Why Do Giant Stars Lose Mass the Way They Do?
Stars lose mass throughout their lives. In the case of the Sun, it’s losing mass right now through the action of the solar wind — which blows material away from our star and out through the solar system.
As they get older, stars shed more mass as part of the “normal” process of stellar aging. For large stars, the mass loss is tremendous. Yet, think about it — a huge, massive star is going to have a pretty strong gravitational self-preservation instinct (so to speak). Its gravitational pull should normally prevent huge amounts of mass loss.
However, in the case of Betelgeuse (in the constellation Orion), which is a red supergiant and approaching the end of its life, the mass loss is more than scientists expected. This big old star is losing the equivalent of one Earth mass (about 6 × 1024 kilograms) each year. That’s a huge amount and astronomers have been puzzling over reasons why this is happening. The best evidence is inside the star, of course, and hidden inside the clouds of material that Betelgeuse has already blown away. However, seeing detail from this distance (640 light-years away) and against the glaringly bright light coming from the star is a tough proposition. It requires high resolution and specialized instruments.
A team of astronomers led by Keiichi Ohnaka at the Max Planck Institute for Radio Astronomy (MPIfR) got around this by using the Very Large Telescope Interferometer in Chile to get a high-resolution view of the scene. This let them see the motions of gas in Betelgeuse’s atmosphere. This is the first time such motions have been seen in a star other than the Sun, and allows them to chart the motions across the star’s surface. The motions show that gas bubbles in Betelgeuse’s atmosphere are large — at least one is the size of the orbit of Mars, which is about the same size as the star itself! They are moving up and down quite actively — the Mar-orbit-sized one is plowing along at at about 40,000 kilometers per hour).
It’s not clear where these bubbles originate, but it does seem that they are part of the mass-loss mystery at Betelgeuse. The observations suggest that these colossal bubbles can expel the material from the surface of the star out to space. It’s not a nice clean stellar wind kind of mass loss, like we see at other stars. This is more violent and spasmodic and may be typical of the way that such a star will behave as it approaches the very end of its stellar existence.
Betelgeuse will explode as a violent supernova sometime in the next hundred thousand years. The view from Earth will be spectacular, and the dying star’s last outburst will be visible even in the daytime hours. Astronomers have been watching this star for years, charting its path to stardeath. Every bit of information they glean helps us understand the mechanics of star death even better.
For more information about the VLT measurements of Betelgeuse’s mass loss, check out the Max Planck web page about the observations. It has links to the original paper and an image of the VLT observatory.