Although black holes emit no light, all those that are several times more massive than the sun came to attention because hot gas falling into them gives off x-rays. That's how astronomers found the first black hole , Cygnus X-1; no less an authority than the rock band Rush declared that "the x-ray is her siren song." Now, for the first time, astronomers have managed to discover a stellar-mass black hole without using x-ray observations. Located 8500 light-years from Earth, the dark object orbits a bright, blue star named MWC 656 that spins so fast it flings gas (colored red in this artist's conception) around itself; additional gas (colored blue) swirls around the black hole but doesn't fall in, explaining the lack of x-rays. By measuring Doppler shifts of the two gaseous disks as they orbit each other every 60.37 days, the scientists deduce that the dark object weighs 3.8 to 6.9 times more than the sun —massive enough to be a black hole, the researchers report online today in Nature. The discovery suggests that many other equally quiescent black holes inhabit the galaxy, unknown to astronomers.
ScienceShot: 'Quiet' Black Holes May Be Lurking Throughout Galaxy