What is a Gamma Ray Burst?

Simply speaking, a gamma ray burst (known in the trade as a 'GRB') is a burst of gamma rays; that is, a relatively short-lived blast of intense gamma radiation*. There is nothing in the universe more powerful than a GRB. There are two types of gamma ray burst, with differing causes:

Long duration gamma ray bursts
Some GRB host galaxy shots from Hubble
These are bursts that last in the region of 20-40 seconds, but contain an energy equivalent to that put out by our Sun over its entire lifetime. These are thought to originate from the high-energy event that is a hypernova (like a supernova but much more violent).

Short duration gamma ray bursts
These are bursts that last up to two seconds and contain significantly less energy than the long-duration bursts (although they still contain a lot of energy compared to other events). These are thought to be produced by collisions of neutron stars with each other, or with black holes.

That sounds dangerous...

Without trying to be alarmist, it is. Even when a gamma ray burst happens as much as 10 billion light years away and is pointing in our direction, it's a pretty noticeable event for anything looking in the right direction with the right kind of telescope. All of the gamma ray bursts detected so far have originated outside of the Milky Way, and have been harmless to Earth.

If a gamma ray burst originated in our own galaxy, however, and were to be shot in our direction, it'd be very bad news indeed: for a gamma ray burst originating about 50,000 light years away (that's about half the width of the Milky Way), most of the life on the side of the planet facing the burst at the time would probably be wiped out fairly quickly, and the rest would have their life span significantly shortened by the effects of radiation damage. Added to this, the burst would cause significant ozone depletion in the Earth's atmosphere: the ozone layer is one of our most important shields against solar radiation.

Around 450 million years ago there was an extinction event** (known as the Ordovician-Silurian extinction event), and it is thought that this was caused by a gamma ray burst: it's possible that this has happened before, and it may happen again. And if it does, there's not a lot we could do about it.

There's some good news though, right?

Yeah; there's always good news! In this case, the best news is that a gamma ray burst doesn't blast out from a hypernova in all directions. Instead, for complicated reasons, a burst is directed out in two relatively narrow beams along the axis of rotation*** of whatever it is that's producing the GRB. This means that it's fairly unlikely that we'll be lying directly in the path of any given GRB, even if it originates close by.

The other good news is that GRBs themselves are pretty rare: on average, it seems that there are only a handful of GRBs expected in any particular galaxy in any particular million-year period.

So don't lose any sleep over it!

If, however, you'd like to lose some sleep, you could try this post which talks about various ways in which the human race could find itself being killed off. Don't have nightmares!

* Gamma rays are a form of radiation, like x-rays, infrared radiation, microwaves and even visible light. These forms of radiation differ from each other in terms of their wavelength and frequency- gamma radiation has a very short wavelength and correspondingly high frequency.
** An 'extinction event' is when a significant number of Earth's species become extinct in a relatively short period of time.
*** That is, it's blasting out from the poles.


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