Antares
Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ Scorpii near the center of the constellation. Distinctly reddish when viewed with the naked eye, Antares is a slow irregular variable star that ranges in brightness from an apparent visual magnitude of +0.6 down to +1.6. It is on average the fifteenth-brightest star in the night sky. Antares is the brightest and most evolved stellar member of the Scorpius–Centaurus association, the nearest OB association to the Sun. It is located about from Earth at the rim of the Upper Scorpius subgroup, and is illuminating the Rho Ophiuchi cloud complex in its foreground.
Classified as spectral type M1.5Iab-Ib, Antares is a red supergiant, a large evolved massive star and one of the largest stars visible to the naked eye. If placed at the center of the Solar System, it would extend out to somewhere in the asteroid belt. Its mass is calculated to be 13 or 15 to 16 times that of the Sun. Antares appears as a single star when viewed with the naked eye, but it is actually a binary star system, with its two components called α Scorpii A and α Scorpii B. The brighter of the pair is the red supergiant, while the fainter is a hot main sequence star of magnitude 5.5. They have a projected separation of about.
Its traditional name Antares derives from the Ancient Greek Ἀντάρης, meaning "rival to Ares", due to the similarity of its reddish hue to the appearance of the planet Mars.
Nomenclature
α Scorpii is the star's Bayer designation. Antares 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 Infrared Astronomical Satellite Sky Survey Atlas catalogue as IRAS 16262–2619. It is also catalogued as a double star WDS J16294-2626 and CCDM J16294-2626. 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.Its traditional name Antares derives from the Ancient Greek Ἀντάρης, meaning "rival to Ares", due to the similarity of its reddish hue to the appearance of the planet Mars. The comparison of Antares with Mars was once thought to have its origin with early Mesopotamian astronomers. This is now considered an outdated speculation, because the name of this star in Mesopotamian astronomy has always been "heart of Scorpion" and it was associated with the goddess Lisin. 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. However, the name "Antares" is already proven in the Greek culture, e.g. in Ptolemy's Almagest and Tetrabiblos. In 2016, the International Astronomical Union organised a Working Group on Star Names 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 the star α Scorpii A. It is now so entered in the IAU Catalog of Star Names.
Observation
Antares is visible all night around May 31 of each year, when the star is at opposition to the Sun. Antares then rises at dusk and sets at dawn as seen at the equator.For two to three weeks on either side of November 30, Antares is not visible in the night sky from mid-northern latitudes, because it is near conjunction with the Sun. In higher northern latitudes, Antares is only visible low in the south in summertime. Higher than 64° northern latitude, the star does not rise at all.
Antares is easier to see from the southern hemisphere due to its southerly declination. In the whole of Antarctica, the star is circumpolar as the whole continent is above 64° S latitude.
History
variations were observed in the spectrum of Antares in the early 20th century, and attempts were made to derive spectroscopic orbits. It became apparent that the small variations could not be due to orbital motion, and they were actually caused by pulsation of the star's atmosphere. Even in 1928, it was calculated that the size of the star must vary by about 20%.Antares was first reported to have a companion star by Johann Tobias Bürg during an occultation on April 13, 1819, although this was not widely accepted and dismissed as a possible atmospheric effect. It was then observed by Scottish astronomer James William Grant FRSE while in India on 23 July 1844. It was rediscovered by Ormsby M. Mitchel in 1846 and measured by William Rutter Dawes in April 1847.
In 1952, Antares was reported to vary in brightness. A photographic magnitude range from 3.00 to 3.16 was described. The brightness has been monitored by the American Association of Variable Star Observers since 1945, and it has been classified as an LC slow irregular variable star, whose apparent magnitude slowly varies between extremes of +0.6 and +1.6, although usually near magnitude +1.0. There is no obvious periodicity, but statistical analyses have suggested periods of 1,733 days or days. No separate long secondary period has been detected, although it has been suggested that primary periods longer than a thousand days are analogous to long secondary periods.
Research published in 2018 demonstrated that Ngarrindjeri Aboriginal people from South Australia observed the variability of Antares and incorporated it into their oral traditions as Waiyungari.
Occultations and conjunctions
Antares is 4.57 degrees south of the ecliptic, one of four first magnitude stars within 6° of the ecliptic, so it can be occulted by the Moon. The occultation of 31 July 2009 was visible in much of southern Asia and the Middle East. Every year around December 2 the Sun passes 5° north of Antares. Lunar occultations of Antares are fairly common, depending on the 18.6-year cycle of the lunar nodes. The current cycle began in 2023 and the last cycle ended in 2010. Shown at right is a video of a reappearance event, clearly showing events for both components.Antares can also be occulted by the planets, e.g. Venus, but these events are rare. The last occultation of Antares by Venus took place on September 17, 525 BC; the next one will be November 17, 2400. Other planets have been calculated not to have occulted Antares over the last millennium, nor will they in the next millennium, as most planets stay near the ecliptic and pass north of Antares. Venus will be extremely near Antares on October 19, 2117, and every eight years thereafter through to October 29, 2157, it will pass south of the star.
Illumination of Rho Ophiuchi cloud complex
Antares is the brightest and most evolved stellar member of the Scorpius–Centaurus association, the nearest OB association to the Sun. It is a member of the Upper Scorpius subgroup of the association, which contains thousands of stars with a mean age of 11 million years. Antares is located about from Earth at the rim of the Upper Scorpius subgroup, and is illuminating the Rho Ophiuchi cloud complex in its foreground. The illuminated cloud is sometimes referred to as the Antares Nebula or is otherwise identified as VdB 107.Stellar system
α Scorpii is a double star that is thought to form a binary system. The best calculated orbit for the stars is still considered to be unreliable. It describes an almost circular orbit seen nearly edge-on, with a period of 1,218 years and a semi-major axis of about. Other recent estimates of the period have ranged from 880 years for a calculated orbit, to 2,562 years for a simple Kepler's Law estimate.Early measurements of the pair found them to be about apart in 1847–49, or apart in 1848. More modern observations consistently give separations around. The variations in the separation are often interpreted as evidence of orbital motion, but are more likely to be simply observational inaccuracies with very little true relative motion between the two components.
The pair have a projected separation of about 529 astronomical units at the estimated distance of Antares, giving a minimum value for the distance between them. Spectroscopic examination of the energy states in the outflow of matter from the companion star suggests that the latter is over beyond the primary.
Antares
Antares is a red supergiant star with a stellar classification of M1.5Iab-Ib, and is indicated to be a spectral standard for that class. Due to the nature of the star, the derived parallax measurements have large errors, so that the true distance of Antares is approximately from the Sun.The brightness of Antares at visual wavelengths 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 true bolometric luminosity is around 100,000 times that of the Sun. There is a large margin of error assigned to values for the bolometric luminosity, typically 30% or more. There is also considerable variation between values published by different authors, for example and published in 2012 and 2013.
The mass of the star has been calculated to be about, or. Comparison of the effective temperature and luminosity of Antares to theoretical evolutionary tracks for massive stars suggest a progenitor mass of and an age of 12 million years, or an initial mass of and an age of 11 to 15 Myr. Comparison of observations from antiquity to theoretical evolutionary tracks suggests an initial mass of, or the possibility that Antares is on a blue loop with an initial mass of . These correspond to ages from 11.8 to 17.3 Myr. Massive stars like Antares are expected to explode as supernovae.
Like most cool supergiants, Antares's size has much uncertainty due to the tenuous and translucent nature of the extended outer regions of the star. Defining an effective temperature is difficult due to spectral lines being generated at different depths in the atmosphere, and linear measurements produce different results depending on the wavelength observed. In addition, Antares pulsates in size, varying its radius by 19%. It also varies in temperature by 150 K, lagging 70 days behind radial velocity changes which are likely to be caused by the pulsations.
The diameter of Antares can be measured most accurately using interferometry or observing lunar occultations events. An apparent diameter from occultations 41.3 ± 0.1 milliarcseconds has been published. Interferometry allows synthesis of a view of the stellar disc, which is then represented as a limb-darkened disk surrounded by an extended atmosphere. The diameter of the limb-darkened disk was measured as in 2009 and in 2010. The linear radius of the star can be calculated from its angular diameter and distance. However, the distance to Antares is not known with the same accuracy as modern measurements of its diameter.
An estimate obtained by interferometry in 1925 by Francis G. Pease at the Mount Wilson Observatory gave Antares a diameter of, equal to approximately, making it the then largest star known. Antares is now known to be somewhat larger; for instance, the Hipparcos satellite's trigonometric parallax of with modern angular diameter estimates lead to a radius of about. Older radii estimates exceeding were derived from older measurements of the diameter, but those measurements are likely to have been affected by asymmetry of the atmosphere and the narrow range of infrared wavelengths observed; Antares has an extended shell which radiates strongly at those particular wavelengths. Despite its large size compared to the Sun, Antares is dwarfed by even larger red supergiants, such as VY Canis Majoris, KY Cygni, RW Cephei or Mu Cephei.
Antares, like the similarly sized red supergiant Betelgeuse in the constellation Orion, will almost certainly explode as a supernova, probably in million years. For a few months, the Antares supernova could be as bright as the full moon and be visible in daytime.