Among all electromagnetic waves, gamma rays have the shortest wavelengths (less than 0.01 nm), highest frequencies (around 1019 Hz), and, consequently, the highest energies (at least 100 keV). They’re even more energetic compared to their more popular cousins, X-rays. Hence, gamma rays have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum. These waves are generated by radioactive atoms and in nuclear explosions. Gamma-rays can kill living cells such as using gamma-rays to kill cancerous cells.
Gamma-rays travel to us across vast distances of the universe, only to be absorbed by the Earth's atmosphere. Different wavelengths of light penetrate the Earth's atmosphere to different depths. Instruments aboard high-altitude balloons and satellites like the Compton Observatory provide our only view of the gamma-ray sky.
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.
This is because most of the gamma radiation observed on the surface of the Earth come from radioactive substances. Due to the wave-particle duality of matter, gamma rays (which are actually electromagnetic waves) are also known as gamma particles. These particles, released during a transition of a radioactive nucleus from a more excited state to a lesser one, bear the energy difference between the two states.
These energy differences are very large, typical of the energies exhibited during interactions in the nucleus. As a consequence, the gamma particles carrying this released energy are very much capable of wreaking havoc on atoms they collide with, rendering the atoms ionized. Thus, like X-rays, gamma rays are considered ionizing radiation.
Gamma particles are actually photons. That means they have zero mass. Despite this, they carry a lot of momentum. Classical physics tells us that this cannot be possible. However, because of the extremely high speeds that these gamma particles have the moment they are released from the nucleus, relativistic effects have to be considered.
One consequence of which is the ability to acquire momentum despite having zero mass. That is why these massless gamma particles can still collide and dislodge loosely attached atom members like electrons. This is precisely the reason why gamma rays, like X-rays, are harmful to the body. Gamma rays can be stopped by high density shielding materials like lead.
Other than for astronomy ,they are used to kill cancer cells without having to resort to difficult surgery. This is so as Gamma rays can kill all living cells.This is called "Radiotherapy", and works because cancer cells can't repair themselves like healthy cells can when damaged by gamma rays.
There's also targeted radiotherapy, where a radioactive substance is used to kill cancer cells - but it's a substance that'll be taken up by a specific part of the body, so the rest of the body only gets a low dose. An example would be using radioactive iodine to treat cancer in the thyroid gland.
Radioactivity is particularly damaging to rapidly dividing cells, such as cancer cells. This also explains why damage is done by radiotherapy to other rapidly dividing cells in the body such as the stomach lining (hence nausea), hair follicles (hair tends to fall out), and a growing foetus (not because of mutations, but simply major damage to the baby's rapidly dividing cells).
Gamma rays are also used to sterilise medical equipment and killed microbes in food so that it will last longer.