Antares, alpha Scorpii, is the brightest star in the constellation Scorpius and the 16th brightest star in the sky. Antares is a class M supergiant star. What does that mean? Well, with a radius of about 800 times the Sun; if it were placed in the center of our solar system, its outer surface would lie between the orbits of Mars and Jupiter.
Antares is big...really big!
Antares is approximately 600 light-years distant. It is a type LC "slow irregular variable" star, whose apparent magnitude slowly varies from +0.88 to +1.16. Its absolute magnitude is -5.28 Mv. Its visual luminosity is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the bolometric luminosity equals roughly 65,000 times that of the Sun.
The mass of the star is calculated to be 15 to 18 solar masses, which isn't very massive considering its bloated dimensions, so Antares has a very low average density.
Antares has a hot blue companion star, Antares B, of spectral type B2.5. It is normally difficult to see in small telescopes due to Antares' glare, but it can be picked out from the glare in 6 inch or larger telescopes on good nights. I've seen it in telescopes on still nights and to me it looks green, but this is probably a contrast effect. The orbit is poorly known, with an estimated period of ~878 years.
What Antares and its companion star might look like from a planet orbiting at a safe distance.
Copyright Don Dixon 1997