WHAT does it take to make a star go bang? The answer may be asymmetry, according to a new computer model of how supernovae occur. The model may have dramatic implications for astronomers who use supernova observations to measure the expansion of the universe.
Alan Calder and colleagues at the University of Chicago modelled what happens to a white dwarf 鈥� a type of dense, dim star 鈥� at the end of its life. Although other groups have tried this before, no one has ever been able to reproduce exactly what astronomers see during the type 1A supernovae that these stars form. For example, they can鈥檛 account for the asymmetric clouds of gas that are sometimes left behind after the star explodes.
Astronomers think white dwarfs go through two phases during their death throes. First they expand, and then they explode. In most models, Calder says, the explosion has to be triggered artificially, 鈥渕uch like putting a second match to it鈥�. But in his team鈥檚 model, described in a pair of papers submitted to Astrophysical Journal Letters, the explosion happens naturally.
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The supernova starts when a small region near the centre of the star erupts spontaneously, creating a huge hot bubble that makes the star swell (see Graphic). The bubble emerges on one side of the star at almost 8000 kilometres per second and this sends a shock wave of material spilling over the surface. Where this converges on the opposite side of the star, the surface is compressed so sharply that it triggers a nuclear reaction. This sets off the supernova, Calder suggests.
Astronomers look for supernovae in distant galaxies to help determine how far away they are. This technique underpins much of cosmology, including the notion that dark energy is driving the expansion of the universe. But crucially it relies on the idea that one exploding white dwarf should look just like another. 鈥淯nderstanding these explosions is important to make sure that we can use supernovae as tools to probe the universe,鈥� comments Craig Wheeler, an astrophysicist at the University of Texas at Austin.
Most models assume the explosions are symmetric to make the calculations easier to cope with. But Calder鈥檚 model, run on a supercomputer, suggests the mechanism is intrinsically asymmetric. Calder and his team plan to extend their model to see what effect this has on the shape of the explosion. If it means the supernovae look different from different directions, astronomers may have to rethink how they use their observations.