A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return.
"If you want to bring a star into a black hole, you can try throwing a single star at it, but like a comet that flies past the sun, nothing of interest will happen unless you get a very nearly direct hit," explained study co-author Ben Bromley, an astrophysicist at the University of Utah.
"On the other hand, if you throw a binary at the black hole, the binary is easily [separated] and, while you don't get a direct hit immediately, you at least capture one of the stars onto a bound orbit."
Applying their theory to the Milky Way, the team estimates that our galaxy's supermassive black hole consumes a star about every thousand years.
That's enough added material for our black hole to have doubled to quadrupled in mass over the past five to ten billion years.
"It only has to eat a typical star every thousand years or so," the CfA's Kenyon said. "It's not fasting, but it's not exactly eating at a high rate."
Current theories predict that all the matter in a black hole is piled up in a single point at the center, but we do not understand how this central singularity works. To properly understand the black hole center requires a fusion of the theory of gravity with the theory that describes the behavior of matter on the smallest scales, called quantum mechanics.
As a body is crushed into a smaller and smaller volume, the gravitational attraction increases, and hence the escape velocity gets bigger. Things have to be thrown harder and harder to escape. Eventually a point is reached when even light, which travels at 186 thousand miles a second, is not travelling fast enough to escape. At this point, nothing can get out as nothing can travel faster than light. This is a black hole.
As heavy as neutron stars are, if they're less than 2 solar masses, they too can only contract so far and no further. That's because, as crushed as they are, the neutrons also resist the inward pull of gravity, just as a white dwarf's electrons do. However, if after a star collapses, the remaining core exceeds approximately 2 solar masses, the outcome is thought to be very different. The precise mass limit is uncertain and depends on the nuclear physics going on within the core, a topic of much debate within the physics community.
Scientists really want to learn more about black holes and other strange and massive objects in the Universe. One space mission that is helping them do just that is a space telescope called XMM-Newton. It was launched into Earth orbit in 1999 by NASA and the European Space Agency. It observes the universe in high-energy x-rays, a type of light that we can't see with our eyes. Matter, such as gas and dust particles, near black holes puts out x-rays as it swirls around at light speed just before the black hole swallows it up. By observing these x-rays, XMM can help scientists understand the black hole.
The astronomers estimated about one percent of the star's mass was ultimately consumed, or accreted, by the black hole. This small amount is consistent with predictions the momentum and energy of the accretion process will cause most of the destroyed star's gas to be flung away from the black hole.
Using a new technique, two NASA scientists have identified the lightest known black hole. With a mass only about 3.8 times greater than our Sun and a diameter of only 15 miles, the black hole lies very close to the minimum size predicted for black holes that originate from dying stars.
There is no limit to how large a black hole can be. However, the largest blackholes we think are in existence are at the centers of many galaxies, and have masses equivalent to about a billion suns (i.e., a billion solar masses). Their radii would be a considerable fraction of the radius of our solar system.
Why do some stars end up as black holes?
The answer involves the gravity and the internal pressure within the star. These two things oppose each other -- the gravitational force of the star acting on a chunk of matter at the star's surface will want to cause that matter to fall inward, but the internal pressure of the star, acting outward at the surface, will want to cause the matter to fly outward. When these two are balanced (i.e., equal in strength) the star will maintain its size: neither collapse not expand. Such is the case for the Sun at the moment, and even, for that matter, for the Earth.