Black holes are regions of space where gravity is so strong that not even light can escape. They form when very massive stars collapse at the end of their life cycles. While black holes cannot be seen directly, astronomers can detect them by observing their effects on nearby objects like stars and gas, and through detection of x-rays emitted during accretion. Black holes come in different sizes, from stellar-mass black holes up to supermassive black holes that may exist at the centers of galaxies.

1. What Is a Black Hole?
Simply put, a black hole is a region of space that is so incredibly dense that not even light
can escape from the surface. However, it is this fact that often leads to miss-understanding.
Black holes, strictly speaking, don't have any greater gravitational reach than any other star
of the same mass. If our Sun suddenly became a black hole of the same mass the rest of
the objects, including Earth, would be unaffected gravitationally. The Earth would remain in
its current orbit, as would the rest of the planets. (Of course other things would be affected,
such as the amount of light and heat that Earth received. So we would still be in trouble,
but we wouldn't get sucked into the black hole.)
There is a region of space surrounding the black hole from where light can not escape,
hence the name. The boundary of this region is known as the event horizon, and it is
defined as the point where the escape velocity from the gravitational field is equal to
the speed of light. The calculation of the radial distance to this boundary can become quite
complicated when the black hole is rotating and/or is charged. For the simplest case (a non-
rotating, charge neutral black hole), the entire mass of the black hole would be contained
within the event horizon (a necessary requirement for all black holes). The event horizon
radius (Rs) would then be defined as Rs = 2GM/c2
.
How Do Black Holes Form?
This is actually somewhat of a complex question, namely because there are different types
of black holes. The most common type of black holes are known as stellar mass black holes as
they are roughly up to a few times the mass of our Sun. These types of black holes are
formed when large main sequence stars (10 - 15 times the mass of our Sun) run out of nuclear
fuel in their cores. The result is a massive supernova explosion, leaving a black hole core behind
where the star once existed. The two other types of black holes are supermassive black
holes -- black holes with masses millions or billions times the mass of the Sun -- and micro
black holes -- black holes with extremely small masses, perhaps as small as 20 micrograms.
In both cases the mechanisms for their creation is not entirely clear. Micro black holes exist
in theory, but have not been directly detected. While supermassive black holes are found to
exist in the cores of most galaxies. While it is possible that supermassive black holes result
from the merger of smaller, stellar mass black holes and other matter, it is possible that they
form from the collapse of a single, extremely high mass star. However, no such star has
ever been observed. Meanwhile, micro black holes would be created during the collision of
two very high energy particles. It is thought that this happens continuously in the upper
atmosphere of Earth, and is likely to happen in particle physics experiments such as CERN.
But no need to worry, we are not in danger.
How Do We Know Black Holes Exist If We Can't "See" Them?
Since light can not escape from the region around a black hole bound by the event horizon,
it is not possible to directly "see" a black hole. However, it is possible to observe these
objects by their effect on their surroundings.
Black holes that are near other objects will have a gravitational effect on them. Going back
to the earlier example, suppose that our Sun became a black hole of one solar mass. An

2. alien observer somewhere else in the galaxy studying our solar system would see the planets,
comets and asteroids orbiting a central point. They would deduce that the planets and other
objects were bound in their orbits by a one solar mass object. Since they would see no such
star, the object would correctly be identified as a black hole.
Another way that we observe black holes is by utilizing another property of black holes,
specifically that they, like all massive objects, will cause light to bend -- due to the intense
gravity -- as it passes by. As stars behind the black hole move relative to it, the light
emitted by them will appear distorted, or the stars will appear to move in an unusual way.
From this information the position and mass of the black hole can be determined.
There is another type of black hole system, known as a microquasar. These dynamic objects
consist of a stellar mass black hole in a binary system with another star, usually a large main
sequence star. Due to the immense gravity of the black hole, matter from the companion
star is funneled off onto a disk surrounding the black hole. This material then heats up as it
begins to fall into the black hole through a process called accretion. The result is the
creation of X-rays that we can detect using telescopes orbiting the Earth.
Hawking Radiation
The final way that we could possibly detect a black hole is through a mechanism known as
Hawking radiation. Named for the famed theoretical physicist and cosmologist Stephen
Hawking, Hawking radiation is a consequence of thermodynamics that requires that energy
escape from a black hole.
The basic (perhaps oversimplified) idea is that, due to natural interactions and fluctuations
in the vacuum (the very fabric of space time if you will), matter will be created in the form
of an electron and anti-electron (called a positron). When this occurs near the event horizon,
one particle will be ejected away from the black hole, while the other will fall into the
gravitational well.
To an observer, all that is "seen" is a particle being emitted from the black hole. The particle
would be seen as having positive energy. Meaning, by symmetry, that the particle that fell
into the black hole would have negative energy. The result is that as a black hole ages it
looses energy, and therefore loses mass (by Einstein's famous equation, E=Mc2
).
Ultimately, it is found that black holes will eventually completely decay unless more mass is
accreted. And it is this same phenomenon that is responsible for the short lifetimes
expected by micro black holes.
http://space.about.com/od/blackholes/a/Information-About-Black-Holes.htm
Astronomers think the object shown in this Chandra X-ray Observatory image (in box) may be an
elusive intermediate-mass black hole. Located about 32 million light-years from Earth in the
Messier 74 galaxy (M74), this object emits periodic bursts of x-rays at a rate that suggests it is
much larger than a stellar-mass black hole but significantly smaller than the supermassive black
holes found at the centers of galaxies. Few such middling black holes have been discovered, and
scientists aren't sure how they form.
Photograph courtesy NASA/CXC/U. of Michigan/J. Liu et al./ NOAO/AURA/NSF/T. Boroson

3. Black holes are the cold remnants of former stars, so dense that no matter—not even light—is able to
escape their powerful gravitational pull.
While most stars end up as white dwarfs or neutron stars, black holes are the last evolutionary stage in
the lifetimes of enormous stars that had been at least 10 or 15 times as massive as our own sun.
When giant stars reach the final stages of their lives they often detonate in cataclysms known
as supernovae. Such an explosion scatters most of a star into the void of space but leaves behind a
large "cold" remnant on which fusion no longer takes place.
In younger stars, nuclear fusion creates energy and a constant outward pressure that exists in balance
with the inward pull of gravity caused by the star's own mass. But in the dead remnants of a massive
supernova, no force opposes gravity—so the star begins to collapse in upon itself.
With no force to check gravity, a budding black hole shrinks to zero volume—at which point it is infinitely
dense. Even the light from such a star is unable to escape its immense gravitational pull. The star's own
light becomes trapped in orbit, and the dark star becomes known as a black hole.
Black holes pull matter and even energy into themselves—but no more so than other stars or cosmic
objects of similar mass. That means that a black hole with the mass of our own sun would not "suck"
objects into it any more than our own sun does with its own gravitational pull.
Planets, light, and other matter must pass close to a black hole in order to be pulled into its grasp. When
they reach a point of no return they are said to have entered the event horizon—the point from which
any escape is impossible because it requires moving faster than the speed of light.
Small But Powerful
Black holes are small in size. A million-solar-mass hole, like that believed to be at the center of some
galaxies, would have a radius of just about two million miles (three million kilometers)—only about four
times the size of the sun. A black hole with a mass equal to that of the sun would have a two-mile (three-
kilometer) radius.
Because they are so small, distant, and dark, black holes cannot be directly observed. Yet scientists have
confirmed their long-held suspicions that they exist. This is typically done by measuring mass in a region
of the sky and looking for areas of large, dark mass.
Many black holes exist in binary star systems. These holes may continually pull mass from their
neighboring star, growing the black hole and shrinking the other star, until the black hole is large and the
companion star has completely vanished.
Extremely large black holes may exist at the center of some galaxies—including our own Milky Way.
These massive features may have the mass of 10 to 100 billion suns. They are similar to smaller black
holes but grow to enormous size because there is so much matter in the center of the galaxy for them to
add. Black holes can accrue limitless amounts of matter; they simply become even denser as their mass
increases.
Black holes capture the public's imagination and feature prominently in extremely theoretical concepts
like wormholes. These "tunnels" could allow rapid travel through space and time—but there is no
evidence that they exist.
http://science.nationalgeographic.com/science/space/universe/black-holes-article/