What is a Black Hole?

Question

Tell me all about them, how are they created, how are they destroyed, how do they work, what's their gravity, how big can they be, what's their density...

Answer 1

black hole is a concentration of mass whose gravitational field is so strong that nothing can escape. Black holes are predicted by general relativity. Under the description provided by general relativity, as an object moves closer to a black hole, the energy required for it to escape continues to increase until it becomes infinite at the event horizon, the surface beyond which escape is impossible. Inside the event horizon, the geometry of spacetime is distorted in a way that makes moving closer to the central singularity inevitable no matter how the infalling object moves.

The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei.

It has been hypothesised that black holes radiate energy due to quantum mechanical effects (known as Hawking radiation).

In theory, no object beyond the event horizon of a black hole can ever escape, including light. However, black holes can be inductively detected from observation of phenomena near them, such as gravitational lensing, galactic jets, and stars that appear to be in orbit around space where there is no visible matter.

The most conspicuous effects are believed to come from matter accreting onto a black hole, which is predicted to collect into an extremely hot and fast-spinning accretion disk. The internal viscosity of the disk causes it to become extremely hot, and emit large amounts of X-ray and ultraviolet radiation. This process is extremely efficient and can convert about 50% of the rest mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few percent of the mass to energy. Other observed effects are narrow jets of particles at relativistic speeds heading along the disk's axis.

Stellar collapse will generate black holes containing at least three solar masses. Black holes smaller than this limit can only be created if their matter is subjected to sufficient pressure from some source other than self-gravitation. The enormous pressures needed for this are thought to have existed in the very early stages of the universe, possibly creating primordial black holes which could have masses smaller than that of the Sun.

Supermassive black holes are believed to exist in the center of most galaxies, including our own Milky Way. This type of black hole contains millions to billions of solar masses, and there are several models of how they might have been formed. The first is via gravitational collapse of a dense cluster of stars. A second is by large amounts of mass accreting onto a "seed" black hole of stellar mass. A third is by repeated fusion of smaller black holes.

Intermediate-mass black holes have a mass between that of stellar and supermassive black holes, typically in the range of thousands of solar masses. Intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources, and in 2004 detection was claimed of an intermediate-mass black hole orbiting the Sagittarius A* supermassive black hole candidate at the core of the Milky Way galaxy. This detection is disputed.

Certain models of unification of the four fundamental forces allow the formation of micro black holes under laboratory conditions. These postulate that the energy at which gravity is unified with the other forces is comparable to the energy at which the other three are unified, as opposed to being the Planck energy (which is much higher). This would allow production of extremely short-lived black holes in terrestrial particle accelerators. No conclusive evidence of this type of black hole production has been presented, though even a negative result improves constraints on compactification of extra dimensions from string theory or other models of physics.

Answer 2

A black hole is a massive object in a relatively small amount of space.

When Einstein's equations for curved space-time are solved for a spherical symmetric mass distribution, a singularity appears when the ratio mass/radius becomes larger than the "critical" value. (See also "Schwarzschild radius", etc.) For outside observers, it is no longer possible to communicate with the inside of the mass distribution. This is called a black hole, because even lightrays from inside will never be able to reach outside observers.

At the same time, black holes exert great forces of gravity on neighboring matter.

Because black holes follow from a very general theory of space, time and mass, they could exist in any size: just cram a relatively large amount of matter in a sufficiently small space.

The standard model for stars predicts that heavy stars may collapse to become a black hole. Black holes as such are not visible, but matter flowing into a black hole should be recognizable because of high frequency radiation (e.g. X-rays). Therefore astronomers search for X-ray radiation that could belong to a double star system: the heavy part of the system would be a black hole that sucks up matter from the small star.

Because black holes continually consume energy, it is not clear how they can be destroyed. In fact, we don't know what happens with the energy that is trapped in a black hole. Fantasies about "worm holes", parallel universes and "white holes" are interesting for SF stories, but nothing is known.

The well-know contemporary physicist Hawking is involved in studies of quantum effects of black holes.

Answer 3

When a really huge star collapses - a really huge one - bigger than our sun at the end of its life - sometimes it forms a black hole - the density of the matter is so great that nothing can escape the gravity of a black hole - including light (which is why we cant see them) - and thus black hole.

Answer 4

A black hole is a depression in space caused by a superdense singularity of matter, sometimes as small as a basketball, created by a collapsing star. They cannot be destroyed, not by our knowledge. No one is sure of their gravitational force or their density, common science has not been able to probe that far into the vast outer reaches of space.

Answer 5

An object whose gravitational pull inside a certain radius is so strong that nothing, not even light can escape it. A black hole forms when the amount of matter in the core of a star undergoing a supernova is great enough to cause a runaway gravitational collapse.

Answer 6

A big star, bigger than 4 times the weight of our sun, fuses the hydrogen in its atmosphere into helium through nuclear reactions in its core akin to a huge hydrogen bomb. Four protons fuse into a helium nuclei, releasing a tremendous amount of energy in the process. That's what makes the sun so hot (10 million degrees above absolute zero in its core). Because of the sheer gravitational weight of this huge star bearing down on its core, the now-helium core starts fusing into carbon and oxygen. When it works its way up through the Periodic Table of Elements and gets to an iron core, that's as far as the Periodic Table will allow more energy to be produced from the nuclear reaction than what is going into that reaction. The nuclear furnace shuts down (nothing more to fuse). Now there's nothing to stop the sheer weight of the star's outer envelope to start compressing the core. The iron core heats up but there is not enough outward pressure from the core to keep the star from collapsing. The iron core implodes and then "bounces" back, blowing off a lot of the star's outer atmosphere in a collosal explosion, called a "nova." If the weight of that what's left of that huge star can still overcome the outward pressure of what's left of the core, the core will start fusing even more exotic elements, like gold and platinum. What happens then, if the star is massive enough, is that the free electrons surrounding the core slam into the postively-charged nuclei of the star's core, rendering the protons there into neutrons, and you have a neutron star. Those neutrons (subatomic nuclei) in the core are pressed cheeck-to-cheek, the densest state of matter (in the form of plasma, the 4th state of matter (gas, liquid, solid, plasma). A spoonful of neutron star stuff on earth might weigh as much as a mountain. Anyway, if the star still weighs too much, even the outward neutron pressure can't hold the star up, and it collapses even further. If that neutron star (about the size of the state of Rhode Island but still weighing more that our sun) is spinning because of the angular momentum of the core's plasma, it is called a pulsar, emitting high energy bursts of gamma rays. Those gamma ray beams are pointed towards the earth at split-second intervals, like a lighthouse beam spinning aroud at some 1000s of rotations a second. Those gamma ray beams are detected by the earth's radiotelescopes. Now, as far as we know, we've never seen a squished neutron before, but the sheer weight of the neutron star means that the escape velocity for light in order to leave the neutron star exceeds the speed of light (186,000 miles per second). By comparison, the escape velocity of the earth's gravitational field is about 7.8 miles per second. Therefore, even light can't escape from that collapsing neutron star, so it becomes black (it doesn't radiate light like an an ordinary star does). Ergo, the name "black hole." Other than the fact that we can detect the gravitational influence of a black hole by how the orbits of the stars near it are affected, we don't know diddly squat about black holes. One famous saying about the properties of a black hole is that "it has no hair." Meaning, there's nothing to tell us about its properties except its apparent mass, spin, and charge. The theory is that the stuff inside the black hole keeps on collapsing until it gets to a point of infinite mass and temperature, called a singularity The singularity is referred to as "cloaked", meaning that even if we could see inside the black hole (which we can't), we would not be able to look at the "singularity". Quantum theory forbids it. Neverthleless, it appears that black holes contribute to some important functions of a galaxy, and contribute towards the overall galactic evolution. So, they perform a necessary function. What that is, we don't know. Nobody's ever witnessed the "destruction" of a black hole, although Stehen Hawking theorizes that black holes do radiate a small amount of energy. After a gazillion yeras, a black hole may well expldode back into ordinary space, but that's not happening. Given the mass of a that black hole, something like to happen would take much longer than the universe is old. To answer the rest of the questions, a black hole can have millions, if not billions, of solar masses in weight. As to their density, as detailed above, their "singularity" is of "infinite" density. The term is meaningless but the math works out that way. Perhaps the singularites are a gateways to another universe. That should relieve the "infinite" pressure. Who knows?

Answer 7

black holes are dense aggreagtions of matter . they mainly consists of heavy elements as they are prodused by repeated fusion. they are formed when a star of mass >1000 times the sun explodes into supernova . they have a such a intense gravitational force that even light is not able to escape as their name says "black hole".

Answer 8

Is this for homework. That is a lot of information for in here, but I still have your answer.

Go to www.google.com

Type define: black hole in the search window. You will find all kinds of information, most of it from Nasa.

Good luck.

Answer 9

An anomaly in space where gravity is strongly centered and absorbs anything within its range.

Answer 10

A collapsed star