On September 16, 2012, NASA’s Swift satellite caught an x-ray outburst, believed to have come a massive cloud of gas plunging toward a previously unknown black hole. This new black hole in our Milky Way galaxy has been designated as Swift J1745-26 by astronomers.
Often when astronomers speak of black holes, they are speaking of supermassive objects thought to be located at the center of most galaxies, including our own Milky Way. Supermassive black holes may have the mass of a billion suns. But stellar-mass black holes are very different, much less massive, formed from individual stars.
The first stellar-mass black hole candidate was Cygnus X-1, which, not coincidentally, is one of the strongest X-ray sources seen from Earth. Cyg X-1 is now estimated to have a mass about 14.8 times the mass of our sun.
Today, by understanding how stars evolve and by estimating how many stars have enough mass to evolve into black holes, astronomers deduce that our galaxy has some 100 million stellar-mass black holes. We don’t see these objects, but astronomers believe they exist.*Astronomers do now study about a dozen stellar-mass black hole candidates in our Milky Way, including Cygnus X-1 and now Swift J1745-26. The nearest one is about 1,600 light-years from Earth, according to hubblesite.org.
There are about 200 globular clusters in the Milky Way that may have already spawned intermediate-sized black holes, which means that hundreds of them would be wandering invisibly around the Milky Way. These could be engulfing the nebulae, stars and planets that are unfortunate enough to cross their paths, but apparently this poses no imminent danger to Earth -- or at least not as far as anyone knows at this point in time.
“These rogue black holes are extremely unlikely to do any damage to us in the lifetime of the universe,” Holley-Bockelmann stresses. “Their danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!”
The Daily Galaxy via www.nasa.gov/swift