Image of the dwarf galaxy Holmberg IX taken with the Large Binocular Telescope. The arrow indicates the approximate location of the new star system. Also see animation below.
Credit: Ohio State University
SYDNEY: Peanut-shaped star systems so rare that only two of their kind have ever been found have surprised their discoverers and may help explain how some supernovae erupt.
The star systems are unusual because they contain two orbiting yellow supergiants - very large stars that are between 15 to 20 times the size of our Sun. The stars are so close that they share some of their mass, appearing as a blurry peanut-like shape.
Researchers led by doctoral student José Prieto from Ohio State University in Columbus, USA, believe the stars may be the progenitors of rare supernovae linked to yellow (or mid temperature) stars. Their results were published in the Astrophysical Journal Letters earlier this year.
The astronomers initially discovered one yellow supergiant binary 13 million light-years away, within the small galaxy Holmberg IX, which orbits the larger galaxy M81. It was found using the 8.4-metre Large Binocular Telescope (LBT) on Mount Graham in Arizona, USA. Reviewing old astronomical data, the researchers spotted another system about 230 000 light-years away within the Small Magellanic Cloud, a small galaxy that orbits our own Milky Way.
Unexpected duo
"We didn't expect to find one of these things, much less two," said Kris Stanek, an astrophysict Ohio State University, part of the group that detected the M81 binary.
"We needed the LBT to spot the first binary, but the second one is so bright that you could see it with binoculars in your backyard," he added. "Yet, if we hadn't found the first one, we may never have found the second one."
Prieto believes these systems may be the progenitors of rare supernova linked to yellow supergiants. Only two such supernova have been detected - most supergiants go supernova when at the blue (or hot) phase or red (or cool) phase, he said.
Stars swing between blue and red phases depending on the chemical elements they consume in their cores. But it had been thought that few supergiants spend long in the transitional yellow phase - until now.
|
Video animation of a new type of star system, one that may be the progenitor of a rare type of supernova, known as a 'yellow supergiant eclipsing binary'. (Credit: Kevin Gecsi, Ohio State University).
|
"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 supergiants 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 supergiant."
Big jump
Ken Freeman, an astrophysicist at the Australian National University's Research School of Astronomy and Astrophysics said the discovery is "cute but not Earth shattering".
"It's a bit of a jump to say the yellow supergiant binaries are related to the supernova," said Freeman, adding that single yellow supergiants could also go supernova.
Stars go supernova when the star's core can't hold itself up and it implodes. Supergiant stars may only have a lifespan of 10 million years (as opposed to the 8 billion year life cycle of our own Sun), Freeman added. "When you get bright stars like that, everything happens very quickly."
While a star is more likely to go supernova during one of the limiting phases (when the star is blue or red), it's not surprising that it could happen during a star's yellow phase, said Freeman. "But you'd have to be pretty quick," he added.
