A pair of yellow supergiant stars, orbiting so close to one another that they form a single peanut-shaped object, has been discovered in a nearby galaxy. The astronomers who discovered it say that similar conjoined giants might be the source of some unusual supernova explosions.
The stellar peanut inhabits a small galaxy called Holmberg IX, around 12 million light years from Earth. It was discovered using the Large Binocular Telescope in Arizona, US.
"This is the most luminous star we detected in Holmberg IX," says the study's lead author, José Prieto of Ohio State University in Columbus, US. 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 behaviour can be explained if there are actually two stars rotating around each other, so each periodically blocks the light from its neighbour. 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 supergiant 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 satellite of the Milky Way galaxy.
The big yellow peanuts might explain a small astrophysical puzzle.
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 has usually expanded and cooled to become a red supergiant.
But two supernova explosions, in 2004 and 2006, were seen to come from yellow stars. A yellow supergiant is just a very brief transitional phase on the way to becoming a red supergiant, and it shouldn't have an iron core big enough to go supernova.
Unless, perhaps, these supernovae were produced by close-contact binary stars that share an outer envelope. If one star's outer layers start to expand, the second star might disrupt the process by hoovering up the material or ejecting it in a stellar wind.
Then an iron supernova bomb could build up while the stars remain relatively small, warm and yellow – et voilá, the giant exploding space peanut.
Journal reference: Astrophysical Journal Letters (vol 673, p L59)
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