The Star Antares

Antares imaged using the VLT (Very Large Telescope) array in Chile. Credit: Ohnaka/ESO

The Red Supergiant star Antares was in the news recently when the VLTI (Very Large Telescope Interferometer) located in Chile was able to resolve structure on the star itself. The VLTI is a configuration of the Very Large Telescope that allows it to be used as an interferometer that has resolution far in excess of the Hubble telescope (at least for small bright objects like stars).

VLT (Very Large Telescope) array. Image Credit: ESO (European Southern Observatory)

Resolving Antares with the VLTI was an extraordinary feat as the size of Antares appears tiny from earth and is just 0.036 arc seconds across, which is equivalent to how large a pea appears from 100km! The title image is the best image we have of a star, apart from the sun, and hints at complex structures . Two bright spots spots visible in the image are thought to be thinning’s in the "MOLsphere", a layer of Carbon Monoxide and Water that surrounds large cool stars.

The following drawing is an Artist’s impression of what the star might look like, although the limb of the star is probably is not as well defined as shown. As instrumentation improves over time it will be fascinating to see more detail come into view.

Artist’s Impression based on VLTI. Credit: SO/M. Kornmesser

Properties of Antares

Antares itself is a M1.5 type Red Supergiant star and is extremely large with a diameter estimated to be 680 times greater than the suns. If Antares were placed at the location the sun, all the inner planets including Mars would be swallowed up by the sun and Jupiter would only be a little outside the star. Despite the size, Antares appears as a tiny object from the earth because of it’s 550 Light Years distance. The temperature of Antares photosphere, or its visible boundary, is 3600 Kelvin which is considerably cooler than the suns 5600 Kelvin and this is why it has a reddish color.

In the visible spectrum the star is an impressive 10,000 times brighter than the sun, but it’s true (Bolometric) luminosity is closer to 100,000 times greater as most the light is emitted in the infrared part of the spectrum. The star has a mass of 12 suns, meaning that it will likely end its life in a spectacular supernova explosion. As the star is in the Red Supergiant phase, it’s already getting near the end of its life and probably has less than a million years left before it goes supernova.

Antares Companion

Amateur image of Antares clearly shows its blue companion Star. Note the actual disk of the primary star is not visible in this image, it looks large because of light scatter and overexposure. Taken with a C14 and QHY183c camera. Credit:Author/Lovejoy

Antares has a blue companion star that can sometimes be seen in a powerful telescope. It is quite hard to see because it is very close to the primary star and requires a good steady sky to see. Although much fainter than the primary star it is far brighter and hotter than the sun and appears to orbit the primary star every 2000 years or so, although the exact period is uncertain. The true distance between the 2 stars is 260 AU (Astronomical Units) which is about 3 times the distance of Neptune from the sun.

Finding Antares

Antares is the brightest star in the Constellation of Scorpio, the Scorpion, and the 15th brightest star in the sky. Credit:Author/Lovejoy

Antares is the brightest star of the Constellation of Scorpius, a distinctive constellation that is currently visible in the east before dawn and is visible in the evening during the middle of the year. Keen eyed people might notice Antares has an Orange hue to the naked eye, but the color is very obvious in binoculars. An amateur telescope of at least 25cm aperture is required to see the companion star and then only when conditions are excellent.

Conclusion

Larger telescopes and improving technology are making it possible to see things in the sky thought to be impossible a couple of decades ago, as can be seen with the VLTI image of Antares. For the first time we are starting to see detail on the surfaces of stars, which will open up our understanding of them immensely.

References

1. Ohnaka, K, Weigelt, G, Hofmann, K.H. (2017). Vigorous atmospheric motion in the red supergiant star Antares. arXiv:1708.06372