Above: a gamma ray burst (GRB) (rendered in Pov-Ray; the background nebula and stars were generated
with Galaxy.inc, see acknowledgments).
Gamma ray bursts (GRBs) are the most luminous explosions seen in the known Universe and are the most
energetic explosions known since the Big Bang! Gamma rays are highly energetic electromagnetic waves.
The electromagnetic (EM) spectrum includes, going from most to least energetic: gamma-rays, X-rays,
Ultraviolet, visible (optical) light, infrared, radio waves, microwaves. The hotter the object, the more intense is
the bulk of the EM radiation that it emits. A human body emits infrared (invisible to unaided human vision, but
visible in night-scopes), a relatively cool star might emit red visible light, a hotter star blue light or even
ultraviolet. To emit gamma-rays in quantity requires something very energetic or very hot! EM radiation is
loosely referred to as light (though strictly only visible EM radiation is light). All EM radiation consists of waves
of electrical and magnetic energy and can be loosely said to comprise particles called photons. The light that
the human eye detects is simply electromagnetic waves in a certain energy range.
Gamma-ray bursts are seen first as a blast of gamma-rays (lasting from milliseconds to several minutes)
followed by an afterglow of less energetic EM radiation (including visible light). If these explosions
emitted energy equally in all directions, then judging by their observed luminosity and tremendous distance,
they would be truly enormous, converting the mass of the Sun into gamma-rays in a matter of seconds!
However, it appears that these explosions are 'collimated' meaning that most of the energy is beamed away
as two bipolar jets, focusing the energy into a small area. Only when one of these jets points directly toward
us do we observe the GRB. Even so, they are still more energetic than the average supernova!
What causes GRBs is still uncertain, but there appear to be three likely causes:
1. A hypernova (collapsar) - the highly energetic supernova explosion of a Wolf-Rayet star. Wolf-Rayet
stars are the old versions of blue hypergiants that have blasted off their outer hydrogen envelope. These
massive stars (which begin with at least 40 times the Sun's mass when on the Main Sequence) are so hot
and luminous that the pressure generated by the sheer intensity of their light blasts off the outer layers,
leaving the helium rich inner atmosphere exposed. Inside the core, nuclear fusion continues until iron is
deposited. Since iron can not be burned by nuclear fusion, reactions in the core cease and the core
accumulates more iron as nuclear fusion continues in shells around it, but eventually the iron core collapses
under its own weight. This collapse is truly catastrophic as the core implodes to form a stellar black hole.
Material that implodes, but is outside the core, forms an accretion disc or torus of material spiralling in to the
black hole, heating up enormously as it does so. Some of this material gets blasted away from the poles of
the black hole (just outside the event horizon) instead of falling in. These polar jets ram into the helium
atmosphere but can punch through this and with the bulky hydrogen envelope removed, there is nothing to
absorb the blast of the jets. The jets break through the star's atmosphere, accelerating as they do so to
almost the speed of light, generating relativistic shock waves travelling at 99.99% the speed of light!
(Relativistic means close to the speed of light). These shock waves radiate intense gamma-rays in two
beams, one from each pole of the star. These hypernova are thought to generate long bursts of gamma-rays
and could account for GRBs lasting several seconds or minutes.
2. A neutron star colliding with another neutron star or black hole. Many neutron stars occur as pairs in old
binary star systems. As they rotate about one another, breaking may slow them down, causing them to slowly
spiral in toward each other. When they collide the two stars are most likely destroyed in an immense
explosion. Also, in regions of densely packed stars, as neutron stars get flung about by supernova
explosions, sooner or later two are going to be drawn together by each others strong gravitational fields,
forming binaries and making collisions much more likely. These are thought to generate short bursts of
gamma-rays and could account for gamma-ray bursts lasting only several milliseconds.
3. Magnetar giant flares (megaflares or hyperflares). Magnetars are highly magnetic neutron stars (with
magnetic fields of about 80 billion Tesla or 800 000 gauss) and emit periodic flares of gamma and X-rays.
These are thought to be slightly less energetic than in the other two cases and emit so-called soft gamma
rays (i.e. less energetic gamma rays) and do so periodically and so may account for what are called soft
gamma ray repeaters.
Just how powerful are gamma-ray bursts?
Very! If one goes off within 500 light years ( about 5 million billion kilometres, or 5 trillion km, 5^15 km!) of the
Earth and hits the Earth for several seconds, then it would cause severe damage to the ozone layer lasting
several years and could trigger a mass extinction of life on Earth. Although satellites detect one per day, on
average, most are in other galaxies and so they are too faraway to pose a threat, but who knows, sooner or
The furthest GRB observed was 12.3 billion light years away (and so occurred 12.3 billion years ago as we
look back in time)!