CASPER
THE FRIENDLY GHOST
As a large star nears the end of its life it begins to shed mass at a tremendous rate.
New observations of the supergiant star Betelgeuse may help scientists finally understand how this feat of weight loss is achieved.
Views from the European Southern Observatory's Very Large Telescope in Chile reveal a vast plume of gas spewing off of Betelgeuse, the shoulder star in the Orion constellation. This behemoth star has about 20 times the mass of our sun, but is losing about one sun's worth of mass every 10,000 to 100,000 years.
That rate of slimming may not sound like a crash diet, but it is about a million times faster than the rate at which the sun loses mass.
"Mostly every star goes through such a phase at the end of its life," said astrophysicist Pierre Kervella of the Observatoire de Paris-Meudon in France. "We know the mass is going out, but we don't know how it is expelled from the star."
The observations of the plume imply that Betelgeuse is not shedding mass evenly in all directions. Perhaps the plume is created by the rotation of the star, which may drive mass toward the poles where it could overflow into a spray of particles.
Or maybe gas is expelled in bubbles by convection, the same process that circulates water in a heating pot. Scientists think convection could transport gas upward toward the star's surface, and that the momentum of this movement could cause some gas to be expelled when it reaches the surface.
There is some data to support both possibilities, and it's possible that they work in concert.
"There is a coincidence between the polar axis of the star and the direction of the plume," Kervella said, lending support to the rotation hypothesis. "But convection appears more probable currently, because there are some observations which show that there is gas motion on the surface of the star."
At this rate, Betelgeuse is speeding through its remaining mass and is set to die in a supernova explosion within the next 10,000 years.
"Maybe it will explode in a few thousand years, but it could also be next year," Kervella told SPACE.com. "We don't know exactly. It has probably lived 90 to 95 percent of its life."
The star has already exhausted its store of hydrogen by burning it up in nuclear fusion reactions, and has moved on to burning helium and heavier elements in a stage called the red giant phase. These reactions tend to produce more heat than occurs when burning hydrogen, with the effect of inflating the star to a much larger volume than when it was young.
Because Betelgeuse is expanding in size while losing mass, its density has become very low: the star is now about a billion times less dense than the air on Earth.
This incredibly low density helps enable the star's mass loss. Gas on the surface of Betelgeuse is so far from the center of the star's gravity that the gravitational hold on it is weak and it can easily slip away.
Kervella, Keiichi Ohnaka of the Max-Planck Institute for Radio Astronomy in Germany, and colleagues detail the new findings in a pair of papers set to be published in the journal Astronomy and Astrophysics.