Stars go kaboom, spilling cosmic secrets
Astronomers hope type 1a supernovas will help in quest to explain dark energy
By Ron Cowen
At least once a second, a dim, elderly star somewhere in the cosmos turns into a thermonuclear bomb. Briefly outshining its home galaxy, the explosion, known as a type 1a supernova, unleashes the equivalent of 1028 megatons of TNT — enough energy to destroy an entire solar system.
Astronomers have marveled at these cosmic firecrackers for centuries. But so far nobody has explained in detail how these supernovas explode. Now, theorists are on the verge of attaining that understanding — and just in time, because astronomers are observing type 1a supernovas with a new urgency. In fact, the story these stars have to tell is a matter of cosmic life and death.
When astronomer Robert Kirshner, now at Harvard University, first began observing these cataclysmic explosions in 1972, it didn’t matter that no one understood how they happen. A lack of knowledge about the explosion process didn’t stop Kirshner and his colleagues, along with another team, from using type 1a supernovas to discover in 1998 that a mysterious entity, later dubbed dark energy, is accelerating the expansion of the universe (SN: 2/2/08, p. 74). But today, ignorance about type 1a supernovas is no longer bliss, say Kirshner and other astronomers. Researchers now are not only relying on supernovas as distance markers to deduce the presence of dark energy, but also to unveil its character.
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 1a supernovas, says Don Lamb of the University of Chicago.
Theorists are beginning to crack the riddle of supernova explosions by borrowing some of the techniques — and computer codes — applied to a surprisingly down-to-Earth system: combustion in gasoline engines. Thanks to these codes, 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.
Simulations developed by supernova expert Stan Woosley, also of UC Santa Cruz, along with Kasen, Fritz Röpke of the Max Planck Institute for Astrophysics in Garching, Germany, and others now suggest that supernovas that erupted a few billion years back in time may be different — intrinsically brighter — than those exploding today. The team has begun to identify several other features that may affect supernova brightness — such as how rapidly a star rotated before it exploded and its abundance of elements heavier than helium — which might confound dark energy measurements if overlooked.
“We’re starting to make meaningful comments about how useful these supernovae can be for precision cosmology,” Woosley says.
Exploding stellar probes
Astronomers rely on type 1a super-novas to probe the expansion history of the universe because these explosions are almost perfect cosmic mile markers.
Since all 1a’s appear to have the same starting point — blowing up the same amount of mass —they all should have roughly the same luminosity. After adjusting for variations by applying the Phillips relation, which holds that intrinsically brighter supernovas take more time to fade than dimmer ones, researchers can, in principle, read off the wattage of these cosmic lightbulbs. Just as the apparent brightness of a 60-watt bulb predictably diminishes with distance, so too should the observed brightness of a supernova.
When astronomers applied this prescription, they found that light from distant supernovas appeared dimmer than it ought to be based on what had been the accepted model of the universe’s evolution. That unexpected result led in 1998 to an astonishing conclusion: Rather than slowing down, the cosmos has recently sped up its rate of expansion, putting extra distance between nearby and remote supernovas — and the galaxies in which they originated.
Now, astronomers want to know the inherent brightness of type 1a supernovas to within a few percent, rather than the previous error margin of 20 percent — and how that brightness varies among different populations. Suppose, for example, that supernovas containing a lower abundance of heavy elements — typical of stars earlier in the history of the universe — areon average intrinsically brighter than supernovas exploding today. The Phillips relation says that the supernovas with fewer metals should remain bright for a longer period of time than others. Indeed, models suggest that such cosmic bulbs would last longer than younger supernovas, but not quite as long as the relation predicts, Woosley and collaborators now find. This effect cannot be ignored if researchers want to use type 1a’s to measure distances to an accuracy of 1 or 2 percent, which will be required to assess whether or not dark energy varies with time, Kasen says.
If type 1a supernovas vary in brightness according to a random statistical distribution, with some explosions brighter and some dimmer than average, simply observing many more of them will beat down the error in using them as standard lightbulbs, Kasen says. But if some type 1a’s, such as distant ones, are systematically different from others, as his team now suggests, a problem emerges.
If such properties aren’t accounted for, “our errors would be greater than we really believe” in using type 1a supernovas to measure the expansion of the universe and the nature of dark energy, says Mike Zingale of Stony Brook University in New York.
Road to kaboom