Astronomers have been observing the visible portion of the electromagnetic spectrum for centuries, but this is just a very small "window" to the universe, and it provides a limited view. Once the space age was established, it opened many new windows to the cosmos. Gamma rays are high-energy particles. They were first detected through the decay of radioactive elements, but are also produced by nuclear reactions. The most efficient nuclear reactors are the stars, and they produce gamma rays deep in their interiors. By observing this radiation, astronomers can "see" into stellar cores.

Fortunately (or unfortunately, depending on your point of view), gamma rays do not penetrate to the surface of the earth; the ozone layer a bsorbs high-energy radiation. If it did not, life on this planet would not last long. The protective atmosphere is a barrier to the study of cosmic rays...