The study of extremely distant objects by means of the
X-ray and
gamma ray photons they emit.
Why would anyone want to do that? After all, neither X-rays nor gamma rays penetrate the atmosphere, so you have to use balloons or rockets to get high enough to detect them.
The answer is that it's the best way to look at really hot (and therefore way cool) stuff. Things like black holes, neutron stars, and supernova remnants.
Here's why: Think about how when you heat something, it eventually starts to glow red. Then as you heat it further, it gets orange, yellow, and eventually looks pretty white. If you keep heating (hard to do on earth, without serious equipment), the light it emits will become blue, then ultraviolet, and keep on going to shorter and shorter wavelengths. So keep going beyond ultraviolet, and you get to X-rays. Once you're at a couple million degrees C. you'll be emitting X-rays. (Don't try this at home, kids.)
So looking at X-rays and gamma rays (which are shorter wavelength still) is a way to look at the hottest things there are. Or if you prefer, they are a window onto the most violent objects in the universe. (That's right, X-ray astronomers are the John Woo's of science.)