, has a color similar to
Mars. Since it is found within the Zodiac, which contains the apparent path of the Sun and planets, it is commonly mistaken for the red planet, a fact shown by its name, Antares, or "Ant-Ares," which means "like Mars," "Ares" being the Greek name for the god of war. This magnificent first magnitude (typically 0.96) star, shining opposite
Betelgeuse, its counterpart in
Orion, is ranked the 15th brightest in the sky. It is, however, a semi-regular variable that can change by several tenths of magnitude over a period of years. Its great distance of 600 light years reveals that it is truly luminous, to the eye over 10,000 times brighter than the Sun. Because it is cool, only about 3600 degrees Kelvin at its surface, it radiates a considerable amount of its light in the invisible infrared. When that is taken into account, the star becomes some 60,000 times brighter than the
Sun. A low temperature coupled with high luminosity tells us that the star must be huge, luminosity and temperature giving a radius of about 3 Astronomical Units. It is so big that astronomers can easily detect and measure the size of its apparent disk, which gives an even bigger radius of 3.8 AU, three-fourths the size of the orbit of Jupiter The difference is caused by uncertainties in distance, temperature, and the actual location of the mass-losing surface, as the star is slowly evaporating under a fierce wind that has encased it in a gas cloud, or nebula, that shines by light scattered from the ultraluminous star within. Buried within the wind is a fifth magnitude (5.5) hot class B (B2.5) companion star (only 3 seconds of arc away) that hides within Antares' bright glare. The two are separated by roughly 550 AU and take perhaps 2500 years to orbit each other. The companion hollows out a small ionized region within the wind, and although blue-white, has the reputation of appearing green as a result of a contrast effect with its brilliant reddish mate. Antares, with a mass of 15 to 18 solar masses, probably does not have much time left to it. It is massive enough someday to develop an iron core and eventually to explode as a brilliant
supernova. The event may be a million years off, an astronomical blink of an eye; or it may occur tonight, so keep a watch on one of the great stars of the nighttime sky. The companion, however, at around 7 to 8 solar masses, seems to be just below the supernova limit and will probably die as a massive
white dwarf.
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