Antares

Antares, Alpha Scorpii (α Sco) is a red supergiant star located in the constellation Scorpius. With an average apparent magnitude of 0.96, it is the brightest star in Scorpius and usually the 15th brightest star in the sky. It is only slightly fainter than Altair in the constellation Aquila, Acrux in Crux, and Aldebaran in Taurus, and it just outshines the bright Spica in Virgo, Pollux in Gemini, and Fomalhaut in Piscis Austrinus. Antares lies at an approximate distance of 550 light years from Earth. It marks the heart of the celestial scorpion.

Star system

Even though it appears as a single star to the unaided eye, Alpha Scorpii is in fact a binary star system. The system consists of the red supergiant Alpha Scorpii A, formally named Antares, and a fainter blue-white main sequence star designated Alpha Scorpii B. Antares has an apparent magnitude that varies between 0.6 and 1.6, while the companion is considerably fainter at magnitude 5.5.

The pair’s orbital period is uncertain, with estimates in the range from 880 to 2,562 years. The separation between the stars was measured at 3.5’’ and 2.5’’ in the 1840s, and more recent measurements have yielded separations between 2.6’’ and 2.8’’. The projected separation between the stars is at least 529 astronomical units. The companion is believed to lie more than 220 astronomical units beyond Antares.

Antares (Alpha Scorpii), image: Wikisky

Alpha Scorpii A

Antares has the stellar classification M1.5Iab-Ib, indicating a supergiant star appearing distinctly red in colour. It has run out of its supply of hydrogen and is currently burning through progressively heavier elements. It has an estimated mass about 12 times that of the Sun. Its exact mass is uncertain, but estimates are mostly in the range from 11 to 14.3 solar masses. The star’s projected rotational velocity is 20 km/s.

Since evolving away from the main sequence, Antares has expanded to a size of 680 to 800 solar radii. It is losing mass through a powerful stellar wind that has enshrouded the supergiant in an envelope of gas illuminated by the star’s light. Antares has lost about 3 solar masses of material from its initial mass. Its estimated age is about 15 million years.

With an effective temperature of 3,660 K, Antares is 75,900 times more luminous than the Sun. Like its other properties, the star’s exact luminosity is uncertain, but believed to be in the range from 44,700 to 128,900 solar luminosities. Even though the bolometric luminosity of Antares is about 100,000 times that of the Sun, a lot of the star’s energy output is in the invisible infrared part of the spectrum. At visual wavelengths, Antares is about 10,000 times more luminous than the Sun. Its luminosity is dimmed by interstellar dust, but the degree of dimming is uncertain.

Antares serves as a spectral standard for its class, which means that its spectrum is used as an anchor point for the Morgan-Keenan (MK) system of stellar classification, used to classify other stars.

Antares is classified as a slow irregular variable (type Lc) and its brightness varies from magnitude 0.6 to 1.6. Type Lc variables are supergiant stars of late spectral types whose brightness typically varies by about 1 magnitude. Other stars in this class include the orange supergiants Enif (Epsilon Pegasi), Suhail (Lambda Velorum), NS Puppis, and Omicron¹ Canis Majoris, the red bright giants Pi Aurigae, BE Camelopardalis and CQ Camelopardalis, and the red supergiants TZ Cassiopeiae, RX Telescopii and NO Aurigae.

The variability of Antares was reported in 1952 by the South African astronomer Alan William James Cousins, who observed variations in the star’s photographic magnitude in the range from 3.00 to 3.16. The American Association of Variable Star Observers (AAVSO) has monitored the star’s brightness since 1945 and recorded slow variations between magnitudes 0.6 and 1.6. However, the brightness of Antares usually stays near magnitude 1.0. The star does not have a clear period, but a study published in 2006 that analysed the AAVSO data over the last century suggested a period of 1650 ± 640 days. A 2009 study of long secondary periods in pulsating red giants did not find a separate long secondary period in Antares.

A 2012 study of stars belonging to the Scorpius-Centaurus OB association calculated an initial mass of 17.2 solar masses and an age of 12 million years (allowing for a margin of error in the range from 11 to 15 million years) for Antares. In 2013, high spectral resolution imaging of the star’s atmosphere found an outer atmosphere between 1.2 and 1.4 R_⋆, an effective temperature of 3,660 ± 120 K, and a mass of 15 ± 5 solar masses, with an estimated age between 11 and 15 million years. Both studies compared the star’s luminosity and temperature to theoretical evolutionary tracks to derive its initial mass. Even if the results vary, Antares is massive enough to end its life as a supernova.

Size

The exact size of Antares is uncertain, but its estimated radius is between 680 and 800 solar radii. If it replaced the Sun in our solar system, it would extend between the orbits of Mars (297-358 R_☉) and Jupiter (1,064–1,173 R_☉). The star’s physical radius is over three astronomical units (three times the average distance from Earth to the Sun).

The extended outer regions of Antares make it difficult to determine its exact size. Additionally, it is a pulsating star and its radius changes by about 19%, or 165 solar radii. A study published in 1990 yielded an angular diameter of 41.3 ± 0.1 milliarcseconds after observations with the ESO 3.6 m telescope using speckly interferometry during a lunar occultation. More recent measurements of the limb-darkened disk gave diameters of 37.38 ± 0.06 mas in 2009 and 37.31 ± 0.09 mas in 2010. While the star’s physical radius could be derived from its angular size and distance, its exact distance is uncertain, which makes the exact size difficult to pinpoint.

The Hipparcos satellite data gave a parallax of 5.89 ± 1.00 mas for Antares, which translates into a radius about 680 times that of the Sun. Older estimates were higher than 850 solar radii, making Antares one of the largest known stars at the time. However, the supergiant is considerably smaller than the former record holder VY Canis Majoris (1,420 solar radii) and nowhere near the red supergiants UY Scuti (1,708 R_☉) and WOH G64 (1,540 R_☉), currently the largest stars known.

Size comparison of planets and stars, image: Wikimedia Commons/ Dave Jarvis, Jcpag2012, JoeyPknowsalotaboutthat (CC BY-SA 4.0)

Supernova

While the exact mass of Antares is uncertain, the star is most certainly massive enough to be a supernova candidate. Like Betelgeuse in Orion, it may go out as soon as in the next 10 thousand years or at some point in the next million years. When it does, the event could rival the full Moon in brightness and be visible in daytime. However, the supernova will not affect Earth because Antares is too distant.

Alpha Scorpii B

Antares B is a bluish main sequence star of the spectral type B2.5V. It has a mass 7.2 times that of the Sun and a radius 5.2 times solar. With a temperature of 18,500 K, it shines with 2,754 solar luminosities. It is partly responsible for energizing the nebulosity around Antares. Alpha Scorpii B is a very fast spinner, with a projected rotational velocity of 250 km/s. It is not massive enough to meet its end as a supernova, but will instead end its life as a white dwarf.

Antares B has an apparent magnitude of 5.5 but makes a challenging target for small telescopes because of its proximity to the considerably brighter supergiant star. It requires a 6-inch or larger telescope. The star can sometimes appear greenish, likely due to the contrast with the orange Antares or to the light of the two stars mixing when they are too close. The spectrum of Alpha Scorpii B shows numerous absorption lines that indicate the presence of matter ejected by the primary component.

Due to its proximity to the bright supergiant, Antares B was not discovered until the 19th century. Austrian astronomer Johann Tobias Bürg discovered it from Vienna during a lunar occultation of the brighter star on April 13, 1819.

Facts

Antares is one of the two stars in the constellation Scorpius selected for navigation. The other navigational star in the constellation is Shaula, Lambda Scorpii.

Antares is a member of the Upper Scorpius subgroup of the Scorpius-Centaurus Association (Sco OB2), the nearest OB association to the Sun. The members of the Upper Scorpius subgroup have a mean age of 11 million years and lie at an average distance of 550 light years from Earth. Antares is the brightest and most massive star in Sco OB2.

Variations in radial velocity were detected in the star’s spectrum in the early 20th century. Astronomers soon realized that the variations were not the result of the star’s orbital motion but were instead caused by the pulsation of its atmosphere. As early as 1928, calculations pointed to variations in the star’s size by about 20 percent.

The existence of the companion, Antares B, was first reported by the Austrian astronomer Johann Tobias Bürg, who observed Antares during a lunar occultation that occurred on April 13, 1819. However, Bürg’s discovery was dismissed by many who believed it to be merely an atmospheric effect. The companion was then observed several decades later, first by the Scottish astronomer James William Grant from India on July 23, 1844, and then by the American astronomer Ormsby M. Mitchel in 1846. In April 1847, it was measured by the English astronomer William Rutter Dawes.

In 2017, astronomers imaged the surface of Antares using ESO’s Very Large Telescope Interferometer (VLTI). They also mapped the velocities of the material in the star’s atmosphere, revealing unforeseen turmoil. Antares was the first star other than the Sun to be a target of such a study and the image of the star’s surface and atmosphere was the best to date of any star other than the Sun.

Using ESO’s Very Large Telescope Interferometer astronomers have constructed this remarkable image of the red supergiant star Antares. This is the most detailed image ever of this object, or any other star apart from the Sun. Image: ESO/K. Ohnaka

The aim of the study was to clarify how evolved red supergiants lose their mass so quickly in the late stage of their life. The astronomers concluded that the process could not result from convection (movement of matter that transfers energy from the stellar core to the outer atmosphere) and that a new process was needed to explain the turbulent motion in the star’s extended atmosphere.

Antares lies only 4.57 degrees south of the ecliptic (the Sun’s apparent path across the sky) and is frequently occulted by the Moon and sometimes by the planets. It is one of several bright stars in zodiac constellations located within six degrees of the ecliptic. The others are Aldebaran in Taurus, Regulus in Leo and Spica in Virgo. The Sun passes near Antares – about 5 degrees north of the star – in early December every year, and the star is invisible during this period. Antares is occulted by planets only rarely. The last occultation, by Venus, was recorded on September 17, 525 BCE, and the next one will occur on November 17, 2400.

Antares is often compared to Betelgeuse, another exceptionally bright red supergiant. Betelgeuse is similar in colour and brightness, with the spectral classification M1–M2 Ia–ab and an apparent magnitude that varies from 0.0 to 1.3. The star’s properties are just as challenging to pin down as those of Antares, but with an estimated radius between 887 and 955 solar radii, Betelgeuse is somewhat larger than Antares and, at a distance between 640 and 724 light years from Earth, it is also more distant. It is brighter than Antares mainly because of its larger size.

Antares is one of the brightest stars in near-infrared wavelengths. With a J-band magnitude of -2.7, it outshines R Doradus (-2.6), Arcturus (-2.2) and Aldebaran (-2.1), but is not quite as bright as Betelgeuse (-2.99).

Antares is one of the 27 stars represented on the flag of Brazil. Each star on the flag symbolizes a Brazilian Federative Unit and Antares represents the state of Piauí. Seven other Scorpius stars are featured on the flag: Shaula (Lambda Scorpii), Acrab (Beta Scorpii), Larawag (Epsilon Scorpii), Sargas (Theta Scorpii), Iota Scorpii, Kappa Scorpii, and Mu Scorpii (Xamidimura and Pipirima).

The Ngarrindjeri people of southern Australia associated Antares and its variability with the myth of Waiyungari (“red man”). In local lore, Waiyungari was a young man who was expected to avoid contact with women during his initiation rite. However, his brother Nepeli’s two wives were attracted to him and came to his hut disguised as emus. Waiyungari followed the emus until the women revealed themselves and seduced him. When his brother discovered the betrayal, he set fire to Waiyungari’s hut, where Waiyungari and the women were sleeping. The three were able to escape and, to avoid punishment, Waiyungari threw a spear into the Milky Way and pulled himself and his brother’s wives up into the sky. He became Antares and the women became two fainter stars flanking him. It is said that when the star’s brightness increases, this marks the beginning of initiation rites, when young men must stay away from women. The two fainter stars representing the two women were likely Alniyat (Sigma Scorpii) and Paikauhale (Tau Scorpii).

In medieval astrology, Antares was one of the 15 Behenian fixed stars, believed to be a source of special power. Each of the 15 stars was associated with a planet, a plant and a gemstone, and the latter two would be used in rituals to bring out the star’s influence, e.g. into an amulet. Antares was linked with Venus and Jupiter, and its plant and gemstone were birthwort and sardonyx.

Like many other bright stars, Antares has often been used and referenced in works of fiction. Notable uses in literature include M. A. R. Barker’s Tékumel novels and games, Alfred Bester’s novel The Stars My Destination (1956), Kenneth Bulmer’s Dray Prescot series (1972–1998), Elliot S. Maggin’s novel Superman: Last Son of Krypton (1978), Douglas Adams’ The Restaurant at the End of the Universe (1980), Michael McCollum’s Antares Dawn (1986), and Larry Townsend’s The Scorpius Equation (1993).

Antares is referenced in the graphic novels in the Antares series (2007—2011) by Luiz Eduardo de Oliveira and the Japanese manga series Saint Seiya. The star is also used in a number of computer games, including Frontier: Elite II (1993), Frontier: First Encounters (1995), Master of Orion II: Battle at Antares (1996), Descent: FreeSpace – The Great War (1998), and Star Control II (1992).

Name

The name Antares (pronunciation: /ænˈtɛəriːz/) comes from the Ancient Greek Ἀντάρης, meaning “rivalling Ares.” The name refers to the star’s similarity to the planet Mars due to its reddish appearance. The comparison of the two possibly dates back to early Mesopotamian astronomy. Mars is normally considerably fainter than Antares but outshines it for a few months every couple of years. It also passes near the star every two years, which is likely what prompted the comparisons.

The name Antares was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on June 30, 2016. It formally applies only to the component Alpha Scorpii A.

Antares has been known by many other names across different cultures. Its ancient Chinese name was 心宿二 (Xīnxiù’èr), or the Second Star of Heart. The Chinese Heart, or Xin (心), mansion is one of the seven mansions of the Azure Dragon and represents the Dragon’s heart. It also contains the stars Alniyat (Sigma Scorpii) and Paikauhale (Tau Scorpii). Antares was the national star of the Shang dynasty (also known as the Yin dynasty), which ruled in the 2nd millennium BCE. Antares was known as 火星 (Huǒxīng), meaning “fiery star.”

In Babylonian astronomy, Antares was known as GABA GIR.TAB, or “the Breast of the Scorpion.” The name was documented in star catalogues dating back to at least 1100 BCE. In the Babylonian astronomy compendium MUL.APIN, compiled around 1000 BCE, the star marks the breast of the goddess Ishara, associated with the underworld and represented by the constellation Scorpius.

Taken by Stéphane Guisard, an ESO engineer and world-renowned astrophotographer, from Cerro Paranal, home of ESO’s Very Large Telescope, this image directly benefits from the quality of Paranal’s sky, one of the best on the planet. The image shows the region spanning the sky from the constellation of Sagittarius (the Archer) to Scorpius (the Scorpion). The very colourful Rho Ophiuchi and Antares region features prominently to the right, as well as much darker areas, such as the Pipe and Snake Nebulae. The dusty lane of our Milky Way runs obliquely through the image, dotted with remarkable bright, reddish nebulae, such as the Lagoon and the Trifid Nebulae, as well as NGC 6357 and NGC 6334. This dark lane also hosts the very center of our Galaxy, where a supermassive black hole is lurking. Image: ESO/S. Guisard

In ancient Greece, Antares was called Καρδία Σκορπίου (Kardia Skorpiū), meaning “the heart of the scorpion.” The name was translated into Arabic as Calbalakrab, derived from the phrase Qalb al-Άqrab. The Latin equivalent of the name was Cor Scorpii.

In Mesopotamian astronomy, Antares was known by several names, including Kakkab Bir (“the Vermilion Star”), Kak-shisa (“the Creator of Prosperity”), Masu Sar (“the Hero and the King”), and Dar Lugal (“The King”).

In ancient Egypt, the star was called tms n hntt, meaning “the red one of the prow.” It was associated with Serket, the goddess of fertility and healing.

Antares was one of the four Royal Stars of Persia, along with Aldebaran, Regulus and Fomalhaut. The four stars were seen as guardians of the four districts of the sky and commonly used for navigation. Antares was known as Satevis, the watcher of the west.

The Māori people of New Zealand know the star as Rēhua, a sacred figure who lives in the highest of heavens, is immortal and has the power to heal any illness. The Wotjobaluk people of the state of Victoria associated Antares with Djuit, son of Marpean-kurrk, a female ancestral figure represented by Arcturus. The Kulin Kooris knew Antares as Balayang, brother of Bunjil, represented by Altair.

Location

Antares lies near the centre of Scorpius constellation and is flanked by two relatively bright stars, the magnitude 2.88 Alniyat (Sigma Scorpii) and magnitude 2.82 Paikauhale (Tau Scorpii). The two stars mark the arteries near the heart of the scorpion. Antares is the brightest star near the distinct pattern of three bright stars that outline the scorpion’s claws, Acrab (Beta¹ Scorpii), Dschubba (Delta Scorpii), and Fang (Pi Scorpii). The bright stars that form the Teapot in Sagittarius can be used for orientation.

Antares location, image: Wikisky

Antares is located in the southern sky and can be seen from all locations south of the latitude 63° N, if not throughout the year, then at least for a period. The star is circumpolar, i.e. it never sets and is visible year-round, from locations south of the latitude 67° S. Northern observers can see it above the southern horizon in the summer months, while observers south of the equator can see it much higher in the sky for most of the year.

Antares reaches its highest point in the sky at dawn in early March and at sundown in early September. Its midnight culmination is around May 31 or June 1, when it is at opposition to the Sun and stays visible throughout the night. The star is invisible for several weeks around November 30 because it is too close to the Sun.

Antares can be used to find several interesting deep sky objects which lie in the vicinity. The brightest of these, the globular cluster Messier 4, appears only 1.3 degrees west of the star. The cluster appears quite large, occupying an area of 26′, and has an apparent magnitude of 5.9. It stretches over an area 75 light years across and its stars can be resolved in medium-sized telescopes. The cluster lies at a distance of 7,200 light years and its estimated age is 12.2 billion years. It is one of the nearest globular clusters to the Sun.

Antares, Messier 4, NGC 6144, IC 4605 and IC 4603, image: Wikisky

The globular cluster NGC 6144 is considerably smaller and fainter, but lies closer to Antares. It has an apparent magnitude of 9.63 and an apparent size of 1′.8.

The reflection nebulae IC 4605 around the star 22 Scorpii and IC 4603 around SAO 184376 lie in the direction of the constellation Ophiuchus.

Antares and the Rho Ophiuchi complex, image: Giuseppe Donatiello (CC0 1.0)

Constellation

Antares is located in the southern constellation Scorpius. Like other constellations in the zodiac family, Scorpius is one of the Greek constellations, first listed by the Greco-Roman astronomer Ptolemy of Alexandria in the 2nd century CE. It represents the scorpion that stung Orion, the Hunter, in Greek mythology. It is said that Orion and the scorpion were placed at the opposite ends of the sky and the two constellations never appear above the horizon at the same time because Orion is still fleeing from the scorpion.

Scorpius constellation map by IAU and Sky&Telescope magazine

Scorpius is located in a rich field of the Milky Way and can be seen in its entirety between latitudes 40° N and 90° S. Observers in mid-northern latitudes can see it above the southern horizon during the summer months. Scorpius is easy to recognize because it lies to the right of the Teapot in Sagittarius, straddling the Milky Way, with its brightest stars forming a distinct claw and tail with a stinger.

Scorpius is known for its bright stars Antares and Shaula, as well as for its many bright deep sky objects. In addition to those already mentioned, these include the globular cluster Messier 80, the open clusters NGC 6821, the Northern Jewel Box Cluster (NGC 6231), Ptolemy’s Cluster (Messier 7), the Butterfly Cluster (Messier 6), the emission nebula NGC 6334, nicknamed the Cat’s Paw Nebula, the diffuse nebula NGC 6357, known as the Lobster Nebula or War and Peace Nebula, and the bipolar planetary nebula NGC 6302, also known as the Butterfly or Bug Nebula.

The best time of year to observe the stars and deep sky objects of Scorpius is during the month of July, when the constellation is prominent in the southern sky.

The 10 brightest stars in Scorpius are Antares (Alpha Sco A, mag. 0.6 – 1.6), Shaula (Lambda Sco A, mag. 1.62), Sargas (Theta Sco A, mag. 1.84), Dschubba (Delta Sco, mag 2.307), Larawag (Epsilon Sco, mag. 2.31), Kappa Sco (mag. 2.39), Acrab (Beta Sco, mag. 2.62), Lesath (Upsilon Sco, mag. 2.70), Paikauhale (Tau Sco, mag. 2.82), and Fang (Pi Sco, mag. 2.89).

Antares – Alpha Scorpii

Alpha Scorpii A

Alpha Scorpii B