Type Ia supernovae leaped into astronomical fame in 1998 when they helped astronomers discover the accelerating universe and suggested the presence of a mysterious anti-gravity force known as dark energy.
Yet even though Type Ia's continue to be astronomy's best cosmological rulers, astronomers are still hotly debating the cause of these powerful explosions, a question that becomes ever more important in this age of precision cosmology.
The more familiar kind of supernova (either Type Ib, Type Ic, or Type II) is pretty well understood. When a massive star runs out of fuel, its core collapses to form a neutron star or black hole. The outer layers ricochet off the collapsing core in a spectacular explosion. But a Type Ia supernova, astronomers agree, is an altogether different beast — a white dwarf that becomes a thermonuclear bomb, leaving nothing behind but wisps of gas.
So what ignites the bomb? Astronomers have argued over the answer for some 40-odd years. One camp holds that white dwarfs explode when they collect too much gas from a "normal" companion star. The extra mass tips the white dwarf past the Chandrasekhar limit, the point at which the pressure between electrons can no longer support the dwarf's weight. The dwarf starts to collapse in on itself, setting off the runaway nuclear fusion reactions that make it explode.
A single exploding white dwarf used to be the textbook answer to Type Ia supernovae, and evidence suggests that, at least in some cases, it's true. But another contentious study, which looked for the X-ray emission expected from white dwarfs accreting matter, found much less than expected.
Meanwhile, another theory is gaining traction. This theory is based on binary systems, where two white dwarfs revolve around each other in an ever-shrinking orbit. When they merge, kaboom.
To see if this theory could be a valid contender in the Type Ia debate, Carlos Badenes (University of Pittsburgh) and Dan Maoz (Tel Aviv University) rifled through the Sloan Digital Sky Survey, a massive archive of images and spectra, to find 4,000 white dwarfs floating in the Milky Way Galaxy. They measured each white dwarf's motion by measuring the Doppler effect in the archived spectra. Fifteen white dwarfs whirled around an unseen companion faster than 250 km/second (560,000 mph), implying a tight orbit around another white dwarf. Such a fast, close pair is destined to spiral together.
Using their observed numbers, Badenes and Maoz compared the rate of such white dwarf mergers expected in our galaxy to the rate of Type Ia supernovae in Milky Way-type galaxies, only to find that the rates are almost exactly the same: on average, one white-dwarf pair merges, and one Type Ia supernova explodes, in each galaxy every century. This doesn't prove that white dwarf mergers produce Type Ia supernovae, but it proves that they exist in sufficient numbers.
"We don't theorize about the existence of white-dwarf pairs, we observe them," Badenes says. "We have to apply models to interpret their velocities in terms of a merger rate, but the 15 short-period binaries in our sample are real. Those 15 systems unavoidably lead to a high merger rate."
The models used by Badenes and Maoz contain significant uncertainties, but the results still represent progress in the field, says Andrew Howell (University of California, Santa Barbara), a supernova expert not involved in the study.
If the study holds up, however, a question remains. Most of the white-dwarf pairs that Badenes and Maoz found add up to less than the Chandrasekhar limit: less than about 1.4 times the mass of our Sun. Astronomers have a handle on what happens in a Type Ia supernova if a white dwarf exceeds the Chandrasekhar limit. But if less massive white-dwarf mergers cause Type Ia supernovae, then "what happens during the merger is very much an open question," says Badenes. "From a modeling point of view, it’s a very messy problem." But that's something they'll leave to the theorists.