The x-ray Universe is very different from the visible one as x-rays are produced only by matter that has been heated to temperatures in the millions of kelvins. And this occurs only in the presence of extremely powerful magnetic or gravitational ﬁelds, or during explosions. See Q. 156 for a view of the Galaxy in x-rays.
(a) image of a galaxy cluster taken in visible light and, (b) in x-rays (false color). The x-ray image reveals that the cluster is enveloped by a cloud of gas at a temperature of 40 million K. Credit: McNamara/NASA.