Spectrum of a black hole, Chandra X-ray Observatory (CXO) image. The CXO is a space telescope orbiting the Earth. It is the most powerful X-ray telescope ever built. It observes X-rays from high-energy regions in space, such as hot gas in the remnants of exploded stars. A black hole is formed when the core of a star collapses under its own weight, increasing its gravitational field to the point where, beyond a boundary known as the event horizon, nothing, not even light, can escape. High-energy radiation such as X-rays are produced as material drawn into the black hole is destroyed. The CXO was launched by NASA in 1999. Image obtained in April 2000.