Gamma-ray jets produced deep within massive stars can blow apart the star when they emerge, creating a supernova. The jets are very light and travel near the speed of light toward the star's surface. They are created by a complex interaction of a black hole, an accretion disk, and very strong magnetic fields that come into being when a massive star depletes its supply of hydrogen fuel and falls into itself. When a highly magnetic black hole forms in the center of a star, it is surrounded by an accretion disk made of stellar material. The black hole typically produces two jets that travel away from it in opposite directions toward the poles of the star. The jets travel through star's outer layers to reach the surface of the star.
Postdoc Davide Lazzati and Fellow Mitch Begelman recently figured out how the jets travel through huge amounts of dense stellar material without being slowed down. They determined that as the jets propagate through the star, they create shock waves ahead of them that push the star aside. At the same time, however, pressure from the star makes the jets narrower. In other words, as the jets try to open up the star, the star tries to compress the jets from the side.
Initially, the star appears to win the contest, narrowing the jets to the width of a pencil beam. The fast, narrow jets heat both the star and their own heads, creating a shock wave that gets partially deflected back to create hot, high-pressure cocoons around the jets. As they reach the surface, the jets open holes for the cocoons to be released and expand, as shown in the figure on the right. Lazzati says it's like popping the cork out of a champagne bottle. The jet expansion then propagates back toward the black hole, blowing up the star. Depending on conditions in a particular star, the emergence of the jets and the supernova can be simultaneous, or the jets can create the explosion. In the end, though, the jets win.
An article describing this research appeared online in April of 2005 and in the Astrophysical Journal in June. - Julie Phillips