Antares

Antares (α Scorpii, α Sco, Alpha Scorpii) is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky. (It is sometimes listed as 15th brightest, if the two brighter components of the Capella quadruple star system are counted as one star.) Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic. Antares is a slow variable star with an average magnitude of +1.09.^[3]

Properties

Antares is a supergiant star with a stellar classification of M1.5Iab-b.^[3] It has a radius of approximately 883 times that of the Sun;^[6] if it were placed in the center of our solar system, its outer surface would lie between the orbits of Mars and Jupiter. Based upon parallax measurements, Antares is approximately 550 light-years (170 parsecs) from the Earth.^[1] 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.^[9] Its large size and relatively small mass give Antares a very low average density.

The size of Antares may be calculated using its parallax and angular diameter. The parallax angle is given in the box to the right, and the angular diameter is known from lunar occultation measurements (41.3 ± 0.1 mas).^[10] This leads to a radius of 822 ± 80 solar radii.

Antares is a type LC slow irregular variable star, whose apparent magnitude slowly varies from +0.88 to +1.16.^[4]

Antares is visible in the sky all night 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. For approximately two to three weeks on either side of November 30, Antares is not visible in the night sky, being lost in the Sun's glare;^[11] 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 celestial equator.

Companion star

Antares has a secondary companion star, Antares B, that changed from an angular separation of 3.3 arcseconds in 1854 to 2.86 arcseconds in 1990. The last is equal to a projected separation of about 529 Astronomical Units (AU) at the estimated distance of Antares, giving a minimum value for the separation of the pair. Spectroscopic examination of the energy states in the outflow of matter from the companion star suggest that it is about 224 AU behind the primary,^[6] giving a combined separation of about 574 AU.^[12] The stellar classification of this star is B2.5,^[9] with numerous spectral lines suggesting it has been polluted by matter ejected by Antares.^[6] At magnitude 5.5, it is only 1/370th as bright visually as Antares A, although it shines with 170 times the Sun's luminosity.^[9]

It is normally difficult to see in small telescopes due to Antares' glare, but can sometimes be seen in apertures over 150 mm (5.9 in).^[13] The companion is often described as green, but this is probably a contrast effect.^[9] 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 by Johann Tobias Bürg during one such occultation on April 13, 1819.^[14]

The orbit is poorly known, with an estimated period of 878 years.

Position on the ecliptic

Antares is one of the 4 first magnitude stars that lies within 5° of the ecliptic and therefore can be occulted by the Moon and rarely by the planets. On 31 July 2009, Antares was occulted by the moon. The event was visible in much of southern Asia and the Middle East.^[15][16] Every year around December 2 the Sun passes 5° north of Antares.^[11]

Traditional names

Antares, the proper name of this star, derives from the Ancient Greek Άντάρης, meaning "(holds) against Ares (Mars)", due to the similarity of its reddish hue to the appearance of the planet Mars. It is the brightest star in the constellation Scorpius. However, it is also thought that Antares may have been named after the ancient Arab warrior-poet Antar.^[17] Its distinctive color has made the star an object of interest to many societies throughout history.

• In Persia, Antares was known as Satevis, one of the four "royal stars".^[18]
• In India, it with σ and τ Sco were one of nakshatra (Hindu lunar mansion), as Jyeshthā "Oldest", or the Seniormost or Chief, and Rohinī "Ruddy". The natives born with their moon in this nakshatra show a combination of power, independence and a sense of danger. ^[17]
• In China, it was named 心宿二 (Mandarin: xīn sù èr), because it was the second star of the asterism 心宿 ("Heart").
• Antares is listed in MUL.APIN as GABA GIR.TAB, meaning "the Breast of the Scorpion:Lishi, Nabu".^[19]
• In Renaissance astrology, Antares is one of the Behenian fixed stars and has the symbol .^[20]
• The Wotjobaluk Koori people of Victoria, Australia, knew Antares as Djuit, son of Marpean-kurrk (Arcturus); the stars on each side represented his wives. The Kulin Kooris saw Antares (Balayang) as the brother of Bunjil (Altair).^[21]

Alternative name of this star, meaning "the Heart of Scorpion":

• Calbalakrab from the Arabic Qalb al-Άqrab.^[22] This had been directly translated from the Ancient Greek Καρδία Σκορπίου Kardia Skorpiū.
• Cor Scorpii translated above Greek name into Latin.^[17]

See also

• Antares in fiction

References

External links

• A comparison of the size of Antares to other celestial objects
• Exploring the Full Stellar Population of the upper Scorpius OB Association
• Antares on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Coordinates: 16^h 29^m 24^s, −26° 25′ 55″