Astro-Lesson: Gamma Ray Bursts.

First: Good to see that I got some votes out of you all and looks like Gamma-Ray Bursts just barely won. I'll have a new poll up tomorrow I suppose for next weeks Astro-Lesson.

Second: I decided to start calling these things Astro-Lessons because they are little lessons in astronomy that are just supposed to help build a pretty basic understanding of the Universe we live in.

Now onto the lesson:

NASA concept art of a gamma-ray burst

Gamma-Ray Bursts, also known as GRBs, are short-lived bursts of gamma ray photons associated with immense explosions that have been observed in distant galaxies. These are the most luminous electromagnetic events known. They typically last only seconds, but can be milliseconds quick, or even as slow as several minutes. After the burst, there is typically an "afterglow" in longer wavelength (X-ray, ultraviolet, optical, infrared, and radio).

Gamma rays, if you're unfamiliar with them, are a form of electromagnetic radiation at high frequency (very short wavelength) produced by subatomic interactions, such as radioactive decay, fusion, and fission. Gamma rays are a health hazard since they are a form of ionizing radiation, which is what makes GRBs scary.

NOT LIKE THAT...

Imagine, if you will, a microwave so powerful that it would cleanse the entire galaxy of life. It would literally fry everything. That would be the power of a gamma-ray burst in our galaxy.

GRBs are caused by a special type of supernovae, hundred of times brighter then typical ones (astronomers can pick out supernovae in distant galaxies) and a million trillion times brighter then the Sun. There aren't a very common event though, GRBs are detected roughly ONCE PER DAY in any random direction of the sky.

Until recently, astronomers knew very little about GRBs. In fact, their discovery was a bit accidental. During the Cold War in the 1960s, U.S. military satellites were watching for Soviet nuclear weapons testing in violation of the test ban treaty. The satellites carried gamma ray detectors since a nuclear explosion produces gamma rays. But they began noticing these huge gamma ray bursts coming from deep space. These bursts remained a mystery up into the early 90s. There were no indications as to how far away the GRBs were, if they originated at the edge of our solar system, or in the Milky Way, or further away.

Hubble catches a GRB in action.

A combination of satellite observations with ground-based follow-up observations and theoretical work began to unveil one of the biggest mysteries in modern astronomy. It turns out that GRBs occur incredibly far away, near the edge of the observable Universe in distant galaxies. (Though one of my favorite explanations was that GRBs were massive nuclear weapons being used in a galactic war.)

As astronomers began to learn more about GRBs, they began to notice differences in individual events. These differences were in the length of the event, in which there were two classifications: long-duration (longer than 2 seconds) and short-duration (less than 2 seconds). The short-duration ones can last for a few milliseconds though, and average about 0.3 seconds (300 milliseconds). Long-duration bursts can last up to several minutes and average around 30 seconds.

Infographic describing theories on both long and short GRBs.

It is believed that entirely different physical properties cause long and short duration GRBs. The long ones are the ones that astronomers feel confident in their knowledge of. The short GRBs are only theoretically described and remain a mystery. So there isn't much else to say about short-duration GRBs, they exist and their are missions to study them, but the data is so fleeting, it is hard to pinpoint how far away they are or what causes them.

For the long-duration GRBs there is a good amount known. In the 1990s is when astronomer's discovered the "afterglow" which allowed the origin of the GRB to be pinpointed. This "afterglow" pointed to galaxies at immense distances, Billions of light years away. Some of these GRBs first occurred before the Earth, being only 4 billion years old, even existed! The most recent likely happened when the Earth was young, perhaps before the first microbes formed.

The faint smudge of a galaxy in the center of this Hubble image is where a GRB exploded in 1997.

In tracing GRBs back to their origin, astronomers began to study the galaxies that were the source of these explosions. Unfortunately there is no definite answer for what causes a GRB, but they are associated with a special sort of supernova seen in these distant galaxies, dubbed hypernova for their immense size and magnitude. The "smoking gun" that linked the two happened in March 2003 when the afterglow from a GRB perfectly matched the optical spectrum of a supernova in that galaxy.

What causes the supernova and the GRB is the mystery though. It would require an incredibly massive amount of energy, either from a incredibly massive star or a black hole or neutron star. Some of the theories suggest neutron star collisions while others suggest the collapse of massive stars.

But even though gamma-ray bursts seem like scary unpredictable events, we got two things going for us. One, these things have all happened very, very far away where they cannot hurt us, although the closest ones have been known to cause satellite interference. And two, there is no indication that these burst are happening in closer, more modern galaxies.

If GRB's were still common in modern galaxies as we know them, they would occur a lot closer and more often. So there is some sort of point where these events must die off. The idea is kind of that star formation hit a certain limit, perhaps the right mixture of elements and the right temperatures are just not available anymore to create the stars responsible for GRBs. So, we are likely safe from any threat of annihilation by these incredibly violent events.

Further Reading: 

Wikipedia entry on gamma-ray bursts

NASA's website on gamma-ray bursts

UC Berkeley's website on gamma-ray bursts 

Addendum: I did some more research (actually I watched the 3rd episode of Phil Plait's Bad Universe on Discovery Channel, which coincidentally happened to premiere yesterday [I watched it after I wrote this post]) and it turns out the safe distance things is a bit iffy. We seem pretty safe from gamma-ray bursts, but there is a very extremely small chance that we might get hit, there is no real good way to know. But there is evidence that an extinction event that happened in the past may have been caused by a GRB. I don't know that it's a definitive theory, but it is a plausible one with reactions of the atmosphere and massive extinction.

Posted in Astro-Lesson,Astronomy,Galaxies,Gamma Ray Bursts,Supernova,Universe 22 comments