Antares (α Scorpii / Alpha Scorpii) is the brightest star in the constellation Scorpius and one of the brightest stars in the nighttime sky. Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic. The similarly colored Aldebaran lies almost directly opposite Antares in the Zodiac.
Antares is a class M supergiant star, with a diameter of approximately 700 times solar. I.e., if in place of our sun, its outer surface would extend between the orbit of Mars and Jupiter. Antares is approximately 600 light years from our solar system. Visually, its luminosity is about 10,000 times that of the Sun but overall, taking into account that the star radiates a considerable part of its energy in the infrared part of the spectrum the luminosity equals roughly 65,000 times that of the Sun. The mass of the star is calculated to be 15 to 18 solar masses.[2] Its large size and relatively small mass give Antares a very low density.
The best time to view Antares is on or around May 31 of each year, when the star is at "opposition" to the Sun. At this time, Antares rises at dusk and sets at dawn, and is thus in view all night (this of course depends entirely on your own position on the globe). For approximately two to three weeks on either side of November 30, Antares is not visible at all, being lost in the Sun's glare; this period of invisibility is longer in the Northern Hemisphere than in the Southern Hemisphere, since the star's declination is significantly south of the ecliptic.
Comparison between the red supergiant Antares and the Sun. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for size comparison.
http://en.wikipedia.org/wiki/Antares
