What is meant by black hole?

Answer 1

Black Hole:
Simply put, a black hole is a super dense object that has an intense gravitational pull. A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. A There are two parts to a black hole, a singularity and a event horizon.

The event horizon is where the force of gravity becomes so strong that even light is pulled into the black hole. Although the event horizon is part of a black hole, it is not a tangible object. If you were to fall into a black hole, it would be impossible for you to know when you hit the event horizon.

The singularity is not really a tangible object either. According to the General Theory of Relativity the Singularity is a point of infinite space time curvature. This means that the force of gravity has become infinitely strong at the center of a black hole. Everything that falls into a black hole by passing the event horizon, including light, will eventually reach the singularity of a black hole. Before something reaches the singularity it is torn apart by intense gravitational forces. Even the atoms themselves are torn apart by the gravitational forces.


Light from a Black Hole:
One of their basic properties of black holes is that they trap light. According to the special theory of relativity (and also according to the general theory since it contains the special theory) that nothing exceeds the speed of light.

The particles of light, called the photons lose energy when travelling out of a gravitational field and appear to be redder to an external observer. . The stronger the gravitational field, the more energy the photons lose because of this gravitational redshift. The extreme case is a black hole where photons from within a certain radius lose all their energy and become invisible.

Another view of the same phenomenon is that the light can not escape out of a black hole as it can not go pass the event horizon. The event horizon is the point outside the black hole where the gravitational attraction becomes so strong that the escape velocity (the velocity at which an object would have to go to escape the gravitational field) equals the speed of light. Since according to the relativity theory no object can exceed the speed of light that means nothing, not even light, could escape the black hole once it is inside this distance from the center of the black hole.

A more fundamental way of viewing this (the same phenomenon) is that in a black hole, the gravitational field is so intense that it bends space and time around itself so that inside the event horizon there are literally no paths in space and time that lead to the outside of the black hole: No matter what direction you went, you would find that your path led back to the center of the black hole, where the singularity is found.

Black Hole Radiation:
Classically, black holes are black. Quantum mechanically, black holes radiate. Recent attempts to understand black holes on a quantum level have indicated that they radiate thermally (they have a finite temperature, though one incredibly low if the black hole is of reasonable size) that is proportional to the gradient of the gravity field.

This radiation known as Hawking radiation, after the British physicist Stephen Hawking who first proposed it. Hawking radiation has a blackbody (Planck) spectrum with a temperature T given by
kT = hbar g / (2 pi c) = hbar c / (4 pi rs)
where k is Boltzmann's constant, hbar = h / (2 pi) is Planck's constant divided by 2 pi, and g = G M / rs2 is the surface gravity at the horizon, the Schwarzschild radius rs, of the black hole of mass M. Numerically, the Hawking temperature is T = 4 × 10-20 g Kelvin if the gravitational acceleration g is measured in Earth gravities (gees).
Hawking Radiation is due to the capture of virtual particles decaying from the vacuum at the horizon. These are created in pairs and one of them is caught in the black hole and the other is radiated externally.

Black Hole Evaporation:
The energy that produces the radiation comes from the mass of the black hole. Consequently, the black hole gradually shrinks. It turns out that the rate of radiation increases as the mass decreases, so the black hole continues to radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely.

Cheers.

Answer 2

Below is a Wikipedia excerpt and is the original concept of a black hole: http://en.wikipedia.org/wiki/Schwarzschild_radius

In 1916, Karl Schwarzschild obtained an exact solution to Einstein's field equations for the gravitational field outside a non-rotating, spherically symmetric body (see Schwarzschild metric). The solution possesed a term in it having the form 1 / (2M − r); the value of r making this term singular has come to be known as the Schwarszchild radius. The physical significance of this singularity, and whether this singularity could ever occur in nature, was debated for many decades, and a general acceptance of the possibility of a black hole did not occur until after World War II.

The Schwarzschild radius of an object is proportional to the mass. Accordingly, the Sun has a Schwarzschild radius of approximately 3 km, while the Earth's is only about 9 mm.

An object smaller than its Schwarzschild radius is called a black hole.

[There are other interesting concepts involving Rotating black holes, etc.]
Note the Earth would have to crush down to 9mm radius (about the size of a baseball) in order to "become" a black hole.

Answer 3

"Black hole" is the name given by astronomers to a cosmic phenomenon that occurs when a very dense star made out of closely packed atomic nuclei (neutrons) collapses under the sheer weight of its own gravity. The result is something with so much gravity that even light can't escape from its surface. That's why it's black. No light can come out of it.

The latest research into black holes suggests that we are very close to actually seeing one with our space-based telescope. That is, we will be able to see the outline of the black hole by the bright matter surrounding it. Scientists right now are studying the center of our galaxy, where there is a very large black hole. They believe they will soon be able to photograph the black hole while it is "feeding" on a star that got too close. When that happens, there will be a disk (an "accretion disk") of very bright star material swirling around the black hole as the material is being drawn into the black hole, sort of like water going down a drain. We may soon have our first photographs of a black hole. We live in interesting times.

Answer 4

Stephen Hawking could probably explain it better, but it's a star that has collapsed in on itself due to it's massive size. All of it's hydrogen fuel has been depleted. It's gravatational pull is unfathomable, a tablespoon of a black hole would weigh tons because of its density. Any matter that is caught in it is spagettified. Or so it appears that way from an observation point. But it also is thought that if you happened to be the person that was sucked in, everything would appear normal to you. Then there was the argument that Stephen Hawking supported for 30 years, that whatever material had been sucked in by the black hole disappeared completely after the black hole dies. Which defies the law of conservation of matter, it can neither be created nor destroyed. He soon proved himself wrong after a near death visit to the hospital. Then you could get into string theory, which says that all of these "strings" resonate and other stuff in harmony to create our universe. Or something of the sorts.

Answer 5

A black hole is an object predicted by general relativity, with a gravitational field so powerful that even electromagnetic radiation (such as light) cannot escape its pull.

A black hole is defined to be a region of space-time where escape to the outside universe is impossible. The outer boundary of this region is called the event horizon. Nothing can move from inside the event horizon to the outside, even briefly, due to the extreme gravitational field existing within the region. For the same reason, observers outside the event horizon cannot see any events which may be happening within the event horizon; thus any energy being radiated or events happening within the region are forever unable to be seen or detected from outside. Within the black hole is a singularity, an anomalous place where matter is compressed to the degree that the known laws of physics no longer apply to it.

Theoretically, a black hole can be any size. Astrophysicists expect to find black holes with masses ranging between roughly the mass of the Sun ("stellar-mass" black holes) to many millions of times the mass of the Sun (supermassive black holes).

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 also been hypothesized that black holes radiate an undetectably small amount of energy due to quantum mechanical effects. This is called Hawking radiation.

Answer 6

I'am a science student so i think i know this pretty well. If the initial mass of a normal star is many times that of the sun, the neutron star formed further contracts. It can contract until the radius becomes as small as 3 kilometres. such an object has a very large desity and also a very large gravitational force. The gravitational force is so high that even light (photons) cannot escape out of its strong attraction. Such an object is called a black hole. Hope this answered your question.

Answer 7

A black hole is a star that has collapsed with sufficient mass and density to create a gravitational pull so strong it won't allow light to escape. When viewed through a telescope, it appears to be a black hole in space since no light can pass through it.

Answer 8

When a star having a large mass & density collapses & creates a gravitational pull so strong that it does not allow even light to escape - we then term is as a Black Hole, since when viewed thro a telescope it appears as a mass of darkness.

Answer 9

A black hole is an object in space who's density, and therefore gravity, is so high that not only does it suck matter into it but also energy, so since light cannot escape it appears black.

They are believed to be formed by stars that burn out and collapse in on themselves, I don't know if they have determined the minimum sized star necessary to form one when it collapses. Maybe that should be your next question ;-)
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Answer 10

A blackhole is a massive star, at least 100 times bigger than our sun. When the star can not longer make energy using it's own power, I believe it's when Iron is it's main fuel instead of hydrogen...it's collapses on itself due to it's mass. It becomes about the size of a large city...when this happens, it's gravitational pull is so strong that not even light escapes...it just sucks everything in.

I like how I got a thumbs done....okay geniuses, what's a blackhole then that I haven't mentioned? Lol