Gorged Star Could Explode

July 21, 2006 — Astronomers have discovered that a piggish, belching little star might just bust a gut — spilling secrets about the universe in the process.

The scene of interstellar swilling is called RS Ophiuchi, a dense and voracious white dwarf star with what looks to be a red giant companion star that is footing all the dining bills.

RS Ophiuchi has a long history of outbursts every 20 years or so, which astronomers — using new radio, infrared and X-ray observations — now ascribe to thermonuclear burps when the white dwarf eats too heartily from the red giant.

Here’s how they think it works: As the red giant material is sucked down onto the white dwarf and piles up on its surface, the pressure at the bottom of the pile builds up until it’s enough to trigger a thermonuclear explosion, said astronomer Jennifer Sokoloski of the Harvard-Smithsonian Center for Astrophysics.

Such an explosion happened on Feb. 12, and astronomers have since been gathering data with an armada of modern space and ground-based telescopes.

"It’s incredible," said Sokoloski, who coauthored one of two papers about the rare, ongoing event, which appear in the July 20 edition of Nature.

At thousands of miles per second, the blast wave from the explosion crashes through the gassy, dusty nebula region created by the red giant, lighting it up in visible light, infrared radiation, radio waves and even X-rays.

The explosion actually made RS Ophiuchi visible to the naked eye, if you knew where to look, she reported.

The multi-spectral lightshow has been giving astrophysicists an unprecedented opportunity to test theories about how such explosions are shaped, and how they propagate through space.

That, in turn, is more broadly important to astronomers because this little white dwarf appears on the verge of a much larger explosion – an obliterating supernova – of a special type used to measure the size of the universe.

"This object is a low-energy analogue of a supernova," explained astrophysicist Michael Bode of Liverpool John Moores University, referencing the Feb. 12 explosion.

Questions like whether a blast is spherical or aligned into narrow jets, or entirely asymmetrical in one direction, have a big effect on how bright its light appears when it reaches Earth.

Getting a better handle on those factors with a nearby object ought to help astronomers make sense of the behavior of more distant stellar explosions, said Bode.

There’s also a very good chance RS Ophiuchi is preparing to become a supernova itself, said Sokoloski. If that’s the case, it could help astronomers better understand just how white dwarfs create what are called Type 1A supernovae — considered the standard 60-watt bulbs of the universe.

Astronomers suspect all Type 1A supernovae behave similarly. So when one blinks on somewhere in the cosmos, astronomers make some theoretical assumptions about its original brightness and spectral qualities, and compare that to what they see.

The comparison allows them to measure the supernova's distance from Earth and determine some qualities of the space in between.

In this way astronomers can measure the size of the universe and calculate things like the "dark energy" that keeps the universe expanding, said Bode.

The trouble is, there is no guarantee that all Type 1A supernovae are alike.

"In RS Ophiuchi we see it evolving before our very eyes," said Bode. "We see it changing hour-by-hour."

And if during some of these 20-year belches, some red giant material is staying down on the white dwarf, it could be building up enough mass to explode the entire star as a Type 1A supernova.

That’s something Bode and his colleagues are trying to figure out right now, he said.

"It appears very much that this could be a Type 1A supernova progenitor," agreed NASA astronomer Richard Barry. If it is, it’s the first chance astronomers have to watch as a star gets ready to blow itself to smithereens.

"And it’s right in our own backyard," remarked Barry.