from the glam-shots dept.
A team of astronomers led by Wouter Vlemmings, Chalmers University of Technology, have used the [Atacama Large Millimeter Array] to make the sharpest observations yet of a star with the same starting mass as the Sun. The new images show for the first time details on the surface of the red giant W Hydrae, 320 light years distant in the constellation of Hydra, the Water Snake. W Hydrae is an example of an AGB (asymptotic giant branch) star. Such stars are cool, bright, old and lose mass via stellar winds. The name derives from their position on the famous Hertzsprung-Russell diagram, which classifies stars according to their brightness and temperature.
[...] Alma's images provide the clearest view yet of the surface of a red giant with a similar mass to the Sun. Earlier sharp images have shown details on much more massive, red supergiant stars like Betelgeuse and Antares. The observations have also surprised the scientists. The presence of an unexpectedly compact and bright spot provides evidence that the star has surprisingly hot gas in a layer above the star's surface: a chromosphere. "Our measurements of the bright spot suggest there are powerful shock waves in the star's atmosphere that reach higher temperatures than are predicted by current theoretical models for AGB stars," says Theo Khouri, astronomer at Chalmers and member of the team. An alternative possibility is at least as surprising: that the star was undergoing a giant flare when the observations were made.
The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA (DOI: 10.1038/s41550-017-0288-9) (DX)
Astronomers have used the European Southern Observatory's Very Large Telescope to observe giant "bubbles" (granulation cells/patterns) extending from the surface of the red giant π1 Gruis (aka Pi1 Gruis):
Astronomers using ESO's Very Large Telescope have for the first time directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π1 Gruis. This remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of this huge star, which has 350 times the diameter of the Sun. Each cell covers more than a quarter of the star's diameter and measures about 120 million kilometres across. These new results are being published this week in the journal Nature.
Located 530 light-years from Earth in the constellation of Grus (The Crane), π1 Gruis is a cool red giant. It has about the same mass as our Sun, but is 350 times larger and several thousand times as bright. Our Sun will swell to become a similar red giant star in about five billion years.
An international team of astronomers led by Claudia Paladini (ESO) used the PIONIER instrument on ESO's Very Large Telescope to observe π1 Gruis in greater detail than ever before. They found that the surface of this red giant has just a few convective cells, or granules, that are each about 120 million kilometres across — about a quarter of the star's diameter. Just one of these granules would extend from the Sun to beyond Venus. The surfaces — known as photospheres — of many giant stars are obscured by dust, which hinders observations. However, in the case of π1 Gruis, although dust is present far from the star, it does not have a significant effect on the new infrared observations.
Also at EarthSky.
Large granulation cells on the surface of the giant star π1 Gruis (DOI: 10.1038/nature25001) (DX)
Open access version of the above paper: Convective pattern on the surface of the giant star π1 Gruis
The European Southern Observatory's Very Large Telescope Interferometer (VLTI) has captured the best ever image of another star. The VLTI was used to image the surface of Antares, a red supergiant star about 550 light years away in the heart of the constellation Scorpius (The Scorpion):
The VLTI is a unique facility that can combine the light from up to four telescopes, either the 8.2-metre Unit Telescopes, or the smaller Auxiliary Telescopes, to create a virtual telescope equivalent to a single mirror up to 200 metres across. This allows it to resolve fine details far beyond what can be seen with a single telescope alone.
[...] 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 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 the atmospheric gas across the entire disc of Antares — the first ever created for a star other than the 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.
Vigorous atmospheric motion in the red supergiant star Antares (DOI: 10.1038/nature23445) (DX)