Gamma rays are the form of electromagnetic radiation with the most energy and the tiniest wavelength. They are defined as waves with a period (wavelength) of less than 1 picometer, which is 0.001 nanometers. By comparison, the diameter of a hydrogen atom is 50 picometers. Therefore the wavelength of gamma radiation is by definition subatomic. Their energy is on the order of 1.25 MeV (mega-electron-volts) or above. Gamma rays are generated in very energetic events such as supernovae, in and around bodies of exotic matter like neutron stars, pulsars, and black holes, or in a less spectacular fashion when radioactive nuclei break down in the interstellar medium.
The wavelength of gamma rays can drop as low as 10-15 or 0.000001 nanometers, around the classic radius of an electron. As the wavelength decreases, the corresponding energy increases. Because of their huge energy, gamma rays are extremely hard to stop. To shield something from gamma rays requires thick shields (1m+) of substances with as high of an atomic number as possible. Lead is one popular substance. Gamma rays have been known to travel through 3 meters of concrete. Because of their high energies and penetration ability, gamma rays are extremely hazardous biologically — they can kill living cells upon contact. The most dangerous initial effect of a nuclear explosion is the gamma ray flash.
Gamma rays were first observed by Paul Ulrich Villard in 1900, while he was studying the radioactivity of uranium. Initially, it was suspected that gamma rays were particles, like its radiative cousins alpha particles and beta particles, but shining them through a crystal proved that they were indeed electromagnetic waves. Like alpha particles and beta particles, gamma rays are ionizing radiation, though (unlike beta particles) are not themselves charged. Ionizing radiation is powerful enough to strip electrons from atoms it hits, giving them as a charge and causing disruption to the resident material.
One of the most amazing phenomena concerning gamma rays is that of the gamma ray burst (GRB). These are massive gamma ray explosions that occur in deep space. They are the most energetic activity in the universe since the Big Bang. (More energetic than supernovae.) In 10 seconds, a large gamma ray burst releases more energy than the Sun will release during its 10 billion year lifetime. Various have been constructed to explain various types of gamma ray bursts. The prevailing theory is that of a collapsar. A collapsar is a special supermassive star that ejects high-energy relativistic jets from its poles while undergoing the final collapse stage. We observe these as GRBs. A different type of GRB is likely explained by degenerate binary stars. Extremely dense neutron stars may occasionally collide, releasing tremendous amounts of gamma rays in the fusion process.
Gamma rays are also used in medicine to kill malignant cells, such as cancer cells. This procedure is called gamma-knife surgery.