Dr. Boaz Katz: A New Model for Stellar Explosions

A threesome is a potentially explosive arrangement. That may be as true for the stars as it is for people: Dr. Boaz Katz believes that a three-body model could explain many of the huge stellar explosions known as supernovae. In this model, which he and his colleagues Drs. Subo Dong and Doron Kushnir developed during his postdoctoral research at the Institute for Advanced Study, Princeton, the disruptive third star sends its two companions hurtling into one another for a head-on collision. Katz hopes to uncover more evidence for this model of supernova formation through research with his new group in the Weizmann Institute’s Particle Physics and Astrophysics Department. 

Katz says he and Dong came to the idea while studying something entirely different: exoplanets. In the last decade, astronomers have detected thousands of planets orbiting stars outside of our solar system; a few of these make up solar systems much wilder than ours. Rather than swinging around their sun in orderly circular paths, all in the same plane; these distant planets have been recorded circling in unstable, elongated orbits in planes that lie at skewed angles to one another. Katz and Dong were investigating the origins of this disorderly behavior, especially theories that these behaviors arise from the gravitational effects of other planets or a companion star in the system. 

These wild orbits often put stars and their planets on collision courses, and Katz and Dong began to wonder whether stars might similarly collide – and what might happen if they did. Could certain stars, in certain circumstances, run into one another head-on with enough energy to ignite the nuclear dynamos at their cores?  

Fundamental questions concerning explosions

Katz and Dong then approached Doron Kushnir, who was also at the Institute for Advanced Study. Kushnir had been working on fundamental questions concerning the explosion mechanisms of supernovae, and he was one of only a few scientists in the world who had the tools to calculate whether such collisions would actually end in giant blasts. 

Most observed supernovae are of the type known as 1a, and the fact that these all look alike suggests they occur in a similar way. Astrophysicists generally agree that the stars that explode to become type 1a supernova are white dwarfs – old stars that are about the circumference of the Earth and the mass of the sun, making them a million times denser than water or the Sun. It was thought, however, that white dwarfs blow up only when they become “overweight,” either from vacuuming up material from a larger companion or merging with another white dwarf. These scenarios turn out to be very difficult to analyze, and neither one has been demonstrated to lead, incontrovertibly, to an explosion. Katz and his colleagues think that the weight limit is something of a red herring; they propose that the explosion is caused by a direct collision between two white dwarfs. The calculations, at least, bear out this idea. Like the exoplanets careening in wild orbits, these two would need a third companion star to create instability in their relationship.