View Full Version : Nearby GRB as depicted on TV

2007-Aug-08, 08:58 PM
Here's a hypothetical question, trivial perhaps but its been sticking in my brain.

Recently PBS Nova - (or was it the History Channel?) ran an episode about how the world could end and one of the more far-flung ways was the dreaded effect of a nearby (i.e. 1000 ly) Gamma Ray Burst aimed by chance at Earth.

In the show they illustrated the GRB as a Star Trek like phaser-beam moving through space, and smashing into Earth destroying the ozone layer and frying everybody on the side of our planet facing the source. This would be a bad thing, especially if one were just driving home after saving up to buy that new convertible.

This show brought to mind two observations.

1. (Minor) If a beam could be seen moving through space, then the beam itself would have to be some kind of sub-light-speed hot plasma emitting light, and the light naturally precedes it, (so we can see the beam moving as in Star Trek.) However, actual GRBs detected by satellites just happen and then astronomers rush to catch the afterglow. (The show's narration did say we'd have no warning, but a beam looks cool visually so they showed it that way.)

2. (My main question.) Suppose we were unlucky enough to be hit with a closer GRB but we were perhaps a little lucky in that the Moon just happened to have moved in front of the source just before it reached Earth. The burst lasts less than a minute by which time the Moon has covered the exploding star during the duration of the burst.

I ask this because the show described the appearance of a closer GRB in Earth's sky as enormously bright, rivaling or surpassing the Sun. Yet, a light source 1000 ly away would still be a point of light would it not? If it were technically a point even though tremendously bright, would it be completely covered or would the effect be like a solar eclipse, with the moons' shadow providing a temporary, though moving safe zone?

In a solar eclipse the sun happens to appear to be the same size in our sky as the moon so it is masked effectively and the small lunar umbra shadow reaches Earth. But a GRB behind the moon is still a point of light, so would the moon totally mask its effect?

2007-Aug-09, 01:03 AM
The star's great distance away means the lines between it and the Earth, or from it to any point on Earth, will be pretty close to parallel, so the fact that this image isn't to scale isn't an issue... unless the people of Habangu object to the use of their national flag...

Green circle: Earth

Blue circle: moon

Black fields: rays that miss (coming from far off at the right or left of this image)

Red fields: rays that hit us if they come from way off at the right because there's no moon blocking them there, or rays that are blocked if they come from way off at the left because they hit the moon, or (between the circles) the gamma shadow where no rays would be

Gray fields: rays that will hit no matter which side they come from; nothing's there on the right to stop them and the moon on the left just isn't big enough

So if the beam comes from the left, the moon protects a moon-sized spot on the Earth, but not the rest of it.

On the other hand, relabel the blue circle "Earth" and the green one "some big planet like Saturn", and you get something interesting... now the red AND gray fields to the left of the green circle are the shadow of a beam coming from the right, and the Earth fits inside the whole thing... it's just that this particular arrangement with the planets being in just the right places at the right time is substantially more unlikely to happen just when it's needed than a similar Earth-moon lineup is.

The Bad Astronomer
2007-Aug-09, 04:46 AM
The beam from a GRB is a cone that is at best 5 degrees wide (and probably wider after time). Even a GRB a light year away would have a beam front many billion of miles across. The entire Earth facing the burst would be consumed.

2007-Aug-09, 06:24 AM
You might see a beam from the sides since the radiation might be intense enough to cause interstellar gas to flouresce or lase and form a plasma. The afterglow they look for isn't the beam itself, it is the glow of the stuff that it hit dropping back to ground state.

2007-Aug-09, 06:16 PM
Thanks BA and all for the replies, (including the flag analogy). D'oh! I didn't think that much about beam spread. So hypothetically a "close" explosion even 1200 ly away would send a GRB that would easily engulf the solar system.

This would make a mess of us though he Sun probably wouldn't notice.
I realize Asteroid impact is likely higher on the list of dangers to Earth but just was wondering about GRBs as a point source in the sky.

2007-Aug-10, 08:33 PM
I seem to remember two specials--one on the ABC end of the world special and the one on The History Channel I think...