How Supergiant Stars Lose Mass

Two teams of astronomers, one from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and one from the Paris Observatory, France, have recently used the European Southern Observatory’s Very Large Telescope to study Betelgeuse, the second brightest star in the constellation of Orion. Their combined research might offer an answer to one of the mysteries involving red supergiants and explain their rapid loss of mass.
 Star Betelgeuse (Credit: ESO)
Star Betelgeuse
(Credit: ESO)

Betelgeuse is a red supergiant, one of the biggest stars we know – it is almost a thousand times larger than our Sun. Unlike many stars, it is easy to find, since it’s in the midst of the Orion constellation (also known as “The Hunter”). The unarmed eye can recognize it easily: it’s the second brightest star in that group. Betelgeuse is also one of the most luminous stars known, emitting light equivalent to more than 100,000 Suns.

Though these extreme properties might crown this star as king of the stars, they also foretell its fall; such stars tend to quickly explode as a supernova. The good news is that this event is likely to be seen easily from Earth, even in broad daylight – a day that many astronomers, professional and amateur as one, look forward to.

Like all Red supergiants, Betelgeuse holds several unsolved mysteries. One of them is how it sheds tremendous quantities of material in such a short while. In order to answer this question, two teams of astronomers have combined to study Betelgeuse using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) and other advanced technologies.

The team, led by Pierre Kervella from the Paris Observatory, used the adaptive optics instrument NACO combined with a so-called “lucky imaging” technique, in order to obtain the sharpest ever image of Betelgeuse. The resulting NACO images almost reach the theoretical limit of sharpness attainable for an 8-metre telescope; the resolution is as fine as 37 milliarcseconds. Such resolution would hypothetically enable a person on Earth to locate a tennis ball on the International Space Station.

Lucky imaging (also called lucky exposures) is a method used for astronomical photography. This technique uses a high-speed camera with exposure times short enough (100ms or less) so that the changes in the Earth’s atmosphere during the exposure are minimal. With lucky imaging, those exposures least affected by the atmosphere (typically around 10 percent) are chosen and combined into a single image by shifting and adding the short exposures. The result usually has a much higher resolution than what would be possible with a single, longer exposure that includes all the frames.

This technique, though subject to criticism, has brought impressive results. “Thanks to these outstanding images, we have detected a large plume of gas extending into space from the surface of Betelgeuse,” says Kervella. The plume extends to at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune. “This is a clear indication that the whole outer shell of the star is not shedding matter evenly in all directions,” adds Kervella.

In order to explain this asymmetry two theories have been proposed. One assumes that the mass loss occurs above the polar caps of the giant star, possibly because of its rotation. The second theorizes that such a plume is generated above large-scale gas motions inside the star, known as convection — similar to the circulation of water heated in a pot. To decide between these two theories, another scientific approach was implemented.

The second technique, interferometry, was used by the German team from the Max Planck Institute for Radio Astronomy, led by Keiichi Ohnaka. Using this method, researchers diagnose the properties of two or more lasers or waves, by studying the pattern of interference created by their superposition. In addition to astronomy, this technique is common in other fields, such as engineering metrology, oceanography, seismology, quantum mechanics and nuclear and particle physics.

These observations revealed that the gas in Betelgeuse’s atmosphere is moving vigorously up and down, and that these bubbles are as large as the supergiant star itself. In their paper, the astronomers review the results yielded by this combination of techniques, emphasizing the images’ sharpness, equal to that of a virtual, gigantic 48-metre telescope. Moreover, they conclude that these large-scale gas motions roiling under Betelgeuse’s red surface are behind the ejection of the massive plume into space.

TFOT has covered other supernova-related stories. The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus, and scientists from the Weizmann Institute of Science in Israel and San Diego State University have observed an Explosion of a Star 50 Times Larger Than the Sun. Other related TFOT stories include the new theories regarding Premature Supernovae, and the Confirmation of Luminous Object as Supernova, made by an international team of astronomers.

For more information about Betelgeuse’s latest images, see ESO’s press release.