Supermassive black holes do not give off any of their own light, hence the word “black” in their name. However, many black holes pull in surrounding material and emit powerful bursts of X-rays. Collectively, these active black holes can be thought of a cosmic choir, singing in the language of X-rays. Their “song” is what astronomers call the cosmic X-ray background.
NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, has captured the best high-energy X-ray view yet of a portion of the Andromeda Galaxy, our nearest large neighbouring spiral galaxy. The space observatory has observed 40 “X-ray binaries” — intense sources of X-rays comprising a black hole or neutron star that feeds off a stellar companion.