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A Hypernova: The Super-charged Supernova and its link to Gamma-Ray Bursts

Supernovae mark the explosive deaths of particularly massive stars. During the few weeks it lasts, a supernova can outshine all the other stars in a galaxy combined. As if this wasn't powerful enough, there is a rare type of super-charged supernova called a hypernova.

With a power about 100 times that of the already astonishingly powerful "typical" supernova, the hypernova has also been linked to the phenomenon of gamma-ray bursts (GRBs), quick bursts of gamma ray photons, the most energetic form of light.

GRBs are believed to occur when a blast wave of material is produced inside a collapsing star about to undergo a hypernova explosion. Both the gamma rays and the blast wave of stellar material erupt from the star -- the gamma ray photons necessarily at the speed of light, the stellar material just a little slower -- along a particular path (axis).

Traveling outward in an expanding cone, the stellar material collides with gas and dust in the interstellar medium, exciting new emissions of photons but at gradually decreasing energies in what is called the "afterglow." This includes X rays, ultraviolet light, visible light, microwaves, and radio waves. The burst and its afterglow are detected on Earth only if the Earth happens to be aligned along or near the blast axis.

While there is, on average, only one supernova per galaxy per century, there is something on the order of 100 billion galaxies in the observable Universe (which refers to the part of the Universe light has had time to reach us). Taking 10 billion years for the age of the Universe (it's actually 13.7 billion, but stars didn't form for the first few hundred million and it's just an estimate anyway), Dr. Richard Mushotzky of the NASA Goddard Space Flight Center, derived a figure of 1 billion supernovae per year. That comes to about 30 supernovae per second in the observable Universe!

Linking the hypernova to the collapsing stellar core to describe the GRB is the "hypernova/collapsar model," concrete proof of which came in early 2003. It was made possible in part to a fortuitously "nearby" burst whose location was distributed to astronomers by the Gamma-ray Burst Coordinates Network (GCN). On March 29, 2003, a burst (dubbed GRB 030329 by the standard naming convention) went off close enough that the follow-up observations were decisive in solving the gamma-ray burst mystery. The optical spectrum of the afterglow was nearly identical to that of supernova SN1998bw. In addition, observations from x-ray satellites showed the same characteristic signature of "shocked" and "heated" oxygen that's also present in supernovae. Thus, astronomers were able to determine the "afterglow" light of a relatively close gamma-ray burst (located "just" 2 billion light years away) resembled a supernova.

It isn't known if every hypernova is associated with a GRB. However, astronomers estimate only about one out of 100,000 supernovae produce a hypernova. This works out to about one gamma-ray burst per day, which is in fact what is observed.

What is almost certain is that the core of the star involved in a given hypernova is massive enough to collapse into a black hole (rather than a neutron star). So every GRB detected is also the "birth cry" of a new black hole.

* Return to "GRBs: A Collapse and the a Spectacular Explosion".

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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