24th August 2017
The most detailed ever image of a star
Astronomers using ESO's Very Large Telescope have produced the most detailed ever image of a star, the red supergiant Antares, and created the first map of surface motion on a star other than our Sun.
Using the European Southern Observatory's (ESO's) Very Large Telescope, astronomers have produced the most detailed ever image of a star – the red supergiant Antares, which lies 550 light years from Earth and is the 15th brightest star in the night sky. They have also made the first map showing the velocities of material in the atmosphere of a star other than our Sun, revealing unexpected turbulence in Antares's huge extended atmosphere.
To the unaided eye, the bright star Antares shines with a strong red tint in the heart of the constellation of Scorpius (The Scorpion). It is a huge and comparatively cool red supergiant, only 12 million years old, but already in the late stages of its life on the way to becoming a supernova. It now has a mass about 12 times that of the Sun and a diameter 700 times larger. If placed in our Solar System, it would engulf the inner planets along with Jupiter and its atmosphere would even come close to reaching Saturn. Antares is thought to have ejected three solar masses of material during its life so far.
A team of astronomers, led by Keiichi Ohnaka from the Universidad Católica del Norte in Chile, has now used ESO's Very Large Telescope Interferometer (VLTI) at the Paranal Observatory in Chile to map Antares's surface and measure the motions of its surface material. This is the best ever image of the surface and atmosphere of any star other than our Sun. The VLTI is a unique facility, able to combine the light from up to four telescopes to create a "virtual" telescope equivalent to a single mirror up to 200m (656 ft) across. This allows it to resolve fine details, far beyond what can be seen with a single telescope alone.
"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 Ohnaka, who is also the lead author of a paper on the study. "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. They then used these 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 atmospheric gas across the entire disc of Antares – the first ever created for a star other than our Sun.
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. 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," concludes Ohnaka. "Our work brings stellar astrophysics to a new dimension and opens an entirely new window to observe stars."
The team's work is published in the journal Nature.
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