Antares

Antares A/B

The position of Antares in the Scorpius constellation.
Observation data Epoch J2000      Equinox J2000
Constellation	Scorpius
Right ascension	16h 29m 24.45970s[1]
Declination	−26° 25′ 55.2094″[1]
Apparent magnitude (V)	+0.96[2]
Characteristics
Spectral type	M1.5Iab + 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[6]
Details
A
Mass	12.4[3] M☉
Radius	883[3] R☉
Luminosity	57,500[7] L☉
Surface gravity (log g)	0.1[3] cgs
Temperature	3400 ± 200[7] K
Rotational velocity (v sin i)	20[2] km/s
B
Mass	7.2[8] M☉
Radius	5.2[8] R☉
Surface gravity (log g)	3.9[8] cgs
Temperature	18,500[7] K
Rotational velocity (v sin i)	250[8] km/s
Other designations
α Scorpii, 21 Sco, Cor Scorpii, Kalb al Akrab, Scorpion's Heart, Vespertilio,[9] HR 6134, CD -26°11359, HD 148478, SAO 184415, FK5 616, WDS 16294-2626, CCDM J16294-2626A/B, HIP 80763[10]
Database references
SIMBAD	data

Antares (/ænˈtɑːriːz/), also known by its Bayer designation Alpha Scorpii (abbreviated to α Scorpii or α Sco), is the fifteenth brightest star in the nighttime sky and the brightest star in the constellation Scorpius, and is often referred to as "the heart of the scorpion". Along with Aldebaran, Regulus, and Fomalhaut, Antares comprises the group known as the 'Royal stars of Persia'. It is one of the four brightest stars near the ecliptic.

Distinctly reddish when viewed with the unaided eye, Antares is a red supergiant of spectral type M0.5Iab and is one of the largest and most luminous observable stars. It is a slow irregular variable star with the quoted magnitude of +0.96. Antares 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 145 parsecs (470 light years).^[11]

Properties

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 is a supergiant star with a stellar classification of M0.5Iab.^[12] With a radius that is approximately 883 times that of the Sun,^[3] if 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 has been calculated to be in the range of 15 to 18 solar masses.^[13] A recent analysis^[11] comparing the effective temperature and luminosity of Antares to theoretical evolutionary tracks for massive stars which include rotation and mass loss yielded a mass of approximately 17 solar masses and an age of 12 million years.

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).^[14] 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.^[15]

Antares is a type LC slow irregular variable star, whose apparent magnitude slowly varies from +0.6 to +1.6.^[4]

Antares near the Sun on 30 November. This date may vary between 30 Nov and 2 Dec every year

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;^[16] 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

Illustration of Antares and its companion star, Antares B

Antares has a secondary, or 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. 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,^[3] giving a combined separation of about 574 AU.^[17] The stellar classification of this star is B3V:,^[12] with numerous spectral lines suggesting it has been polluted by matter ejected by Antares A.^[3] 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.^[13]

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 mm (6 inches).^[18] The companion is often described as green, but this is probably either a contrast effect^[13] 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,^[19] 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 theorized to exist).^[20] When observed by itself during such an occultation, the companion appears a profound blue or bluish-green color.^[20]

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.^[15] Orbital periods of 1,200^[21] - 2,562 years.^[22]

Antares, like the similarly-sized red giant Betelgeuse in the constellation Orion, could explode as a supernova in 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.

Position on the ecliptic

Relative sizes of some planets in the Solar System and several well-known stars, including Antares
1. Mercury < Mars < Venus < Earth
2. Earth < Neptune < Uranus < Saturn < Jupiter
3. Jupiter < Proxima Centauri < Sun < Sirius
4. Sirius < Pollux < Arcturus < Aldebaran
5. Aldebaran < Rigel < Antares < Betelgeuse
6. Betelgeuse < VY Canis Majoris < NML Cygni < UY Scuti.

Antares is one of the four first magnitude stars that lies within 5° of the ecliptic (like Spica, Regulus and Aldebaran) and therefore can be occulted by the Moon and, though rarely, 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.^[23][24] Every year around December 2 the Sun passes 5° north of Antares.^[16]

Membership of the Scorpius-Centaurus Association

Antares 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 145 parsecs (470 light years).^[11]

Traditional names

Antares, the proper name of this star, derives from the Ancient Greek Άντάρης, meaning "equal to-Ares" ("equal to-Mars"), due to the similarity of its reddish hue to the appearance of the planet Mars.^[25] The comparison of Antares with Mars may have originated with early Mesopotamian astronomers.^[26] 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.^[26]

• 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").^[26]
• In Persia, Antares was known as Satevis, one of the four "royal stars".^[27]
• In India, it with σ and τ Sco were Jyeshthā (the eldest or biggest), one of the nakshatra (Hindu lunar mansions).^[26]
• 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).^[28]
• The Māori 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 Māori calendar.
• The ancient Chinese called Antares (Chinese: 心宿二; pinyin: Xīn Sùèr; literally: "second brightest"), because it was the second brightest star of the mansion Xin (心). It was the national star of the Shang Dynasty, and it was sometimes referred to as (Chinese: 火星; pinyin: Huǒxīng; literally: "fiery star") because of its reddish appearance.

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

• In ancient Egypt, Antares represented the scorpion goddess Serket (and was the symbol of Isis in the pyramidal ceremonies).^[26]
• Antares is listed in MUL.APIN as GABA GIR.TAB, meaning "the Breast of the Scorpion: Lishi, Nabu".^[29]
• Calbalakrab from the Arabic Qalb al-Άqrab.^[30] This had been directly translated from the Ancient Greek Καρδία Σκορπίου Kardia Skorpiū.
• Cor Scorpii translated above Greek name into Latin.^[26]

See also

• Antares in fiction

Footnotes

References

External links

Wikimedia Commons has media related to Antares.

• Antares on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
• Antares at Constellation Guide

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