Cosmic Beacons That Flash and Fade
An X-ray telescope points at a neighboring galaxy, and sees a point that wasn't there yesterday. A month later, it blazes hundreds of times brighter than the Sun, and a year after that, it fades again. Such points are called ultraluminous X-ray sources, and each one is a close pair: an ordinary star and a black hole or neutron star, whirling around each other.