No two gamma-ray bursts are the same, as can be seen from this sample of light curves. (Larger image) (Credit: J.T. Bonnell (NASA/GSFC))|
Long or Short Duration?
Gamma-ray bursts are separated into two classes: long-duration bursts and short-duration bursts. Long duration ones last more than 2 seconds and short-duration ones last less than 2 seconds. However, this doesn't tell the whole story. That is because short duration bursts range from a few milliseconds to 2 seconds with an average duration time of about 0.3 seconds (300 milliseconds). The long-duration bursts last anywhere from 2 seconds to a few hundreds of seconds (several minutes) with an average duration time of about 30 seconds.
Astronomers think that long and short duration GRBs are created by fundamentally different physical properties. And whereas they now are fairly confident of what drives the long GRBs, there are only theories when it comes to what drives short-duration bursts. Here we will concern ourselves with long-duration bursts and address short-duration bursts later on.
GRBs: What Astronomers Now Know
Astronomers now know that long-duration gamma-ray bursts can originate
near the farthest edges of the observable Universe. The stars linked to them are typically on the order
of billions of light
years away. This means the light from them traveling at "the speed
of light" (about 186,000 miles per second or 300,000 kilometers per
second) took that many years to reach us. The Earth itself is about 4
billion years old, so some GRBs occurred when our planet was still a
fiery newborn, before the first microbes formed, even before the
oceans had formed. Some GRBs that we have observed actually originated
while the universe was only a few billion years old.
These stars are so far away that we don't actually see the light from
them before they explode. They belong to an early generation of stars
(e.g. maybe the second or third generation of stars) in the Universe. Although such stars long ago died, only now is the light from their explosive deaths reaching us.
That's not to say astronomers have no idea what kind of stars produce gamma-ray bursts. Working with large amounts of data collected over the past 15 years with special instruments aboard satellites, such as NASA's Compton Gamma-Ray Observatory and the joint Italian-Dutch BeppoSAX, and using computer simulations, astronomers have developed a working model of the kind of star that produces a GRB.
Next: A Collapse and then a Spectacular Explosion