This composite image of the Medusa galaxy (also known as NGC 4194) shows X-ray data from NASA's Chandra X-ray Observatory in blue and optical light from the Hubble Space Telescope in orange. Located above the center of the galaxy and seen in the optical data, the "hair" of the Medusa -- made of snakes in the Greek myth -- is a tidal tail formed by a collision between galaxies. The bright X-ray source found towards the left side of Medusa's hair is a black hole.
Most bright X-ray sources in galaxies are binaries containing either stellar mass black holes or neutron stars that remain after the supernova explosion of a massive star. Because these compact objects can generate X-rays for much longer periods of time than the lifetime of their massive progenitor stars, X-ray binaries may be used as "fossils" to study the star formation history of their host galaxies. In this Medusa image, the X-ray binaries are seen as the bright blue point-like objects.
-Kimberly Arcand, CXC