An X-ray telescope is a telescope that is used to observe distant objects in the X-ray portion of the electromagnetic spectrum. X-ray telescopes are typically much larger than optical telescopes and are usually made up of several nested mirrors that focus the incoming X-rays onto a detector. The mirrors must be made of highly reflective materials such as gold, silver, or nickel in order to effectively reflect the X-rays. These telescopes are used to study some of the most energetic phenomena in the universe, such as supernovae and black holes.