By Xinhua
Washington : A pair of yellow super giant stars, orbiting so close to one another that they form the shape of a peanut, has been discovered in a nearby galaxy.
The U.S. astronomers who discovered it announced Tuesday in Astrophysical Journal Letters that similar conjoined giants might be the source of some unusual supernova explosions.
The stellar peanut inhabits a small galaxy called Holmberg IX, about 12 million light years from Earth. It was discovered using the Large Binocular Telescope in Arizona.
“This is the most luminous star we detected in Holmberg IX,” said the study’s lead author, Jose Prieto of Ohio State University.
It is about 100,000 times as bright as the sun at visible wavelengths — and its brightness varies in a telling pattern.
Over a period of 270 days, the team saw the star dim twice. This behavior can be explained if there are actually two stars rotating around each other, so each periodically blocks the light from its neighbor. The precise shape of this “light curve” reveals that the two stars are bloated enough to share their outer regions, forming the peanut shape.
This is the first double yellow super giant to be identified, but looking through archived data the team found a second, almost identical pair of yellow supergiants in the Small Magellanic Cloud, a small galaxy that orbits our own Milky Way.
The team said that the big yellow peanuts might explain a small astrophysical puzzle. Astronomers had thought such massive stars tend to end their lives in supernova explosions when their nuclear furnaces build up iron cores that are dense enough to implode, releasing a colossal amount of energy and blowing apart the rest of the star.
Long before enough iron has built up, however, the star usually expands and cools to become a red supergiant. A yellow supergiant is just a very brief transitional phase on the way to becoming a red supergiant, and it should not have an iron core big enough to go supernova.
However, two supernova explosions, in 2004 and 2006, were seen to come from yellow stars.
Now, based on the new peanut-shaped system, astronomers think that these unusual supernovae were produced by close-contact binary stars sharing an outer envelope.
“When two stars orbit each other very closely, they share material, and the evolution of one affects the other,” Prieto said. “It’s possible two super giants in such a system would evolve more slowly and spend more time in the yellow phase — long enough that one of them could explode as a yellow super giant.”