Antares (), also designated Alpha Scorpii (? Scorpii, abbreviated Alpha Sco, ? Sco), is on average the fifteenth-brightest star in the night sky, and the brightest star in the constellation of Scorpius. Distinctly reddish when viewed with the naked eye, Antares is a slow irregular variable star that ranges in brightness from apparent magnitude +0.6 to +1.6. Often referred to as "the heart of the scorpion", Antares is flanked by Sigma and Tau Scorpii in the centre of the constellation.
Classified as a red supergiant of spectral type M1.5Iab, Antares is one of the largest known stars. It is the brightest, most massive, and most evolved stellar member of the nearest OB association (the Scorpius-Centaurus Association). Antares is a member of the Upper Scorpius subgroup of the Scorpius-Centaurus Association, which contains thousands of stars with mean age 11 million years at a distance of approximately 170 parsecs (550 ly). It is thought to be between 15 and 18 times as massive as the Sun, and have around 883 times its radius. Hence, if placed in the center of the Solar System, its outer surface would lie between the orbits of Mars and Jupiter.
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Nomenclature
? Scorpii (Latinised to Alpha Scorpii) is the star's Bayer designation. It also has the Flamsteed designation 21 Scorpii, as well as catalogue designations such as HR 6134 in the Bright Star Catalogue and HD 148478 in the Henry Draper Catalogue. As a prominent infrared source, it appears in the Two Micron All-Sky Survey catalogue as 2MASS J16292443-2625549 and the Two Micron All-Sky Survey catalogue as IRAS 16262-2619. It is also catalogued as a double star WDS J16294-2626 and CCDM J16294-2626.
The traditional name Antares derives from the Ancient Greek ???????, meaning "equal to-Ares" ("equal to-Mars"), due to the similarity of its reddish hue to the appearance of the planet Mars. The comparison of Antares with Mars may have originated with early Mesopotamian astronomers. However, some scholars have speculated that the star may have been named after Antar, or Antarah ibn Shaddad, the Arab warrior-hero celebrated in the pre-Islamic poems Mu'allaqat. In 2016, the International Astronomical Union organised a Working Group on Star Names (WGSN) to catalog and standardise proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Antares for this star. It is now so entered in the IAU Catalog of Star Names.
Antares is a variable star and is listed in the General Catalogue of Variable Stars but as a Bayer-designated star it does not have a separate variable star designation.
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Properties
Antares is a supergiant star with a stellar classification of M1.5Iab. With a radius that is approximately 883 times that of the Sun, it is one of largest stars, if placed in the center of the 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 Sun. 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 has been calculated to be in the range of 15 to 18 solar masses. A 2012 analysis by Pecaut and colleagues comparing the effective temperature and luminosity of Antares to theoretical evolutionary tracks for massive stars which include rotation an initial progenitor mass of approximately 17 solar masses and an age of 12 million years. Initial mass must also take into account the mass loss from powerful winds, and this would yield a less massive star not stated in this reference paper.
The size of Antares may be calculated using its parallax and angular diameter. The parallax angle is given in the adjacent box, and the angular diameter is known from lunar occultation measurements (41.3 ± 0.1 milliarcseconds). This implies a radius of 890 ± 150 solar radii at this distance. By analysing its radial velocity from its spectrum, Pugh and colleagues calculated a period of 5.93 ± 0.01 years and considered whether this change was orbital or pulsational. If the latter, then the radius of the star changes by 165 ± 22 solar radii (19% ± 4%). However, if this were the case, Antares' brightness would vary by a greater amount.
Antares is a type LC slow irregular variable star, whose apparent magnitude slowly varies from +0.6 to +1.6.
Position
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 as seen at the equator. 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; 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 is one of the four first magnitude stars that lies within 6.5° of the ecliptic (the others are Spica, Regulus and Aldebaran) and therefore can be occulted by the Moon. It can also, rarely, be occulted by Venus. The last occultation of Antares by Venus took place on September 17, 525 BC; the next one will take place on November 17, 2400. Other planets have not occulted 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. Every year around December 2 the Sun passes 5° north of Antares.
Companion star
Antares has a magnitude 5.5 companion star, Antares B, that changed from an angular separation (from its primary, Antares A) of 3.3 arcseconds in 1854 to 2.86 arcseconds in 1990. It was first observed by Scottish astronomer James William Grant FRSE while in India on 23 July 1844. 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 suggests that it is about 224 au beyond the primary. Antares B is a blue-white main-sequence star of spectral type B2.5V; it also has numerous unusual spectral lines suggesting it has been polluted by matter ejected by Antares A.
The companion star is normally difficult to see in small telescopes due to glare from Antares A, but can sometimes be seen in apertures over 150 millimetres (5.9 inches). The companion is often described as green, but this is probably either a contrast effect, or the result of the mixing of light from the two stars when they are seen together through a telescope and are too close to be completely resolved. Antares B can sometimes be observed with a small telescope for a few seconds during lunar occultations while Antares A is hidden by the Moon. It was discovered by Johann Tobias Bürg during one such occultation on April 13, 1819, but until its existence was confirmed in 1846 it was thought by some to be merely the light of Antares viewed through the Moon's atmosphere (which at the time was theorised to exist). When observed by itself during such an occultation, the companion appears a profound blue or bluish-green color.
The orbit of the companion star is poorly known, as attempts to analyse the radial velocity of Antares need to be unravelled from the star's own pulsations. Orbital periods are possible within a range of 1,200 to 2,562 years.
Supernova progenitor
Antares, like the similarly-sized red giant Betelgeuse in the constellation Orion, will almost certainly explode as a supernova, probably within the next few hundred thousand years. For a few months, the Antares supernova could be as bright as the full moon and be visible in daytime.
Other names
In the Babylonian star catalogues dating from at least 1100 BCE, Antares was called GABA GIR.TAB, "the Breast of the Scorpion". In MUL.APIN, which dates between 1100 and 700 BC, it is one of the stars of Ea in the southern sky and marks breast of the Scorpion goddess Ishhara. Later names that translate as "the Heart of Scorpion" include Calbalakrab from the Arabic Qalb al-?qrab. This had been directly translated from the Ancient Greek ?????? ???????? Kardia Skorpi?. Cor Scorpii translated above Greek name into Latin.
In ancient Mesopotamia, Antares may have been known by the following names: Urbat, Bilu-sha-ziri ("the Lord of the Seed"), Kak-shisa ("the Creator of Prosperity"), Dar Lugal ("The King"), Masu Sar ("the Hero and the King"), and Kakkab Bir ("the Vermilion Star"). In ancient Egypt, Antares represented the scorpion goddess Serket (and was the symbol of Isis in the pyramidal ceremonies).
In Persia, Antares was known as Satevis, one of the four "royal stars". In India, it with Sigma and Tau Scorpii were Jyeshth? (the eldest or biggest), one of the nakshatra (Hindu lunar mansions). The ancient Chinese called Antares (Chinese: ???; pinyin: X?nxiù'èr; literally: "second-brightest of mansion Heart"), because it was the second-brightest star of the mansion Xin (?). It was the national star of the Shang Dynasty, and it was sometimes referred to as (Chinese: ??; pinyin: Hu?x?ng; literally: "fiery star") because of its reddish appearance.
The M?ori people of New Zealand call Antares Rehua, and regard it as the chief of all the stars. Rehua is father of Puanga/Puaka (Rigel), an important star in the calculation of the M?ori calendar. 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).
Source of the article : Wikipedia
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