Exploding Stars: Asymmetry with Cosmological Consequences

6 August 2003

Austin, Texas—University of Texas at Austin astronomers, along with colleagues in California and Europe, have discovered new facts about the exploding stars called supernovae that could have effects on the big cosmic questions -- how old is the universe, how large, and how is it changing over time?

The research announced today concludes that the exploding stars called Type Ia supernovae -- the "standard candles" of astronomical research -- don't exactly behave the way astronomers thought.

Former University of Texas researcher Lifan Wang (now of Lawrence Berkeley National Lab), his Texas colleagues J. Craig Wheeler and Peter Hoeflich, Dietrich Baade of the European Southern Observatory and other colleagues on their team have established that Type Ia supernovae do not explode in a perfectly spherical manner. This study was published in the July 10 edition of the Astrophysical Journal.

Type Ia supernovae are called standard candles because astronomers know -- to a certain extent -- how intrinsically bright these exploding stars are. A 100 watt bulb is always the same brightness, but looks dimmer if you are further away from it. Similarly, by comparing the intrinsic brightness of a Type Ia supernova to how bright it appears in the sky, astronomers can figure the distance to it -- and its host galaxy.

Knowing precise distances to galaxies outside our own Milky Way is critical to astronomers' calculations about the universe's age, size, and fate.

The researchers used the European Southern Observatory's Very Large Telescope (VLT) in Chile to measure the polarization of light coming from a Type Ia supernova (SN 2001el) in the galaxy NGC 1448, as it brightened and dimmed. Polarization is a technique to measure the "shape" of an astronomical object. Some Type Ia are little brighter than normal, others a little less bright.

The researchers were able to show that at peak brightness the exploding star was slightly flattened, with one axis shorter by about 10 percent. By a week later, however, the visible explosion was virtually spherical.

"This is the first time the intrinsic polarization, and hence shape, of a normal Type Ia supernova has been detected," Wheeler says.

According to Wheeler, this has two main consequences:

First, there are consequences for its uses in cosmological studies. "Standard candles may not look the same from all angles," he says. So, astronomers will need to account for the fact that different supernovae can be viewed from different aspects before being able to use them for precise distance measurements.

Wheeler uses the example of a carton of eggs to explain how asymmetry can affect brightness measurements. All the eggs in the carton are similar, but each egg looks different depending on how you view it. The egg shape is only apparent when they are viewed from the side. Viewed end-on, each egg looks round. Likewise, if supernovae are not spherically symmetric, they will shine more brightly in one direction than in others --even though each supernova may be virtually identical.

"A Type Ia supernova is a standard egg," Wheeler says. "Sometimes you look at it sideways and sometimes you look at it from the top or bottom, but it's a standard egg."

Even with a telescope as powerful as the VLT, distant supernovae appear only as point-sources of light, so asymmetric shapes cannot be seen directly. Instead they must be inferred from the way the light is polarized. (Polarization refers to the orientation of the plane of the electric wave component of light.)

The second consequence of supernova asymmetry has to do with seeking a better understanding how the thermonuclear explosions of supernovae work. "Astronomers are convinced that what's happening is a star in binary system is pulling mass off its companion star onto itself -- enough to cause it to explode. But there is zero direct observational evidence to prove that," Wheeler says.

"Now, we can look for direct evidence of this mass transfer process in the asymmetry of the light coming from a Type Ia -- this is the holy grail that people have been after for more than 30 years," he concludes.

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Note to Editors:

A low-resolution image of SN 2001el in the galaxy NGC 1448 can be found online here, along with a news release from Lawrence Berkeley National Laboratory.