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Illustration of GRB supernova.
A computer animation of a gamma-ray burst / supernova viewed from a distance. (3 MB QT)

(Credit: NASA / SkyWorks Digital)

A Collapse and then a Spectacular Explosion

The theory describing how gamma-ray bursts originate is called the "collapsar" model. Dr. Stan Woosley of the University of California, Santa Cruz, and one of the architects of the model, coined this term because the model involves the collapse of the core of a special kind of star. This core collapse occurs while the outer layers of the star explode in an especially energetic supernova dubbed a "hypernova" by astronomers. (Here we'll refer to the theory as the "collapsar/hypernova" model to keep in mind both the core collapse and the supernova explosion.)

In looking for the stellar candidates capable of producing a hypernova, astronomers are confronted with the fact that gamma-ray bursts are so far away not even the most powerful telescopes can see the stars thought to be responsible for those observed so far.

But proponents of the collapsar/hypernova model think they have an idea. The kind of star is very heavy, very hot, and prone to episodic fits in which large amounts of material is ejected from it. Such a star is called a "Wolf-Rayet" star after two 19th Century French astronomers, Charles Wolf and Georges Rayet, who studied the first example.

Dissecting an Explosion

Wolf-Rayet stars are linked to hypernovae, which in turn are associated with gamma-ray bursts.

Although the exact picture has not been worked out, astronomers think the gamma-ray photons are probably produced inside the star. The explosion originates at the center of these massive stars. While a black hole forms from the collapsing core, this explosion sends a blast wave moving through the star at speeds close to the speed of light. The gamma rays are created when the blast wave collides with stellar material still inside the star. These gamma rays burst out from the star's surface just ahead of the blast wave. Behind the gamma rays, the blast wave pushes the stellar material outward.

A black hole forms inside surrounded by a disk of accreting matter
The initial stage of a gamma-ray burst. The core of the star has collapsed deep inside the star. A black hole has formed within the star, and within a few seconds launches a jet of matter away. (Larger image)
(Credit: NASA / SkyWorks Digital)
Erupting through the star surface, the blast wave of stellar material sweeps through space at nearly the speed of light, colliding with intervening gas and dust, producing additional emission of photons. These emissions are believed responsible for the "afterglow" of progressively less energetic photons, starting with X rays and then visible light and radio waves. (Whether additional gamma rays are also produced in this "afterglow" phase is still not settled, although some evidence indicates they are.) The afterglow phase can last for days or even weeks. Under the collapsar model, we detect both the GRB and the afterglow when the Earth happens to lie along or very near the axis of the blast. In general, there are many more GRBs than are detected simply because we are not favorably aligned to see them.

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