M33 X-7 binary star system. Composite X-ray and optical image of a binary star system (blue, upper centre) in the nearby galaxy M33, 2.6 million light years from Earth. It consists of a black hole (16 times the mass of the Sun), which is in orbit around a huge companion star. The black hole is the largest 'stellar mass' black hole, meaning that it is the largest black hole to have formed from a collapsed star. The companion star also has an unusually large mass, 70 times that of the Sun. This makes it the most massive companion star known in a binary system containing a black hole. The bright objects surrounding M33 X-7 are young, massive stars. Data captured with NASA's Chandra X-ray Observatory and the Hubble Space Telescope between 21st September and 26th November 2005.