Gamma rays are the most energetic known form of electromagnetic radiation, with each gamma-ray being at least one hundred thousand times more energetic than an optical light photon. Very high energy (VHE) gamma rays pack energies a billion times this amount, or even more. Astronomers think that VHE gamma rays are produced in the environment of the winds or jets of the compact, ultra-dense remnant ashes of massive stars left behind from supernova explosions. There are two kinds of compact remnants: black holes and neutron stars (stars made up predominantly of neutrons, with densities equivalent to the mass of the Sun packed into a volume about 10 kilometers in radius). The winds or jets from the environments of such objects can accelerate charged particles to very close to the speed of light, and radiation that scatters off such energetic particles can become energized, as well, sometimes turning into VHE gamma rays.