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Antares, the brightest star in the constellation Scorpio, is a red supergiant about 550 light-years from Earth. It is 700 times larger than our Sun in visible light and 12 times more massive, and we now have the best map of its atmosphere. In fact, it's the best radio map of any star other than the Sun.
Red supergiants are huge cold stars at the end of their life, spewing out heavy elements into space via vast stellar winds. It's unknown how these huge winds are launched so astronomers looked at Antares, the closest supergiant star to Earth.
The exceptional achievement is reported in Astronomy & Astrophysics, and was possible thanks to the combined use of two world-class observatories. The National Science Foundation’s Karl G. Jansky Very Large Array (VLA) looked at the external part of Antares' atmosphere, even highlighting the stellar wind released by this gigantic star. The Atacama Large Millimeter/submillimeter Array (ALMA) in Chile went in closer, studying the atmosphere just around its surface.
The observations revealed how large the atmosphere actually is. The region known as the chromosphere, which was targeted by ALMA, has a radius 2.5 times of Antares and is cooler than previous studies had suggested, with a temperature around 3,500°C (6,300°F). The VLA observations show how far out the outer layers of this star stretch.
“The size of a star can vary dramatically depending on what wavelength of light it is observed with,” lead author Eamon O’Gorman of the Dublin Institute for Advanced Studies explained in a statement. “The longer wavelengths of the VLA revealed the supergiant’s atmosphere out to nearly 12 times the star’s radius.”
Previous observations of the star were conducted in UV and visible light. That approach tends to pick up the hottest bits of plasma in and around a star. Radio and submillimeter instruments are sensitive to cooler material and for this reason, they have been able to see the atmosphere extending further out.
“We think that red supergiant stars, such as Antares and Betelgeuse, have an inhomogeneous atmosphere,” said co-author Keiichi Ohnaka of the Universidad Católica del Norte in Chile, who previously observed Antares’ atmosphere in infrared light. “Imagine that their atmospheres are a painting made out of many dots of different colors, representing different temperatures. Most of the painting contains dots of the lukewarm gas that radio telescopes can see, but there are also cold dots that only infrared telescopes can see, and hot dots that UV telescopes see. At the moment we can’t observe these dots individually, but we want to try that in future studies.”
The work also shows a clear distinction between the chromosphere and the region where the stellar wind is accelerated. The VLA observation also caught some of that wind being illuminated by Antares B, which has a mass about 7 times of the Sun and 5 times its radius.
