Learning About the Star Antares - The Heart of Scorpius

Learning About the Star Antares - The Heart of Scorpius
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Description and Physical Characteristics

The star Antares, also known as Alpha Scorpii (α Scorpii or α Sco), is a class M red supergiant belonging to the Scorpius constellation. Its name derives from the ancient Greek words “Ant-Ares” meaning the “rival of Mars”. Since it lies in the ecliptic - the path the Sun, the Moon and the planets trace on the celestial sphere - it could be easily mistaken for Mars due to its color and size resemblance to the red planet.

Despite its relatively small mass, the size of this supergiant is enormous, and the average density is considerably low. Another characteristic is that it pulsates irregularly causing its apparent magnitude to vary considerably over the years (variable star). Regarding its luminosity, it is classified as either the 15th or 16th brightest star in the sky. This actually depends on whether we regard the two brighter stars in the Capella quadruple system as one or not. It is 10,000 times brighter than our Sun in the visual spectrum, however since most of its energy is radiated in the infrared, its bolometric luminosity is almost 65,000 times that of the Sun.

Surface temperature: ~ 3,500°K

Spectral type: M1

Scorpius Constellation. The Red Arrow Indicates the Position of Antares

Apparent magnitude: 0.9 - 1.8 (variable star)

Distance: ~ 600 ly (light-years)

Luminosity: ~ 10,000 times the luminosity of the Sun

Mass: 15-18 times the mass of the Sun

Radius: ~ 800 times the radius of the Sun

Radial velocity: - 1.4 km/s

Constellation: Scorpius

Nature, Evolution and a Companion Star

As mentioned above, Antares is a red supergiant entering the final stages of its life. It is surrounded by an egg-shaped nebula which consists of gas thrown off by the giant star. This means that it will soon run out of fuel and explode as a supernova. The explosion may actually take place tomorrow or in a million years and a neutron star or maybe a black hole will be the only object remaining from the once gigantic star.

Currently however, the supergiant star of Scorpius is not alone. It is accompanied by a smaller blue companion known as Antares B, which orbits the main star and lights up its nebula. This hot blue companion was discovered by J. Tobias Bürg in 1819 during a lunar occultation of Antares A for a few seconds. It lies only 550 AUs away from the giant star and it is 170 times brighter than our Sun. It is estimated that it has a period of 878 years, although its orbit is not yet well defined. Some of its general characteristics are:

Spectral type: B2.5

Mass: ~ 3 times the mass of the Sun

Apparent magnitude: + 5.5

Luminosity: 188 times the luminosity of Sun

Interesting Facts and Comparisons

  • One of the most gigantic stars known today.

  • It is one of the four brightest stars near the ecliptic along with Aldebaran, Spica, and Regulus.

  • Antares is isolated in the night sky being separated from other first-magnitude stars, the nearest is Alpha Centauri.

  • If it could take the place of the Sun in the center of our Solar System, its outer surface would lie somewhere between the orbits of Mars and Jupiter.

  • Mars passes near the supergiant every couple of years, making it possible to admire and compare these two celestial bodies.

  • Antares is also similar to another red star, Betelgeuse in the constellation Orion. Although Betelgeuse is smaller, it appears slightly brighter due to its smaller distance from us ( 428 light-years ).

    Size Comparisons Among Antares, Arcturus, the Sun and the Orbit of Mars

How to Spot It

The characteristic color and brightness of the star are two of its properties that make it very easy to spot it in the sky. Another one is the fact that it belongs to a rather distinctive constellation; Scorpius (also indicated as the Antares constellation), where the bright star occupies the ‘heart of the scorpion’.

Around June 1 we can see it reaching its midnight culmination (its highest point in the sky at midnight). At about dawn in early March and at about sunset in early September it can be found in its highest position in the sky. However the best time to view it is around May 31, when it is visible throughout the night for most of the northern hemisphere. For those who live in the southern hemisphere, from about 67 degrees south latitude, the star is circumpolar, meaning that it never sets and is visible every night of the year.

Antares lies within 5° of the ecliptic and this is the reason why it is possible to be occulted by the Moon and rarely by other planets of our Solar System. Actually it happened recently; the Moon cast its shadow on the giant star on 31 July 2009 and regions from southern Asia to the Middle East were able to observe the event. A more interesting event will unfortunately happen much further in the future, Antares will be occulted by Venus on 17 November 2400.


Image Credits:

  • “Antares, the Red Giant of Scorpius Constellation” by c a palmer
  • “Scorpius Constellation. The Red Arrow Indicates the Position of Antares” by Till Credner
  • “Size Comparisons Among Antares, Arcturus, the Sun and the Orbit of Mars” by Sakurambo

This post is part of the series: Bright and Famous Stars

Amazing facts and details on some of the most beautiful stars in the night sky.

  1. Learning About Deneb: The Supergiant!
  2. One of the Biggest and Brightest Stars: Antares