Pulsars are neutron stars that emit radio waves, not gamma rays. AGN, GRBs, and supernovae are all known sources of gamma rays in extragalactic astronomy.

In AGN, gamma rays are primarily produced through the process of inverse Compton scattering, where high-energy electrons interact with low-energy photons, boosting their energy to gamma-ray levels.

Long GRBs are associated with the collapse of massive stars and are believed to be caused by the formation of a black hole or neutron star.

In supernovae, gamma rays are produced during the gravitational collapse of the core of a massive star, resulting in the formation of a neutron star or black hole.

Gamma-ray astronomy is primarily concerned with studying high-energy phenomena in the universe, such as AGN, GRBs, and supernovae. It does not directly address the composition of the Earth's atmosphere, which is studied by atmospheric science.

Gamma rays have the ability to penetrate dense interstellar matter, allowing astronomers to study regions of the universe that are obscured by dust and gas in other wavelengths.

Radio telescopes are used to detect radio waves, not gamma rays. Cherenkov telescopes, Compton telescopes, and pair-production telescopes are all types of gamma-ray telescopes.

The Compton Gamma Ray Observatory (CGRO) was the first gamma-ray telescope launched into space in 1991.

Dark matter has not been directly detected using gamma-ray astronomy. The existence of gamma-ray bursts, the detection of gamma rays from AGN, and the identification of the first black hole candidate are all major discoveries made using gamma-ray astronomy.

The Swift Gamma-Ray Burst Mission is a NASA mission launched in 2004 that is designed to study gamma-ray bursts.

Gamma-ray astronomy has made significant progress in developing suitable detectors for gamma rays. The low flux of gamma rays from astronomical sources, the difficulty in focusing gamma rays, and the high background radiation in the gamma-ray band are all major challenges associated with gamma-ray astronomy.

The Gamma-ray Large Area Space Telescope (GLAST) is a planned gamma-ray telescope that is designed to study the early universe.

The Fermi Gamma-ray Space Telescope is a NASA mission launched in 2008 that is designed to study the Milky Way galaxy.

Searching for signs of extraterrestrial life is not a major scientific goal of gamma-ray astronomy. Understanding the origin and evolution of galaxies, studying the nature of dark matter and dark energy, and detecting gravitational waves are all major scientific goals of gamma-ray astronomy.

The High Energy Stereoscopic System (H.E.S.S.) is a system of five Cherenkov telescopes located in Namibia that is designed to study the most energetic gamma rays.