A simulated snapshot of a Type Ia supernova simulation taken very shortly after the moment of detonation.

Credit: DOE NNSA ASC/Alliance Flash Center

AT LEAST ONCE A SECOND, a dim, elderly star somewhere in the cosmos turns into a colossal thermonuclear bomb. Briefly outshining their entire home galaxies, theses explosion, known as a Type Ia supernovae, unleash the equivalent of 1028 megatons of TNT; enough energy to destroy an entire solar system.

Astronomers have marvelled at these cosmic firecrackers for centuries. But, so far, nobody has explained in detail how they explode. Now, theorists are on the verge of understanding - and just in time, because astronomers are observing Type Ia supernovae with a new urgency. In fact, the story these stars have to tell is a matter of cosmic life and death.

Within the two broad categories of supernovae, Type Ia supernovae, which are caused by the explosion of white dwarf stars, are the only type that have a regular, known brightness. This makes them ideal 'standard candles' for measuring distances in the universe - the further they are away, the dimmer they appear from Earth.

When astronomer Robert Kirshner, now at Harvard University, in Boston, first began observing these cataclysmic explosions in 1972, it didn't matter that nobody understood how they happen. A lack of knowledge about the explosion process didn't stop Kirshner and his colleagues, along with another team, in 1998, using Type Ia supernovae to discover that a mysterious entity, later dubbed dark energy, is accelerating the expansion of the universe. But today, ignorance about Type Ia supernovae is no longer bliss, say Kirshner and other astronomers. Researchers now are not only relying on supernovae as distance markers to deduce the presence of dark energy, but also to unveil its character.

Dark energy is a strange substance that cosmologists predict makes up 70% of the substance of the universe and generates a repulsive force that is accelerating its expansion. One of the deepest mysteries in all of physics and astronomy, the nature of dark energy determines the fate of the universe. If its density across the universe increases over time, the cosmos will end in a Big Rip, with every atom torn asunder. If it somehow vanishes, cosmic expansion will continue but at a slower rate. And if its strength remains fixed in time, akin to the cosmological constant that Einstein inserted into his equations of general relativity, every galaxy will someday become its own island universe.

To determine whether dark energy varies or remains the same throughout time, astronomers need to measure its 'equation of state', defined as the ratio of its density to its pressure. And to measure the equation of state at different epochs in the universe, researchers urgently need more detailed information on Type Ia supernovae, says Don Lamb, a professor of astrophysics at the University of Chicago.

Theorists are beginning to crack the riddle of supernova explosions by borrowing some of the techniques - and computer models - applied to a surprisingly down-to-Earth system: combustion in gasoline engines. Thanks to these models, which require the processing power of supercomputers, researchers can now view the full three-dimensional evolution of a stellar explosion instead of a muted, one-dimensional facsimile.

On the computer screen, "it's like watching a fire consume a forest, you just see these flames working through the star, with all this structure to it," says theoretical astrophysicist Daniel Kasen of the University of California, Santa Cruz.