Discovered in the early 1960s by rocket-borne X-ray detectors, Cygnus X-1 is a binary system containing a supergiant star and a stellar-mass black hole. That black hole is both accreting matter — ...

There's an ultimate speed limit in the Universe: the speed of light in a vacuum, c. If you don't have any mass — whether you're a light wave (a photon), a gluon, or even a gravitational wave — that's ...