Should You Be Worried about Gamma-ray Bursts?

artist's concept of gamma-ray burster
Artist's illustration of a bright gamma-ray burst occurring in a star-forming region. Energy from the explosion is beamed into two narrow, oppositely directed jets. NASA

Of all the cosmic catastrophes that could affect our planet, an attack by radiation from a gamma-ray burst is certainly one of the most extreme. GRBs, as they're called, are powerful events that release huge amounts of gamma rays. These are among the most deadly radiation known. If a person happened to be near a gamma-ray producing object, they'd be fried in an instant. 

The good news is that Earth being blasted by a GRB is a pretty unlikely event.

That's because these bursts occur so far away that the chances of being harmed by one are quite small. Still, they are fascinating events that grab the attention of astronomers whenever they occur. 

What are Gamma-ray Bursts? 

Gamma-ray bursts are giant explosions in distant galaxies that send out swarms of powerfully energetic gamma rays. Stars, supernovae and other objects in space radiate away their energy in various forms of light, including visible light, x-rays, gamma-rays, radio waves, and neutrinos, to name a few. Gamma-ray bursts focus their energy onto a specific wavelength. As a result, they are some of the most powerful events in the universe, and the explosions that create them are quite bright in visible light, too.

The Anatomy of a Gamma-ray Burst

What causes GRBs? Astronomers now know it takes something very weird and massive to create one of these outbursts. They can occur when two highly magnetized objects, like black holes or neutron stars collide, their magnetic fields join together.

That action creates huge jets that focus energetic particles and photons streaming out from the collision. The jets extend across many light-years of space. Think of them like Star Trek-like phaser bursts, only a lot more powerful and reaching out on an almost cosmic scale. 

The energy of a gamma-ray burst is focused along a narrow beam.

Astronomers say it is "collimated". When a supermassive star collapses, it can create a long-duration burst. The collision of two black holes or neutron stars creates short-duration bursts. Oddly enough, short-duration bursts may be less collimated or, in some cases, not highly focused at all. Astronomers are still working to figure out why this might be. 

Why We See GRBs 

Collimating the energy of the blast means that a lot of it gets focused into a narrow beam. If Earth happens to be along the line of sight of the focused blast, instruments detect the GRB right away. It actually produces a bright blast of visible light, too.  A long-duration GRB (which lasts more than two seconds) can produce (and focus) the same amount of energy that would be created if 0.05% of the Sun were instantaneously turned into energy. Now, that's a huge blast!

Understanding the immensity of that kind of energy is difficult. But, when that much energy is beamed directly from halfway across the universe, it can be visible to the naked eye here on Earth. Luckily, most GRBs are not that close to us.

How Often do Gamma-ray Bursts Occur?

In general, astronomers detect about one burst a day. However, they only detect those that beam their radiation in the general direction of Earth.

So, astronomers are likely seeing only a small percentage of the total numbers of GRBs that occur in the universe.

That raises questions about how GRBs (and the objects that cause them) are distributed in space. They heavily rely on the density of star-forming regions, as well as the age of the galaxy involved (and perhaps other factors as well). While most seem to occur in distant galaxies, they could happen in nearby galaxies, or even in our own. GRBs in the Milky Way seem to be fairly rare, however.

Could A Gamma-ray Burst Effect Life on Earth?

Current estimates are that a gamma-ray burst will happen in our galaxy, or in a nearby galaxy, about once every five million years. However, it's pretty likely that the radiation would not have an impact on Earth. It has to happen pretty close to us for it to have an effect.

It all depends on the beaming. Even objects very close to a gamma-ray burst can be unaffected if they're not in the beam path. However, if an object is in the path, the results can be devastating. There is evidence that suggests that a somewhat nearby GRB could have occurred about 450 million years ago, which might have led to a mass extinction. However, the evidence for this is still sketchy.

Standing in the Way of the Beam

A gamma-ray burst, beamed directly at Earth, is pretty unlikely. However, if one did occur, the amount of damage would depend on how close the burst is. Assuming one occurs in the Milky Way galaxy, but very far away from our solar system, things might not be too bad. If it happens relatively nearby, then it depends on how much of the beam Earth intersects.

With the gamma-rays beamed directly at Earth, the radiation would destroy a significant portion of our atmosphere, specifically the ozone layer. The photons streaming from the burst would cause chemical reactions leading to photochemical smog. This would further deplete our protection from cosmic rays. Then there are the lethal doses of radiation that surface life would experience. The end result would be mass extinctions of most species of life on our planet.

Luckily, the statistical probability of such an event is low. Earth seems to be in a region of the galaxy where supermassive stars are rare, and binary compact object systems aren't dangerously close. Even if a GRB happened in our galaxy, the likelihood that it would be aimed right at us is pretty rare.

So, while GRBs are some of the most powerful events in the universe, with the power to devastate life on any planets in its path, we are generally very safe.

 

Edited and updated by Carolyn Collins Petersen.