The black hole at the center of the Milky Way is 3 million times the mass of the
A super-massive black hole with 2 billion times the mass of the Sun apparently lurksin the nearby giant galaxy M87.
There are two other black holes in the Milky Way only 6 times the size of our sun.
theoretical physicist Stephen Hawking (1942-) has devoted much of his life to
probing the space-time described by general relativity and the singularities
where it breaks down. And he’s done most of this work while confined to a
wheelchair, brought on by the progressive neurological disease amyotrophic
lateral sclerosis, or Lou Gehrig’s Disease. Hawking is the Lucasian Professor
of Mathematics at Cambridge, a post once held by Isaac Newton
In the late 1960s, Hawking proved that if general
relativity is true and the universe is expanding, a singularity must have
occurred at the birth of the universe. In 1974 he first recognized a truly
remarkable property of black holes, objects from which nothing was supposed to
be able to escape. By taking into account quantum mechanics, he was able to show
that black holes can radiate energy as particles are created in their vicinity.
But perhaps his most impressive feat was writing the international bestseller A
BRIEF HISTORY OF TIME. The book spent more than four years on the London Sunday
Times bestseller list—the longest run for any book in history.
Pile enough matter
into a small enough volume and its gravitational pull will grow so strong that
nothing can escape from it. That includes light, which travels at the absolute
cosmic speed limit of 186,000 miles per second. In a stroke of descriptive
genius, physicist John Wheeler named these objects “black holes.” The radius
of a black hole is called the event horizon because it marks the edge beyond
which light cannot escape, so any event taking place inside the event horizon
can never be glimpsed from outside—in effect, the inside of the black hole is
cut off from our universe. It has even been speculated that black holes could be
pathways into other universes. Gravity is so strong at the center of a black
hole, that even Einstein’s gravitational laws must break down. The theory that
governs the incredibly dense matter and strong gravitational fields at the
center of a black hole is not yet known.”
For decades, black holes were the darlings of science
fiction writers but treated with perhaps a little less respect by physicists.
Although general relativity predicted that black holes could exist, many
scientists thought they were too bizarre to exist in the real universe. That’s
all changed. Astronomers have now detected several black holes in X-ray-emitting
binary star systems, where a normal star orbits a massive yet invisible
companion that theory says must be a black hole. Even more convincing evidence
has come from the centers of several large galaxies, where stars move about so
quickly that they must be caught in the grips of a massive object. By
calculating the size and mass of these objects, the only conclusion seems to be
that the center of these galaxies harbor supermassive black holes.
Black holes are usually thought of as objects with such strong gravity that nothing, not even light, can escape from them. However, Stephen Hawking has shown that black holes can radiate energy. The reason goes back to quantum mechanics and the uncertainty principle. For very brief periods of time, matter or energy can be created from “empty” space because no such thing as truly empty space exists. Hawking realized that if a particle/anti-particle pair came into existence near the event horizon of a black hole, one might fall into the hole before annihilating its anti-particle. The other particle could then escape the gravitational clutches of the black hole, appearing to an outside observer as radiation.
A black hole is a region of space in which the matter is so compact that nothing can escape from it, not even light; the "surface" of a black hole, inside of which nothing can escape, is called an event horizon. The matter that forms a black hole is crushed out of existence. Just as the Cheshire Cat disappeared and left only its smile behind, a black hole represents matter that leaves only its gravity behind.
Black holes are usually formed when an extremely massive star dies in a supernova. However, some people think small black holes were formed during the Big Bang, and that the resulting "mini black holes" may be in great abundance in our galaxy.
In principle, black holes can have any mass; black holes formed by stellar death have at least twice the mass of our Sun. Unlike ordinary things (e.g., rocks), which have a size roughly proportional to the cube root of their mass, black holes have radii proportional to their mass. The event horizon of a nonrotating black hole the mass of our Sun has a radius about 3~km. Thus, large black holes aren't very dense! A black hole a billion times as massive as our Sun, such as is thought to exist in the center of some galaxies, has an average density just twenty times the density of air.
Black holes, like any gravitating objects, exert a tidal force. If you approach a black hole feet first, the gravitational force at your feet is greater than the force at your head. The tidal force at the event horizon is smaller for larger black holes: you would get torn to shreds far outside a black hole the mass of our sun, but at the event horizon of a billion solar mass black hole the tidal force would only be a millionth of an ounce!
A singularity is a region of space-time in which gravitational forces are so strong that even general relativity, the well-proven gravitational theory of Einstein, and the best theory we have for describing the structure of the universe, breaks down there. A singularity marks a point where the curvature of space-time is infinite, or, in other words, it possesses zero volume and infinite density. General relativity demands that singularities arise under two circumstances. First, a singularity must form during the creation of a black hole. When a very massive star reaches the end of its life, its core, which was previously held up by the pressure of the nuclear fusion that was taking place, collapses and all the matter in the core gets crushed out of existence at the singularity. Second, general relativity shows that under certain reasonable assumptions, an expanding universe like ours must have begun as a singularity.
Massive Star starts to collapse when it exhausts its nuclear fuel and can no
longer counteract the inward pull of gravity.
The crushing weight of the star's overlying layers implodes the core, and the star digs deeper into the fabric of space-time.
Although the star remains barely visible, its light now has a difficult time climbing out of the enormous gravity of the still collapsing core.
The star passes through its event horizon and disappears from our universe, forming a singularity of infinite density.
Black holes don't radiate light, and an object that falls inside a black hole doesn't emit light either, so detecting them can be challenging.
Just as with neutron stars, if a black hole is in a binary and it strips gas from its companion, we can detect X-rays from the resulting accretion disk. The light from accretion disks around black holes looks very similar to the light from disks around neutron stars, and it is not always possible to tell with certainty which object lurks at the center of the disk, although in six cases so far we're sure that the central object is a black hole.
You can also infer the presence of a black hole in the center of some galaxies. This is done by observing stars near the center of the galaxy. If the stars are moving very rapidly around some unseen object, Kepler's laws can be used to estimate the mass in the center. In some cases the mass must be at least a hundred million times our Sun's mass, in a region only a few light years across. Astronomers are virtually certain that the only explanation is a black hole, but we lack direct evidence.
The detection of black holes is very difficult and controversial, and it is being studied actively by many research groups.