
[Greek Antarēs : anti-, rival of; see anti- + Arēs, Ares, the planet Mars.]
α Scorpii, a cool supergiant star of spectral type M1Ib, whose red color stands out in the midsummer sky. With an effective temperature of approximately 6000°F (3600 K), Antares resembles Betelgeuse, the brightest of the red supergiants, and would fill the solar system beyond the orbit of Mars if it replaced the Sun. Antares is only about 100 parsecs (325 light-years) from the Sun, and its angular diameter of about 0.045 arc-second has been measured by interferometric and lunar occultation methods. Red supergiants of this type originate as stars with mass at least 20 times that of the Sun. Such stars quickly evolve through successive stages of thermonuclear fusion. Eventually, the supergiant star implodes in a type II supernova explosion. This fate is likely for Antares in less than a million years. Prior to this dramatic event, Antares will have shed up to 50% of its mass through a stellar wind of material blown away from the star into the surrounding interstellar medium. See also Betelgeuse; Scorpius; Spectral type; Stellar evolution; Supergiant star; Supernova.
Antares is a member of an association of young and primarily hot stars, and is gravitationally bound in a binary star system with a hot blue star of spectral type B3V. The two stars orbit each other with a period of about 900 years, from which their masses can be determined to be about 15 and 7 times that of the Sun for the red and blue components respectively. The interaction of the strong wind of matter from the supergiant with the radiation from the less massive, hot companion produces an unusual nebulosity surrounding the hot star. See also Binary star; Nebula; Star.
| Observation data Epoch J2000 Equinox J2000 |
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|---|---|
| Constellation | Scorpius |
| Right ascension | 16h 29m 24s[1] |
| Declination | -26° 25′ 55″[1] |
| Apparent magnitude (V) | +0.96[2] |
| Characteristics | |
| Spectral type | M1.5Iab-b + B2.5V[3] |
| U−B color index | +1.34[2] |
| B−V color index | +1.83[2] |
| Variable type | LC[4] |
| Astrometry | |
| Radial velocity (Rv) | –3.4[5] km/s |
| Proper motion (μ) | RA: –12.11[1] mas/yr Dec.: –23.30[1] mas/yr |
| Parallax (π) | 5.89 ± 1.00[1] mas |
| Distance | approx. 550 ly (approx. 170 pc) |
| Absolute magnitude (MV) | –5.28 |
| Details | |
| A | |
| Mass | 12.4[6] M☉ |
| Radius | 883[6] R☉ |
| Luminosity | 57,500[7] L☉ |
| Surface gravity (log g) | 0.1[6] |
| Temperature | 3400 ± 200[7] K |
| Rotational velocity (v sin i) | 10[8] km/s |
| B | |
| Mass | 10 M☉ |
| Radius | 4 R☉ |
| Temperature | 18,500[7] K |
| Other designations | |
| Database references | |
| SIMBAD | data |
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]
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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.[10]
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).[11] This leads to a radius of 822 ± 80 solar radii.[clarification needed]
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, because it is near conjunction with the Sun;[12] 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.
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.[13] The stellar classification of this star is B2.5,[10] 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.[10]
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).[14] The companion is often described as green, but this is probably a contrast effect.[10] 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.[15]
The orbit is poorly known, with an estimated period of 878 years.
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 very rarely by Venus. The last occultation of Antares by Venus took place on September 17th, 525BC; the next one will take place on November 17th, 2400. Other planets did not occult Antares in the last millennium nor will they do so in the next millennium, as they pass as a result of their actual node position and inclination always northward of Antares. On 31 July 2009, Antares was occulted by the Moon. The event was visible in much of southern Asia and the Middle East.[16][17] Every year around December 2 the Sun passes 5° north of Antares.[12]
Antares, the proper name of this star, derives from the Ancient Greek Άντάρης, meaning "anti-Ares" ("anti-Mars"), due to the similarity of its reddish hue to the appearance of the planet Mars.[18]
Alternative name of this star, meaning "the Heart of Scorpion":

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