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For decades astronomers have gauged the breadth of the universe using a measuring stick made of embarrassingly mysterious stuff - but no longer.

Researchers have taken an x-ray look at the universe's most common and useful sort of exploding stars, type 1a supernova, and have found these 'standard candles' used to fix the distance of objects in the universe, are caused by the merger of two small dead stars called white dwarfs.

For some time there have been two most likely scenarios for what is causing these most generic explosions in the universe.

One hypothesis involves the merging of two white dwarfs, while the other sees a white dwarf stealing material from a Sun-like companion star, and the accumulating material causes the dwarf to become unstable and explode.

Figuring out exactly which is more common, and where, is essential for fine-tuning cosmic distances.

"For almost three decades astrophysicists have been arguing about this," says Marat Gilfanov of the Max Planck Institute for Astrophysics in Garching, Germany.

Gilfanov is the lead author of a paper in a recent issue of the journal Nature reporting the first direct evidence of the merger hypothesis being the most common.

Their evidence comes in the form of what Chandra x-ray telescope scientist Peter Edmonds calls the missing "x-ray fuse."
X-ray fuse

When a white dwarf is ripping away gases from companion star, the violence of that act causes the gases to scream out copious amounts of x-rays. Since the thieving should go on for literally millions of years before enough material built up on the dwarf to cause it to explode, the x-ray emissions of this sort of situation should be a dead giveaway of what's going on.

"One has an X-ray fuse, the other does not," says Edmonds.

And because we know the rate at which type 1a supernovae pop off in these other galaxies, it's not very hard to estimate the amounts of x-rays that should be expected pouring out of the galaxies if thieving dwarfs are behind them.

But when Gilfanov and his colleagues searched the bulging centre of the Andromeda galaxy and five older galaxies in our cosmic neighbourhood with the Chandra x-ray space telescope, they found 1/30th to 1/50th times less x-ray activity than expected if thieving dwarfs were the cause of the supernovae.

That suggests there's a lot more white dwarf merging out there than expected.
Not all the same

It also underscores how much work needs to be done to sort out the details of type 1a supernovae if they are to continue to be useful as researchers start doing "precision cosmology," says Gilfanov.

According to the researchers, it's not a simple matter of saying all supernovae are the same, and that the dimmer they are, the further away they are.

"If there are different detonation scenarios they will all have different relationships (between their luminosity and the decay of their light)," says Gilfanov. "So we have to sort out all of the diversity of type 1a supernovae out there."

Edmonds agrees. "The gist is about better understanding these tools for doing cosmology. Understanding (supernova) origins is an important question."

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