Gamma Rays

Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the CGRO spacecraft. Bright spots within the galactic plane are pulsars while those above and below the plane are thought to be quasars.

Gamma-rays are the most energetic form of light and are produced by the hottest regions of the universe. They are also produced by such violent events as supernova explosions or the destruction of atoms, and by less dramatic events, such as the decay of radioactive material in space. Things like supernova explosions (the way massive stars die), neutron stars and pulsars, and black holes are all sources of celestial gamma-rays.

Gamma radiation, also known as gamma rays, and denoted by the Greek letter Y refers to electromagnetic radiation of high frequency and therefore high energy per photon. Gamma rays are ionizing radiation, and are thus biologically hazardous. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay), but are also created by other processes. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard's radiation was named "gamma rays" by Ernest Rutherford in 1903.

Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes, and secondary radiation from atmospheric interactions with cosmic ray particles. Rare terrestrial natural sources produce gamma rays that are not of a nuclear origin, such as lightning strikes and terrestrial gamma-ray flashes. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced, that in turn cause secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth's atmosphere and must be detected by spacecraft.

Gamma rays typically have frequencies above 10 exahertz and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom). However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV. In astronomy, gamma rays are defined by their energy, and no production process need be specified.

The energies of gamma rays from astronomical sources range over 10 TeV, at a level far too large to result from radioactive decay. A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay. These bursts of gamma rays, thought to be due to the collapse of stars called hypernovas, are the most powerful events so far discovered in the cosmos.

Gamma Rays Wikipedia

A hypernova. Artist's illustration showing the life of a massive star as nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a black hole. Theoretically, energy may be released during the collapse along the axis of rotation to form a long duration gamma-ray burst.

Articles in the News

Five billion light years across: The largest feature in the universe   Science Daily - August 4, 2015

Astronomers have found what appears to be the largest feature in the observable universe: a ring of nine gamma ray bursts -- and hence galaxies - 5 billion light years across. Gamma-ray bursts (GRBs) are the most luminous events in the universe, releasing as much energy in a few seconds as the Sun does over its 10 billion year lifetime. They are thought to be the result of massive stars collapsing into black holes. Their huge luminosity helps astronomers to map out the location of distant galaxies, something the team exploited. The GRBs that make up the newly discovered ring were observed using a variety of space- and ground-based observatories. They appear to be at very similar distances from us -- around 7 billion light years -- in a circle 36 across on the sky, or more than 70 times the diameter of the Full Moon.

Gamma-Ray Earth and Sky   NASA - December 6, 2013

For an Earth-orbiting gamma-ray telescope, Earth is actually the brightest source of gamma-rays, the most energetic form of light. Gamma-rays from Earth are produced when high energy particles, cosmic rays from space, crash into the atmosphere. While that interaction blocks harmful radiation from reaching the surface, those gamma-rays dominate in this remarkable Earth and sky view from the orbiting Fermi Gamma-ray Space Telescope's Large Area Telescope. The image was constructed using only observations made when the center of our Milky Way galaxy was near the zenith, directly above the Fermi satellite. The zenith is mapped to the center of the field. The Earth and points near the nadir, directly below the satellite, are mapped to the edges of the field resulting in an Earth and all-sky projection from Fermi's orbital perspective. The color scheme shows low intensities of gamma-rays as blue and high intensities as yellowish hues on a logarithmic scale. Our fair planet's brighter gamma-ray glow floods the edges of field, the high intensity yellow ring tracing Earth's limb. Gamma-ray sources in the sky along the relatively faint Milky Way stretch diagonally across the middle. Launched June 11, 2008 to explore the high-energy Universe, this week Fermi celebrated its 2,000th day in low Earth orbit.

Universe's Biggest Explosions Shaped by Extreme Magnetic Fields   Live Science - December 4, 2013

Scientists have captured their best view yet of how extreme magnetic fields shape superfast jets from the most powerful explosions in the universe. The new research tracked polarized light from cosmic explosions, known as gamma-ray bursts, and offered an unprecedented glimpse into how intense magnetic fields shape the evolution of the outbursts.

How a 'monster' gamma-ray blast started - and why we were spared   NBC - November 21, 2013

What happened was a gamma-ray burst, an explosion that happens when a massive star dies, collapses into a brand-new black hole, creates a supernova and ejects energetic radiation that is as bright as can be as it travels across the universe at the speed of light.

Earth's Gold Came from Colliding Dead Stars   Science Daily - July 17, 2013

This artist's conception portrays two neutron stars at the moment of collision. New observations confirm that colliding neutron stars produce short gamma-ray bursts. Such collisions produce rare heavy elements, including gold. All Earth's gold likely came from colliding neutron stars. Gold is rare on Earth in part because it's also rare in the universe. Unlike elements like carbon or iron, it cannot be created within a star. Instead, it must be born in a more cataclysmic event -- like one that occurred last month known as a short gamma-ray burst (GRB). Observations of this GRB provide evidence that it resulted from the collision of two neutron stars -- the dead cores of stars that previously exploded as supernovae. Moreover, a unique glow that persisted for days at the GRB location potentially signifies the creation of substantial amounts of heavy elements -- including gold.

A Violent History of Time NASA - January 24, 2008

From mother Earth, the night sky can look peaceful and unchanging, but the universe as seen in gamma-rays is a place of sudden and chaotic violence. Using gamma-ray telescopes, astronomers witness short but tremendously intense explosions called gamma-ray bursts, and there is nothing more powerful.

Astronomers find first ever gamma ray clock PhysOrg - November 28, 2006

Astronomers using the H.E.S.S. telescopes have discovered the first ever modulated signal from space in Very High Energy Gamma Rays the most energetic such signal ever observed. Regular signals from space have been known since the 1960s, when the first radio pulsar (nicknamed Little Green Men-1 for its regular nature) was discovered. This is the first time a signal has been seen at such high energies 100,000 times higher than previously known.

Astronomers ponder weird cosmic burst ABC - February 24, 2006

A new kind of cosmic explosion has been spotted in Earth's celestial neighbourhood, scientists report. The blast seems like a gamma-ray burst, one of the most distant and powerful type of explosion known to astronomers. But when scientists first detected it with NASA's Swift satellite on 18 February, the explosion was about 25 times closer and lasted 100 times longer than a typical gamma-ray burst. The explosion originated in a star-forming galaxy about 440 million light-years away towards the constellation Aries. A light-year is about 10 trillion kilometres, the distance light travels in a year. This would be the second-closest gamma-ray burst ever detected, if indeed it is one. The burst lasted for nearly 2000 seconds, or about 33 minutes, astronomers say. Most bursts last a few milliseconds to tens of seconds. The burst was also surprisingly dim.

Weak Version of Most Powerful Explosions Found - August 2004

Gamma-ray bursts are the most powerful events in the universe, temporarily outshining several galaxies and likely signaling the birth of a black hole. But astronomers have been puzzled by one burst, noted in 1998, that was comparatively dim. Was the weakling a loner, a total "freak" as one astronomer termed it? Or have others like it simply gone undetected.