The biggest star that is known to exist. Comparison between the red supergiant Antares and the Sun, shown as the tiny dot toward the upper right. The black circle is the size of the orbit of Mars. Arcturus is also included in the picture for size comparison.
Antares (α Scorpii / Alpha Scorpii) is a star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky (sometimes listed as fifteenth 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. 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 that of the sun; if it were placed in the centre of our solar system, its outer surface would lie between the orbits of Mars and Jupiter. Antares is approximately 600 light years from our solar system. 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. Its large size and relatively small mass give Antares a very low average 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. 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 celestial equator.
Antares has a hot blue companion star, Antares B, of spectral type B2.5 at a separation of about 2.9 arcseconds, or 550 AUs at Antares' estimated distance. 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. It is normally difficult to see in small telescopes due to Antares' glare, but becomes easy in apertures over 150 mm (6 in.). The companion is often described as green, but this is probably a contrast effect. 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 orbit is poorly known, with an estimated period of 878 years.
Position on the ecliptic
Antares is one of the 4 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. Every year around December 2 the Sun passes 5° north of Antares.
Of the 21 first-magnitude stars, Antares now lies farthest in angular distance from any other first-magnitude star; i.e. it is possible to draw a larger circle centered around Antares without including any other first-magnitude star inside that circle, than around any other first-magnitude star. The nearest first-magnitude star to Antares is Alpha Centauri, lying approximately 39°6.75′ away. The high proper motion of Alpha Centauri is gradually increasing this angle. Before about March 2000, Achernar and Fomalhaut held this distinction of being the most isolated from other first-magnitude stars.
Antares in ancient cultures
Antares' name derives from the Greek Αντάρης, 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. Its distinctive coloration has made the star an object of interest to many societies throughout history. 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. In the religion of Stregheria, Antares is a fallen angel and quarter guardian of the western gate.
An old Arabic name was Ķalb al Άķrab, the 'Scorpion's heart'. This had been directly translated to the Greek Kardia Scorpiou/Καρδια Σκορπιου and Latin Cor Scorpii.
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