Antares is the brightest star in the constellation Scorpius and the sixteenth brightest star in the sky, at magnitude 1.06. It marks the heart of the Scorpion.
Observation
The best time to view Antares is on or around May 31st of each year, when the star is at opposition to the Sun. Every year, around December 2 the Sun passes 5° north of Antares. Antares is one of the four first magnitude stars which lie within 5° of the ecliptic, and therefore can be occulted by the Moon (and rarely by the planets). On 17 November 2400 Antares will be occulted by Venus.
A fierce stellar wind blowing from the surface of Antares has resulted in a circumstellar gas cloud. In the sky, Antares lies embedded in a complex region of emission and reflection nebulosity, surrounding both it and nearby ρ Oph. This nebulosity is catalogued as IC 4604, IC 4605, IC 4606, and is nicknamed the "Antares Nebula" or the "Rho Ophiuchi Nebula". The globular cluster M 4 also appears nearby, but is more than ten times further away.
History and Mythology
Antares takes its name because of its distinctly red color, which compares to that of Mars. The Greek name Antares means "Rival of Mars", the god of war. The Romans called the star Cor Scorpionis, meaning Heart of the Scorpion. However, they had been anticipated in this by the Assyrians, Babylonians, and Sumerians, who for at least two millennia had also known the star as the Heart of the Scorpion. According to ancient Arab tradition, Antares is the warrior-poet Antar's star. Many of the old Egyptian temples are oriented so that the light of Antares plays a role in the ceremonies performed there. Antares was also known as Satevis in ancient Persia and was one of the four "royal stars" of the Persians around 3000 BC. It was also known as Jyeshtha in ancient India.
Properties
Antares is a red supergiant star of spectral class M2 Iab, with a low surface temperature of about 3,500 degrees. Of the 22 first-magnitude stars, Antares and Betelgeuse are the only red M-type supergiants. Antares is about 550 light years from us and has an absolute magnitude of -5.1. Its visual luminosity is about 9,900 times the Sun's, but because the star radiates a considerable part of its energy in the infrared, its total luminosity is roughly 65,000 times the Sun's. The star's radius is 3.4 AU, or about 400 times the Sun's; if Antares were at the center of our solar system, it would swallow all planets from Mercury to Mars. Its large size and relatively small mass give Antares a very low average density. The star contains 15 to 18 solar masses, and probably does not have much time left to it (astronomically speaking). It is massive enough to develop an iron core and eventually explode as a supernova. Like all M-type supergiants, Antares is variable, but less so than Betelgeuse: its range is between magnitudes 0.86 and 1.06, and its period is irregular.
Companion
Antares is a well known double star. Its hot blue companion star is of spectral type B2.5 V, at a separation of about 2.9", or 550 AU at Antares' estimated distance. The orbit is poorly known, with an estimated period of 878 years. At magnitude 5.5, α Sco B is only 1/370th as bright visually as α Sco A, although it shines with 170 times the Sun's luminosity. The companion is often described as greenish due to contrast effects, but it is difficult to see in small telescopes due to Antares' glare. On rare nights when conditions are perfect, Antares is one of the most beautiful of all doubles: a tiny emerald point touching the edge of a brilliant reddish-orange disk. Antares B can be observed with a small telescope for a few seconds during lunar occultations while Antares itself is hidden by the Moon; it was discovered during one such occultation on April 13, 1819. The companion, contains around 7 to 8 solar masses, just below the supernova limit, and will probably die as a massive white dwarf.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]