Cosmic Ruler Finally Explained

THE GIST:

• The "standard candles" used to measure cosmic distances are ignited by dead stars merging.
• Many scientists favored another scenario in which a dead white dwarf star steals from a sun-like star.
• The multi-million-year X-ray "fuses" that would accompany the stealing scenario was found lacking in six nearby galaxies.

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

An X-ray look at the universe's most common and useful sort of exploding stars -- so-called type 1a supernova -- in a half-dozen neighboring galaxies has finally revealed that these "standard candles" used to fix the distance of objects everywhere are caused -- unlikely as it seems -- 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: A merger of white dwarfs (loving dwarfs, if you will) or a white dwarf stealing material from a Sun-like companion star, and that material accumulates until the dwarf becomes unstable and explodes (a thieving dwarf, so to speak).

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

WATCH VIDEO: Discovery News unlocks the mysteries of stars and finds out why a star's age matters.

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"For almost three decades astrophysicists have been arguing about this," said Marat Gilfanov of the Max Planck Institute for Astrophysics in Garching, Germany. Gilfanov is the lead author of a paper in the Feb. 18 issue of the journal Nature reporting the first direct evidence of the former scenario 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."

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," Edmonds told Discovery News summing up the two supernova scenarios.

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 which should be expected pouring out of the galaxies if thieving dwarfs are behind them.

When Gilfanov and his colleagues searched the bulging center of the Andromeda galaxy and five older galaxies in our cosmic neighborhood with the Chandra X-ray space telescope, however, they found 30 to 50 times less X-ray activity than expected if thieving dwarfs were the cause of the supernovae there.

That suggests there's a lot more dwarf love out there than ever expected.

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," said Gilfanov. It's not a simple matter of saying all these supernovae are the same, so the dimmer they are, the further they are.

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

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