On Earth, gamma-rays are notorious as penetrating radiation from radioactive materials. Some radioactive atoms occur naturally, others are produced in nuclear reactors. The gamma-rays from outer space are blocked by the Earth's atmosphere - fortunately, because this powerful radiation is lethal. Gamma-ray telescopes in space detect radioactive materials in the Universe. They also can pick out signals due to enormous releases of energy, usually created by intense gravity fields rather than nuclear forces. There are a variety of mechanisms producing gamma radiation:
Decay of nucleons
Some radioactive atoms found on Earth emit gamma-rays when they decay. Such atoms also exist in the Universe. They were formed during the supernova explosions of massive stars. Examples are aluminium-26, titanium-44, nickel-56 and iron-60. During their decay they emit gamma-rays of a specific energy. This process is called nuclear spectral line emission.
When a proton collides with another proton or a nucleon, a new nucleon can be formed and part of the energy is emitted as gamma-rays. This radiation can have various energies. Stars generate their energy by nuclear collisions (nuclear fusion). In the Sun protons fuse to helium. However, nuclear interactions can also be triggered when cosmic rays hit interstellar gas in our Galaxy.
Annihilation between matter and anti-matter
When an electron and its anti-particle, a positron, collide, they annihilate each other, emitting a pair of gamma rays. The energy emitted is equal to the mass of the particles, that is, 511 keV each. The line at 511 keV observed in the centre of the Milky Way originates from this process.
The German word 'Bremsstrahlung' means 'slowing-down radiation'. Fast electrons are slowed down when they pass through matter. When an electron flies very closely by an atomic nucleus, the strong positive charge of the nucleus heavily deflects the electron's flight path.
Inverse Compton Scattering
If a particle of light encounters a fast electron, it may gain energy and can be boosted to gamma-rays. This may happen in the case of a compact star with a hot accretion disc glowing in X-rays. When the compact star generates beams of charged particles, the X-rays from the accretion disc, that is, a rotating disc of matter around a body due to gravitation, may be transformed into gamma-rays by 'Inverse Compton Scattering'.
Synchrotron radiation occurs when an electron moves in a magnetic field at almost the speed of light. The change in trajectory of the particle along its spiral path triggers the emission of photons, reaching the gamma range in extreme cases. Synchrotron radiation was first detected in a particle accelerator called a synchrotron.