Computers `Detonate' Massive Stars in Quest To Copy Supernovas
LIKE auto mechanics trying to jump-start an engine, astrophysicists have huddled around computer terminals, trying to mimic the cataclysmic end of massive stars, with little success ... until now.
This week, two teams of researchers announced that they have simulated such events with remarkable regularity, yielding fresh insights into the most violent displays in nature since the Big Bang. ``This is a real breakthrough,'' says Alex Filippenko, an astronomer at the University of California at Berkeley, of the high-tech ``detonations'' being modeled on supercomputers. ''People have been trying to blow up stars for decades'' in an attempt to test notions of how supernovas explode.
The quest to understand these events is driven by the vital role supernovas play in the universe. The nuclear reactions that take place during a supernova supply the chemical elements heavier than helium - such as oxygen, iron, and carbon. They also stimulate star formation as their shock waves move through interstellar gas. They can even change the dynamics of a galaxy's evolution, says Willy Benz, an astronomy professor at the University of Arizona and a member of one of the two research teams. Yet the rarity of these events occurring close enough for regular study - an average of one supernova every 30 to 100 years in the Milky Way - leaves scientists with only supercomputers to test theories.
That changed in 1987, when a supernova appeared in the Large Magellanic Cloud, a satellite galaxy to the Milky Way. The supernova, only about 187,000 light-years away, came at a time when astronomers had an array of high-tech equipment that they could aim at it. What researchers saw suggested a more complex mechanism of explosion than models at the time contained.
When a star with at least 10 times the sun's mass exhausts its hydrogen fuel, it's fusion furnace begins burning layers of ever- heavier elements, which themselves are byproducts of earlier burning. Finally, all that remains is an iron core, which cannot sustain the fusion reaction; the furnace shuts down. In less than a second, gravity forces the core to collapse from roughly 4,000 miles in diameter to 20 miles, reaching the density of atomic nuclei. It can collapse no further than this ``maximum scrunch,'' so it suddenly bounces back, creating a shock wave.
Until now, many models held that the shock wave was responsible for blasting the star's material into space and that it radiated out in a symmetrical fashion. But observations of Supernova 1987A indicated that once the core collapsed, elements created in the star's final stages were more mixed than believed. In addition, computer simulations using this approach had to fine-tune stellar conditions to a degree not seen in nature. Without those adjustments, the shock wave would stall and the star would collapse into a black hole. Nor could the models supply the ``kick'' necessary to turn the remnant, a neutron star, into some of the spinning pulsars that have been observed.
This week at a meeting of the American Astronomical Society in Tucson, University of Arizona astrophysicist Adam Burrows and colleagues released the results of new simulations that solve these problems by introducing convection into the process. Dr. Benz and Marc Herant, an astrophysicist at Los Alamos National Laboratory, have developed similar models. Both consistently ``blow up'' stars that are 10 to 14 times as massive as the sun.
Dr. Filippenko says a next step would be to apply these models to stars with masses 40 or more times that of the sun to see what the models may tell us about the formation of black holes - objects so massive that light cannot escape their gravity.