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Astronomers Image the Surface and Atmosphere of the Red Supergiant Star Antares

08/25/2017 08:11PM

Astronomers Image the Surface and Atmosphere of the Red Supergiant Star Antares
To the unaided eye the famous bright star Antares shines with a strong red tint in the heart of the constellation Scorpius. It is a huge and comparatively cool red supergiant in the late stages of its life, on the way to becoming a supernova.

Antares is considered by astronomers to be a typical red supergiant star. These huge dying stars are formed with between nine and forty times the mass of the Sun. When a star becomes a red supergiant, its atmosphere extends outward so it becomes large and luminous, but low density. Antares now has a mass about 12 times that of the Sun and a diameter about 700 times larger than the Sun's. It is thought that it started life with a mass more like 15 times that of the Sun and has shed three solar masses of material during its life.

A team of astronomers, led by Keiichi Ohnaka, of the Universidad Catolica del Norte in Chile, used ESO's Very Large Telescope Interferometer (VLTI) at the Paranal Observatory in Chile to map Antares' surface and to measure the motions of the surface material. This is the best image of the surface and atmosphere of any star other than the Sun.

The VLTI is a unique facility that can combine the light from up to four telescopes, either the 8.2 meter Unit Telescopes, or the smaller Auxiliary Telescopes, to create a virtual telescope equivalent to a single mirror up to 200 meters across. This allows it to resolve fine details far beyond what can be seen with a single telescope alone.

Using ESO's Very Large Telescope Interferometer astronomers have constructed the most detailed image ever of a star -- the red supergiant star Antares. (Video Credit: ESO)

"How stars like Antares lose mass so quickly in the final phase of their evolution has been a problem for over half a century," said Keiichi Ohnaka. "The VLTI is the only facility that can directly measure the gas motions in the extended atmosphere of Antares -- a crucial step towards clarifying this problem. The next challenge is to identify what's driving the turbulent motions."

Using the new results, the team has created the first two dimensional velocity map of the atmosphere of a star other than the Sun. They did this using the VLTI with three of the Auxiliary Telescopes and an instrument called AMBER to make separate images of the surface of Antares over a small range of infrared wavelengths. The team then used this data to calculate the difference between the speed of the atmospheric gas at different positions on the star and the average speed over the entire star. This resulted in a map of the relative speed of the atmospheric gas across the entire disc of Antares -- the first ever created for a star other than the Sun.

This video starts from a wide field view of the Milky Way, including the prominent constellation Scorpius (The Scopion). It zooms in towards Scorpius' bright red heart -- the red supergiant star Antares. The final view shows an image of the surface of Antares taken with ESO's Very Large Telescope Interferometer. (Video Credit: ESO, K. Ohnaka, N. Risinger - Music: Astral Electronic)

The velocity of material on the surface of Antares that is moving towards or away from Earth can be measured by the Doppler Effect, which shifts spectral lines either towards the red or blue ends of the spectrum, depending on whether the material is receding from or approaching the observer.

The astronomers found turbulent, low density gas much further from the star than predicted and concluded that the movement could not result from convection, that is, from large scale movement of matter which transfers energy from the core to the outer atmosphere of many stars. Convection is the process whereby cold material moves downwards and hot material moves upwards in a circular pattern. The process occurs on Earth in the atmosphere and ocean currents, but it also moves gas around within stars. They reason that a new, currently unknown, process may be needed to explain these movements in the extended atmospheres of red supergiants like Antares.

"In the future, this observing technique can be applied to different types of stars to study their surfaces and atmospheres in unprecedented detail. This has been limited to just the Sun up to now," concludes Ohnaka. "Our work brings stellar astrophysics to a new dimension and opens an entirely new window to observe stars."

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