Astronomers have long been curious about how red supergiant stars, like the bright star, Betelgeuse, manage to shed so much matter into space.
Now, thanks to a collaborative effort which gave scientists a detailed view of the distant star's surface, they have an answer - it's boiling.
Observations with a trio of 1.8-meter radio telescopes show giant bubble-like structures bobbing on the surface of Betelgeuse, a massive star located 640 light-years away in the constellation Orion.
Emitting about 100,000 times more light than our sun, Betelgeuse is the bright orange star on the shoulder of Orion, also known as The Hunter.
The observations, which are being reported in an upcoming issue of Astronomy & Astrophysics, are the first to spatially resolve the motion of gas on the surface of a star other than the sun, said Keiichi Ohnaka, with the Max-Planck-Institut fur Radioastronomie in Bonn, Germany.
Ohnaka and colleagues found giant gas bubbles - some as large as the star itself - moving vigorously up and down in the star's atmosphere.
Betelgeuse is nearing the end of its relatively short life and is expected to explode as a supernova in the next few thousand to hundred thousand years. When it blows, it should be visible from Earth even in daylight.
Scientists aren't sure about the bubbles' origins, but they do point to a likely mechanism for the release of gas and heavy elements into space. These materials become fodder for new stars and planets.
University of California at Berkeley astronomers reported in June that Betelgeuse shrank 15 percent over the past 15 years. At the time, scientists speculated that giant convection cells on the star's surface might be to blame.
Previous observations with the Hubble Space Telescope also gave astronomers hints of unusual surface features on Betelgeuse.
"We could sort of tell there was something in terms of structures on the star, but we couldn't tell anything specific," said Ron Gilliland, with the Space Telescope Science Institute in Baltimore.
Combining the light-gathering power of three telescopes gave scientists an instrument capable of seeing detail as small as a tennis ball from 200 miles away. They believe the giant bubbles seen on Betelgeuse's surface are flinging clumps of material from the surface of the star into space.
"These bubbles may be a result of an eruptive phenomenon in which material is flung out," Ohnaka told Discovery News. "Another possibility is that the bubbles are a result of convection of Betelgeuse's atmosphere."
Scientists hope to buttress their findings with observations of a second red supergiant Antares, located about 600 light-years away in the constellation Scorpius.
"We are working on the data now," Ohnaka said.