In cosmology, a special type of supernovae, known as a standard candle, is used to help determine distances on cosmological length scales. These standard candles were what revealed that the expansion of the universe is accelerating. The inquisitive mind might start asking, "well what makes these 'standard'"? Standard candles are type Ia supernovae, white dwarfs that have reached masses near the Chandrasekhar limit (about 1.4 times the mass of our sun). There are a couple of ways for a white dwarf to reach this mass, but a new study indicates that the most common is also the most violent: a collision between two white dwarfs.

Standard candles play an extremely important role in astronomy due to the challenge of estimating distance. Determining how far an object is from you often requires comparison against a second object of known distance. It's easy to estimate how far my TV is from me when I know that my coffee table is about 18 inches from my sofa. However, when you have nothing absolute to compare against—a dot on a completely white field—figuring out the distance to an object is much more difficult. Even determining if something is a mile wide and ten miles away or ten thousand miles wide but a million miles away can be daunting when you have nothing to compare against.

Standard candles provide a frame of reference for distance. Because all type Ia supernovae are the product of the same process in similar progenitor stars, they provide a frame of reference for nearby objects. Since we know what to expect from such an explosion, we can determine the distance from Earth that it occurs given its peak luminosity. The key to the similarity is the relationship between the Chandrasekhar limit and the supernova process.

White dwarfs are stable at masses up to the limit. As they near this mass, however, the pressures and temperatures in the core of the star are hot enough to ignite uncontrolled carbon and oxygen fusion reactions that blow the star apart.

A white dwarf cannot exist at masses above the Chandrasekhar mass, as the electron degeneracy pressure is no longer powerful enough to push against the massive gravity, so a neutron star or black hole results. Obviously, that means that the white dwarf must approach the Chandrasekhar limit from below. Historically, there have been two main hypotheses for how this happens: either the massive white dwarf is the result of two smaller white dwarfs merging, or the white dwarf siphons matter off a nearby stellar companion.

This week's issue of Nature contains observational results that conclusively favor to one mechanism over another. The key lies in the X-ray emission signatures of the two scenarios. If two white dwarf stars merge, the process doesn't produce many X-rays until right before the thermonuclear blast. In contrast, the accretion scenario should produce a significant amount of X-rays for 10 million years preceding the supernova event.

Using archival data from Chandra (X-ray), Spitzer and 2MASS (near-infrared) observatories, researchers from the Max Planck Institute examined the X-ray flux from a half dozen nearby galaxies and the galactic bulge of Andromeda. They found X-ray emissions were nearly 30 to 50 times less than what would be predicted if the accretion scenario were common. This lead the authors to conclude that the process of drawing matter from companion stars accounts for no more than five percent of the observed type Ia supernovae in early-type galaxies.

Note the caveat at the end there: this result holds true for young galaxies. For the accretion scenario to work, the companion star that is providing mass would need to be considerably larger than the white dwarf, something that would not be common in young galaxies such as those included in the survey. In older galaxies, these massive companions may be more prevalent, so the alternate accretion scenario may be more plausible. With this work, however, astronomers finally seem to be closer to having a standard mechanism that is capable of explaining the cosmologically important standard candles.

Nature, 2010. DOI: 10.1038/nature08685

Listing image by NASA/CXC/MPA/M.Gilfanov & A.Bogdan