Event horizon - definition of event horizon by The Free Dictionary Event horizon - definition of event horizon by The Free Dictionary Also found in: Acronyms , Encyclopedia , Wikipedia . event horizon n. The region, usually described as spherical, marking the outer boundary of a black hole, inside which the gravitational force is strong enough to prevent all matter and radiation from escaping. event horizon n (Astronomy) astronomy the surface around a black hole enclosing the space from which electromagnetic radiation cannot escape due to gravitational attraction. For a non-rotating black hole, the radius is proportional to the mass of the black hole event′ hori`zon n. Black Holes Questions and Answers about Black Holes 1. Is it possible for black holes to lead to other universes? Using black holes to get from one place to another is an interesting story. Here, people sometimes talk about wormholes. �Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity.

Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working). �In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless technojargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity. �Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely. �The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative. �This isn't quite as impossible as it sounds, but it still isn't all that likely!

By the way, one tricky thing about wormholes, and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity). �Therefore, if you have a wormhole you can time-travel. �This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox. �In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person. �I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother. �Since he's dead, he can't produce my father, who can't produce me, which means that I could not have built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture! �Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems.

Cole Miller 2. What happens to all the stuff that get's sucked into the black hole? Would it pop out randomly at some other place or something? Or would things not survive a black hole? It is thought that the matter that goes into a black hole gets crushed into a tiny point at the center called a "singularity". �That's the only place that matter is, so if you were to fall into a black hole you wouldn't hit a surface as you would with a normal star. �Once it's there, it's there. �As far as we know, nothing would survive going into a black hole. �People sometimes talk about "wormholes" as portals to other universes, but it is now thought to be very likely that these can't exist. Cole Miller 3. If nothing escapes a black hole not even light, and �if gravity is perhaps a form of electromagnetic radiation (G.U.T.) how does gravity escape the black hole to pull things in? The idea (in Einstein's conception) is that space and time are parts of one entity, spacetime.

The presence of mass distorts and warps this spacetime; normally the warping is minor, but around a very compact object such as a black hole or a neutron star the warping is dramatic and all sorts of funky effects happen. �A good analogy to think of is a heavy ball on a rubber sheet. The ball distorts the sheet, so if something light rolls by its path is deflected. �Net result: the light thing curves, which is the normal expected effect of gravity. �Therefore, the key is that the effect of gravity does not emanate from inside the black hole; instead, it comes from the overall warping of spacetime *outside* the hole. ��Assuming people eventually unify gravity with the other forces, how would this work with the idea that particles (gravitons) carry gravity? �You're right that these particles couldn't come from inside the black hole. �At the moment, people are working with ideas that just outside the event horizon of the black hole is a "brane" that contains particles that transmit gravity. �This would solve the paradox, in principle. �Sorry I can't be clearer about this part, but this is very much an unsolved problem at this time.

�Mainly, you're right, gravity can't be transmitted from inside the hole. Cole Miller 4. I was wondering, and I'm sure you've been asked this before, but I've heard that maybe in the center of the universe there is a black hole, and that's how the Big Bang came about. �Have you ever heard of this? �Anyway, if that is the case, then wouldn't the universe be pulled towards the black hole instead of expanding? ��You have to be careful when you talk about the center of the universe, because in our current understanding there *isn't* any center. �All portions of the universe are equivalent. �There is, however, a *beginning* to the universe (the Big Bang), and at that time the universe was extremely hot and dense; indeed, so hot and dense that it qualified as a "singularity" (infinite density). This term is also used for the central point of a black hole, which may be where you heard the association. �When thinking about the expansion of the universe, an analogy may help. �Imagine painting dots on a balloon, and blowing the balloon up.

�Every dot moves away from every other dot, and no dot is more the "center" of the expansion than any other. �The balloon does, of course, have a center. In our analogy, that center represents not a place but a *time*; the beginning of the universe. �So, all of space expands as time goes on. �A weird concept, but there you have it! ��You are right that the gravity of the universe is working against the expansion of the universe. �However, although this gravity tries to slow down the expansion, there may be enough speed in the expansion to make it go forever; this isn't known yet. �An analogy is when you throw a ball upwards. �Gravity slows it down, and for a real ball will bring it back eventually. �But if you had a super strong arm, you could in principle throw the ball all the way off the Earth! Similarly, we don't know if the gravity of the universe will cause it to recollapse or whether it will expand forever. Cole Miller 5. If we had some rope the length or ten light years how long would it take a black hole to completely suck the rope in? �It depends on how much mass the black hole had.

�If the black hole had the mass of our Sun, it would take a long time, around a hundred million years. �For a really big black hole, with a hundred million times the mass of our Sun (as is thought to exist in the centers of some galaxies), it would take about ten thousand years. �One thing to remember is that at such great distances the gravity of a black hole acts like the gravity of anything else with that mass (e.g., if the Sun were magically turned into a black hole with the Sun's mass, the Earth would orbit as it always does. We'd just get cold without the Sun's radiation!). �The weird effects you read about for black holes happen only very close in. Cole Miller 6. Black holes are supposedly started from a neutron star collapsing on itself so if the center of galaxies are powered by a super massive black hole would it still have been caused by a neutron star? �And if that is the case, how big of a neutron star would it have to be in order to cause a black hole? There are actually several different ways to form black holes.

�One of these is the possibility you mention, that enough matter could pile onto a neutron star or other object, causing it to exceed some critical mass and collapse to form a black hole. �Black holes can also form as stars die (in "supernova" explosions), or from 2 or more objects (stars or neutron stars or black holes) that are orbiting each other and eventually collide, or from other mechanisms early in the life of the universe. Astronomers are actually not sure how supermassive black holes form. �It was not a giant neutron star that collapsed, though . . . if a neutron star is bigger than a certain size, it can't exist--it will collapse to form a black hole. �The size at which it will become a black hole is much smaller than the size of a supermassive black hole. �It is possible that the supermassive black holes started out small, however. �As time passed, other stars and gas joined with the black hole to make it larger, until it became the huge black hole that we observe now. Dave Rupke 7. Has there been a case where a black hole has sucked it self in? And is there a possible way that could happen?

A black hole can't suck itself in, but it can suck in other black holes (as well as sucking in gas, dust and stars). When a black hole swallows another black hole (or anything else) it grows larger, and this is one of the ways to grow super-massive black holes that are millions of times more massive than our sun, and often found in the centers of galaxies. In addition, black hole mergers create "gravity waves" or ripples in space that spread out from the black hole and can be observed on earth (in principle, at least -- experiments sensitive enough to measure them are only just being built). Andy Young 8. I was just wondering where the matter goes when it is sucked in a black hole? �Also where does a white hole get its matter? �Are these related in any way? Any matter that disappears across the event horizon of a black hole makes the hole more massive and larger. �Inside the event horizon, the matter probably keeps on falling towards the very center of the black hole. �What happens when it gets to the center, we don't really know.

Several years ago, it was thought that matter reaching the center of a black hole might reappear somewhere else in the Universe by being thrown out of a "white hole". �No good evidence for white holes has been found, and they might not exist. Neal Turner. 9. Do white holes exist? Thank you for your interesting question about black holes. As you mentioned in your question, the gravitational pull of a black hole is so strong that anything that gets too close to it cannot escape. The "event horizon" is the closest you can get to the black hole and still escape (but it gets much more difficult to escape as you get close to the event horizon). If you get to the horizon or cross it you can't escape -- even if you travel at the speed of light (and that's the highest speed possible). Since nothing can escape from the event horizon we can never see what is going on inside -- a form of "cosmic censorship". If you were unlucky enough to fall into a very big black hole you might drift past the event horizon without too much trouble (although it would be impossible to go back), and you'd find yourself falling towards the "singularity" at the center.

As you get closer to the singularity the strong gravity would try to stretch you out like spaghetti, and when you reached the singularity you'd be squashed. Unfortunately the laws of physics as we know them don't work at the singularity (when everything gets squashed to a point). There is a lot of ongoing research to figure out a new theory of gravity that explains what happens at the singularity. Things may not be so bad, however, if the black hole is spinning. There is a theory that a spinning black hole may have an "Einstein-Rosen" bridge at the center (a bit like a tunnel) that leads from the center of a black hole to the center of a "white hole" in another universe. So, with a very strong spaceship you might be able to enter a black hole here and emerge from a white hole in another universe. A "white hole" is the opposite of a black hole, and has no "event horizon" so you would be able to see the singularity at the center -- and we'd be able to view something that our physics theories cannot explain. If they do exist, white holes should be easy to detect, but they have never been seen.

Most physicists think what white holes do not exist, and that something falling into a black hole gets squashed at the singularity and the black hole grows a little larger. Andy Young. 10. One question that I've always wondered about is, what would happen if two black holes or neutron stars become close together? If two black holes orbit each other (perhaps because their progenitor stars orbited each other), then they can gradually spiral closer and closer until they merge. �The reason has to do with general relativity. In general relativity, we imagine spacetime as a rubber sheet that is flat in the absence of any masses. �If you add a mass, then spacetime is warped, in the same way that a rubber sheet will be dented if you add a bowling ball. �Now imagine that you have two bowling balls nearby, circling around each other. �They will produce ripples in the sheet that carry away energy, and the bowling balls will get closer together until they hit. �Similarly, two black holes (or any other two masses) that orbit each other create ripples in spacetime called gravitational waves. These ripples carry away energy, and cause the black holes to get closer until they hit. What is the result?

�A single, big black hole! �Black holes can merge in this way, but they can never split apart. �There are now several instruments that exist or are planned that will look for the signs of the ripples in spacetime. �These will be very tiny but might be detectable. Two neutron stars orbiting each other will also spiral closer together, for the same reason. �However, since neutron stars have surfaces, when they hit they might produce more than gravitational radiation. �In fact, some people think that this will produce a burst of radiation that might be seen in the gamma rays. �It is likely that the merged remnant of the two neutron stars will be too heavy to remain as a neutron star, and will then collapse into a black hole. Cole Miller 11. One day in class Mr. Edwards stated that Dr. Packer calculated when 2 singularities are close enough together, your able to pass through safely. Could that ever be a possibility? Your question relates to wormholes: is it possible to have a spacetime in which you can not only go through the throat of what looks like a black hole, but do it safely?

Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity. Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working). �In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless technojargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity. �Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely. �The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative.

�This isn't quite as impossible as it sounds, but it still isn't all that likely! By the way, one tricky thing about wormholes, and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity). �Therefore, if you have a wormhole you can time-travel. �This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox. �In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person. �I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother. �Since he's dead, he can't produce my father, who can't produce me, which means that I could not have built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture!

�Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems. Cole Miller 12. I've heard it said that whenever interstellar matter reaches or touches a black hole's event horizon, it is known to "belch" light and radiation. �What does this mean and why does it happen? �Also, if nothing can travel at the speed of light except light, how can a black hole pull light into itself? It sounds like you may be confusing a few things about black holes: they don't actually "belch" radiation, because nothing can escape from a black hole (in a reasonable amount of time, anyway). �Infalling material typically forms an accretion disk around the event horizon, similar to water spiraling down the drain. �This material loses energy as it spirals around the disk closer to the hole, and radiates this lost energy away. �The amount of emission increases dramatically as one nears the event horizon, because This is the region where the gravitational potential well of the black hole is the steepest, so material falls in faster and loses more energy as a result.

You're right: nothing with mass can move at speeds at or greater than light, as a result of special relativistic effects. �However, black holes can still "pull in" light if it gets within the event horizon. �This is because by definition, the event horizon is the distance away from the central singularity that one would have to be for a black hole of a given mass to have an escape velocity equal to the speed of light. �So something would have to be traveling faster than c to escape the gravity of the hole at that distance! �This is not possible, since light moves at a maximum velocity of c, so even light will get pulled in when it's this close. �The escape velocity of a black hole is sqrt(2GM/c^2), where G is Newton's gravitational constant and M is the mass of the black hole. �So as the mass of the hole increases, so does the velocity an object would need to escape it. -Laura Brenneman 13. What are the difference between Black holes and White holes? We need to think about black holes before we talk about white holes. How do we know black holes exist? Well, we have two kinds of proof.

� Firstly, we have real, hard evidence from telescopes. Telescopes have been used to observe light emitting by material in orbit around something dark (that we can't see with telescopes) and very heavy. These dark heavy objects seem to be very small relative to objects of similar or greater mass that we can see. These dark, massive objects are probably black holes. Secondly, we have a really good theory of gravity, which has been well-tested, which predicts that we should see such objects in the universe. So where do white holes come in? Well, the theory of gravity that we have (Einstein's General Relativity), predicts black holes as a solution to a set of equations. Solutions to Einstein's equations can have a "+" (plus or 'positive') �or "-" (minus or 'negative') sign in front of them. We see this sort of thing all the time in mathematics. The solution to y*y=9 can be y=+3 or y=-3. Depending on your choice of sign in Einstein's equations, you get a black hole which grows by sucking in matter from far away. Or you get the opposite, a "white hole". It's a black hole running backwards in time.

What would a white hole look like? Well, if you had a camera recording a black hole sucking in matter, and could then play the recording in reverse, you'd see a white hole. You'd see matter being blown out of the 'white' hole and the hole would gradually decrease in size as it spits out matter. But are white holes real? Well, just because an equation that describes the world has a 'negative' solution as well as a 'positive' solution, doesn't mean that it's the correct description of things. If I tell you that I have the square of the number of baseball tickets that I have is 9, that means that either I have three tickets or minus three tickets. But minus three baseball tickets doesn't make sense. There's no such thing. There are plenty of examples of equations in dynamics where there are both positive and negative solutions in time, but only one describes the real world. This is because we can see just by looking around us that time moves in one direction. Time never seems to reverse itself and replay an hour, or a day. So we think that the white hole solution to Einstein's equations is one of these solutions that don't apply in the real world.

It's like having minus three baseball tickets...it just doesn't happen. The final word goes to observations, since you can have all the pretty equations you want, but they have to agree with what we see otherwise they're useless. So far observations tell us that we're right. We see a lot of good evidence for black holes, but none so far, for white holes. Barry McKernan 14. I know that the Black Hole has a great gravitational field. Everything will be disappearing if they go through the Black Hole. What would happen if the Sun went through the Black Hole? The Sun also has a great gravitational field. Would the Sun disappear? If the Sun went through a black hole, it would disappear into the black hole just like anything else. When anything (light, a space ship, even the Sun) comes too close to a black hole, the gravity of the black hole is so strong that the object cannot escape, even if it traveled at the speed of light! The distance from a black hole inside of which nothing can escape is called the event horizon. If you flew a space ship close to a black hole, you could remain safely in orbit around the black hole so long as you flew outside of the event horizon.

However, if you went too close and passed inside the event horizon, there would be no escape! Similarly, if the Sun got so close to a black hole that it was inside the event horizon, it would be unable to escape and would disappear. If you want to learn more about black holes or most anything else in physics, astronomy, chemistry, or math, check out this website: Matthew Knight 15. If we could control a singularity, it's placement, size, and/or destination, what do you suppose we would do with it? First, it's useful to remember that most people think there is a form of "cosmic censorship" that means one cannot have a "naked" singularity. That means that a real singularity in the universe will have an event horizon around it, so that (1) you can't see the singularity directly, and (2) what you really have is a black hole. If, therefore, one could control a black hole, then a constructive use would be to generate tremendous amounts of energy. �Gas spiraling into a black hole from far away will typically release about 10% of its mass-energy before finally falling into the event horizon.

�The reason is that the gas is forced to move really fast, so that it experiences friction with itself and heats up, therefore radiating. �This is about 10-20 times as much energy per mass as is released by nuclear fusion, and applies to any matter at all. �That is, you could imagine throwing garbage at the black hole (maybe toxic waste!) and then using the energy generated in that way to power civilization. However, I'm afraid that if the technology existed to control a black hole in the way you mentioned, it would be used for military purposes... Cole Miller 16. On what evidence does it suggest that black holes can be warp holes? I'm not aware of anything called a "warp hole" outside of Star Trek :). �Perhaps you were thinking of wormholes? �If so, let me describe what those are and what people think of them. Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity.

Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working). �In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless techno jargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity. �Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely. �The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative. �This isn't quite as impossible as it sounds, but it still isn't all that likely!

By the way, one tricky thing about wormholes and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity). �Therefore, if you have a wormhole you can time-travel. �This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox. �In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person. �I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother. �Since he's dead, he can't produce my father, who can't produce me, which means that I could not have built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture! �Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems.

Cole Miller 17. When matter is sucked in, it must be destroyed because black holes are only a point but the Conservation of Mass theory says that matter can't be destroyed, it must be converted into another form of matter. �What does the matter in a black hole convert to? Black holes also have x-ray jets that are shot out from the middle of it. Is it possible that this is what becomes of the matter? �Thanks for your knowledge. At the simplest level, one can say that what is really conserved is mass-energy, rather than just matter. �That is, for example, that when a nuclear reaction occurs the energy that is released means that the mass left behind is a little less to compensate. That's what Einstein's equation E=mc^2 means, where E is energy, m is mass, and c^2 is the speed of light squared. �Therefore, you can look at this as the matter being converted into gravitational energy. �Another way of putting this is that like the Cheshire Cat in Alice in Wonderland (which disappeared but left its smile behind), matter falling into a black hole disappears but leaves its gravity behind. However, at a deeper level we really don't know what happens.

�The problem is that according to our current theories, the matter would go to the center of the black hole, where at the singularity it would reach infinite density. �Now, infinities in one's theory usually mean that something has gone wrong. �Maybe it turns into some kind of super dense quantum nugget, but we'll need extensions to our current theories to have a better idea. The jets of matter you mentioned are another interesting phenomenon. The matter in the jets never gets inside the black hole; in fact, in practice, nothing that goes inside a black hole ever comes out again. �However, the rapid motions near (but outside!) a black hole produce huge amounts of energy, so some of the matter can be flung away in a jet. �Along with two of my colleagues (Chris Reynolds and Anita Krishnamurthi), I have an article in the April Sky and Telescope magazine (page 42) talking about these jets, if you're interested. Cole Miller 18. My question is what do scientists think is inside of a black hole or on the other side? Or is there anything that could have possible survived that extreme conditions of the gravitational pull throughout the black hole?

I'm a big science fiction fan, and I'd love it if it were possible to go through a black hole to travel across the universe. �But in reality it almost certainly isn't possible. �Sorry about that :). Specifically, as we understand it now, if you fall into a black hole you are guaranteed to hit the center, which is called the singularity. At the singularity you would be crushed into a ball of almost infinite density, which would destroy anything, even atoms, protons, or quarks. Some people have considered the idea of a "wormhole", where you would never get to that density, but to make a long story short it appears that such things probably can't exist in reality. �Oh, well! Cole 19. Is there a radius around a black hole where light can flow around? If I understand your question, you're asking whether there exists a radius at which light can have a circular orbit about a black hole. �The answer is yes, but the circling photons are subject to a few constraints.

Firstly, the photons must keep to their trajectories VERY precisely, since the photon sphere is really just a very precise, spherically symmetric radius at which objects moving at the speed of light (i.e. only photons) can orbit a black hole. �If a photon interacts with another photon and changes its trajectory it can easily get flung out of the system or plunge into the black hole. �So it is important to note that photons are not stable in this orbit for very long! Secondly, the nature of the photon sphere depends on the spin of the black hole. �The easiest case is for a non-rotating hole: here the radius of the photon sphere is 1.5 times the radius of the event horizon (2GM/c^2). Things get a bit more complex if the black hole is spinning: in this case, there are actually two photon spheres-- one that rotates with the hole (closer in), and one that rotates in the opposite direction as the hole (further out). �The faster the hole spins, the more distance exists between the two photon spheres.

�So photons with the right trajectory that orbit in the same direction as the black hole are temporarily stable in the inner photon sphere, and photons with the right trajectory that orbit in the opposite direction are temporarily stable in the outer photon sphere. �It also helps if the photons in question come in on equatorial trajectories with respect to the black hole... things get a bit more complex if they do not I've included a few links that should help you visualize all this. �Check out the movies in particular! Black Holes: Facts Black Holes: Facts, Theory & Definition By Nola Taylor Redd, Space.com Contributor | April 9, 2015 05:47pm ET MORE Black holes are some of the strangest and most fascinating objects found in outer space. They are objects of extreme density, with such strong gravitational attraction that even light cannot escape from their grasp if it comes near enough. Albert Einstein first predicted black holes in 1916 with his general theory of relativity . The term "black hole" was coined in 1967 by American astronomer John Wheeler , and the first one was discovered in 1971.

There are three types: stellar black holes, supermassive black holes and intermediate black holes. Stellar black holes — small but deadly When a star burns through the last of its fuel, it may find itself collapsing. For smaller stars, up to about three times the sun's mass, the new core will be a neutron star or a white dwarf. But when a larger star collapses, it continues to fall in on itself to create a stellar black hole . Black holes formed by the collapse of individual stars are (relatively) small, but incredibly dense. Such an object packs three times or more the mass of the sun into a city-size range. This leads to a crazy amount of gravitational force pulling on objects around it. Black holes consume the dust and gas from the galaxy around them, growing in size. According the Harvard-Smithsonian Center for Astrophysics , "the Milky Way contains a few hundred million" stellar black holes. Supermassive black holes — the birth of giants Small black holes populate the universe, but their cousins, supermassive black holes, dominate. Supermassive black holes are millions or even billions of times as massive as the sun, but have a radius similar to that of Earth's closest star.

Such black holes are thought to lie at the center of pretty much every galaxy, including the Milky Way . Scientists aren't certain how such large black holes spawn. Once they've formed, they can easily gather mass from the dust and gas around them, material that is plentiful in the center of galaxies, allowing them to grow to enormous sizes. Illustration of a young black hole, such as the two distant dust-free quasars spotted recently by the Spitzer Space Telescope. More photos of black holes of the universe Credit: NASA/JPL-Caltech Supermassive black holes may be the result of hundreds or thousands of tiny black holes that merge together. Large gas clouds could also be responsible, collapsing together and rapidly accreting mass. A third option is the collapse of a stellar cluster, a group of stars all falling together. Intermediate black holes – stuck in the middle Scientists once thought black holes came in only small and large sizes, but recent research has revealed the possibility for the existence of midsize, or intermediate , black holes (IMBHs). Such bodies could form when stars in a cluster collide in a chain reaction. Several of these forming in the same region could eventually fall together in the center of a galaxy and create a supermassive black hole.

In 2014, astronomers found what appeared to be an intermediate-mass black hole in the arm of a spiral galaxy. "Astronomers have been looking very hard for these medium-sized black holes," co-author Tim Roberts, of the University of Durham in the United Kingdom, said in a statement . "There have been hints that they exist, but IMBH's have been acting like a long-lost relative that isn't interested in being found." Black hole theory — how they tick Black holes are incredibly massive, but cover only a small region. Because of the relationship between mass and gravity, this means they have an extremely powerful gravitational force. Virtually nothing can escape from them — under classical physics, even light is trapped by a black hole. Such a strong pull creates an observational problem when it comes to black holes — scientists can't "see" them the way they can see stars and other objects in space. Instead, scientists must rely on the radiation that is emitted as dust and gas are drawn into the dense creatures. Supermassive black holes, lying in the center of a galaxy, may find themselves shrouded by the dust and gas thick around them, which can block the tell-tale emissions.

Sometimes as matter is drawn toward a black hole, it ricochets off of the event horizon and is hurled outward, rather than being tugged into the maw. Bright jets of material traveling at near-relativistic speeds are created. Although the black hole itself remains unseen, these powerful jets can be viewed from great distances. Black holes are strange regions where gravity is strong enough to bend light, warp space and distort time. [ See how black holes work in this SPACE.com infographic . Credit: Karl Tate, SPACE.com contributor Black holes have three "layers" — the outer and inner event horizon and the singularity. The event horizon of a black hole is the boundary around the mouth of the black hole where light loses its ability to escape. Once a particle crosses the event horizon, it cannot leave. Gravity is constant across the event horizon. The inner region of a black hole, where its mass lies, is known as its singularity , the single point in space-time where the mass of the black hole is concentrated. Under the classical mechanics of physics, nothing can escape from a black hole. However, things shift slightly when quantum mechanics are added to the equation.

Under quantum mechanics, for every particle, there is an antiparticle, a particle with the same mass and opposite electric charge. When they meet, particle-antiparticle pairs can annihilate one another. If a particle-antiparticle pair is created just beyond the reach of the event horizon of a black hole, it is possible to have one drawn into the black hole itself while the other is ejected. The result is that the event horizon of the black hole has been reduced and black holes can decay, a process that is rejected under classical mechanics. Scientists are still working to understand the equations by which black holes function. Interesting facts about black holes If you fell into a black hole, theory has long suggested that gravity would stretch you out like spaghetti, though your death would come before you reached singularity. But a 2012 study in Nature suggests that quantum effects would cause the event horizon to act much like a wall of fire, instantly burning anyone to death. Black holes do not "suck." Suction is caused by pulling something into a vacuum, which the massive black hole definitely is not. Instead, objects fall into them. The first object considered to be a black hole is Cygnus X-1 .

Rockets carrying Geiger counters discovered eight new X-ray sources. In 1971, scientists detected radio emissions coming from Cygnus X-1, and a massive hidden companion was found and identified as a black hole. Cygnus X-1 was the subject of a 1974 friendly wager between Stephen Hawking and a fellow physicist Kip Thorne , with Hawking betting that the source was not a black hole. In 1990, he conceded defeat. [ VIDEO: Final Nail in Stephen Hawking's Cygnus X-1 Bet? ] Miniature black holes may have formed immediately after the Big Bang. Rapidly expanding space may have squeezed some regions into tiny, dense black holes less massive than the sun. If a star passes too close to a black hole, it can be torn apart . Astronomers estimate there are anywhere from 10 million to a billion stellar black holes, with masses roughly thrice that of the sun, in the Milky Way. The interesting relationship between string theory and black holes gives rise to more types of massive giants than found under conventional classical mechanics. Black holes remain terrific fodder for science fiction books and movies.

Check out the science behind the movie " Interstellar ," which relied heavily on theoretical physicist Kip Thorne to bring real science to the Hollywood feature. In fact, work with the special effects of the blockbuster lead to an improvement in the scientific understanding of how distant stars might appear when seen near a fast-spinning black hole. Additional resources Parts of a black hole | Article about Parts of a black hole by The Free Dictionary Parts of a black hole | Article about Parts of a black hole by The Free Dictionary Related to Parts of a black hole: Ergosphere black hole, in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse (the final stage in the life history of certain stars; see stellar evolution stellar evolution, life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration. ..... Click the link for more information. ) by the American physicist John A.

Wheeler Wheeler, John Archibald, 1911–2008, American physicist and educator, b. Jacksonville, Fla. Educated at Johns Hopkins (Ph.D., 1933), he joined the faculty at Princeton in 1938, and after 1976 was director of the Center for Theoretical Physics at the Univ. ..... Click the link for more information. . Gravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure pressure, in mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid. ..... Click the link for more information. , that supports the star against the compressive force of its own gravitation gravitation, the attractive force existing between any two particles of matter. The Law of Universal Gravitation Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called ..... Click the link for more information. .

As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf white dwarf, in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be easily ..... Click the link for more information. , while a more massive one would become a supernova supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than ..... Click the link for more information. . If the mass is less than three times that of the sun, it will then form a neutron star neutron star, extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933. ..... Click the link for more information.

. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer Oppenheimer, J. Robert , 1904–67, American physicist, b. New York City, grad. Harvard (B.A., 1925), Ph.D. Univ. of Göttingen, 1927. He taught at the Univ. of California and the California Institute of Technology from 1929 (as professor from 1936) until his appointment ..... Click the link for more information. and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the "singularity." At the point of singularity the effects of Einstein's general theory of relativity relativity, physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference. ..... Click the link for more information. become paramount.

According to this theory, space becomes curved in the vicinity of matter; the greater the concentration of matter, the greater the curvature. When the star (or supernova remnant) shrinks below a certain size determined by its mass, the extreme curvature of space seals off contact with the outside world. The place beyond which no radiation can escape is called the event horizon, and its radius is called the Schwarzschild radius after the German astronomer Karl Schwarzschild, who in 1916 postulated the existence of collapsed celestial objects that emit no radiation. For a star with a mass equal to that of the sun, this limit is a radius of only 1.86 mi (3.0 km). Even light cannot escape a black hole, but is turned back by the enormous pull of gravitation. It is now believed that the origin of some black holes is nonstellar. Some astrophysicists suggest that immense volumes of interstellar matter interstellar matter, matter in a galaxy between the stars, known also as the interstellar medium. Distribution of Interstellar Matter Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the interstellar matter ..... Click the link for more information.

can collect and collapse into supermassive black holes, such as are found at the center of large galaxies. The British physicist Stephen Hawking Hawking, Stephen William, 1942–, British theoretical physicist, b. Oxford, England, grad. University College, Oxford, 1962, Ph.D. Trinity Hall, Cambridge, 1966. In 1962 Hawking was diagnosed as having an incurable muscular disease, amyotrophic lateral sclerosis (ALS). ..... Click the link for more information. has postulated still another kind of nonstellar black hole. Called a primordial, or mini, black hole, it would have been created during the "big bang," in which the universe was created (see cosmology cosmology, area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories ..... Click the link for more information. ). Unlike stellar black holes, primordial black holes create and emit elementary particles elementary particles, the most basic physical constituents of the universe. Basic Constituents of Matter Molecules are built up from the atom, which is the basic unit of any chemical element.

The atom in turn is made from the proton, neutron, and electron. ..... Click the link for more information. , called Hawking radiation, until they exhaust their energy and expire. It has also been suggested that the formation of black holes may be associated with intense gamma ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation gamma radiation, high-energy photons emitted as one of the three types of radiation resulting from natural radioactivity. It is the most energetic form of electromagnetic radiation, with a very short wavelength (high frequency). ..... Click the link for more information. . Also released is longer-lasting electromagnetic radiation electromagnetic radiation, energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field. ..... Click the link for more information. in the form of X rays X ray, invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light.

The wavelength range for X rays is from about 10−8 m to about 10−11 ..... Click the link for more information. , radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance. Because light and other forms of energy and matter are permanently trapped inside a black hole, it can never be observed directly. However, a black hole can be detected by the effect of its gravitational field on nearby objects (e.g., if it is orbited by a visible star), during the collapse while it was forming, or by the X rays and radio frequency signals emitted by rapidly swirling matter being pulled into the black hole. The first discovery (1971) of a possible black hole was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope Hubble Space Telescope (HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P.

Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km) ..... Click the link for more information. announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. Since then others have been found, and in 2011 astronomers announnced the discovery of one, in NGC 4889 in the constellation Coma, whose mass may be as great as 21 billion times that of the sun. The first evidence (2002) of a binary black hole, two supermassive black holes circling one another, was detected in images from the orbiting Chandra X-ray Observatory. Located in the galaxy NGC6240, the pair are 3,000 light years apart, travel around each other at a speed of about 22,000 mph (35,415 km/hr), and have the mass of 100 million suns each.

As the distance between them shrinks over 100 million years, the circling speed will increase until it approaches the speed of light, about 671 million mph (1,080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe. The Chandra observatory has also discovered that massive black holes were associated with galaxies that existed 13 billion years ago. Bibliography See S. W. Hawking, Black Holes and Baby Universes and Other Essays (1994); P. Strathern, The Big Idea: Hawking and Black Holes (1998); J. A. Wheeler, Geons, Black Holes, and Quantum Foam: A Life in Physics (1998); H. Falcke and F. W. Hehl, The Galactic Black Hole: Studies in High Energy Physics, Cosmology and Gravitation (2002); M. Bartusiak, Black Hole (2015). black hole An object so collapsed that its escape velocity exceeds the speed of light.

It becomes a black hole when its radius has shrunk to its Schwarzschild radius and light can no longer escape. Although gravity will make the object shrink beyond this limit, the ‘surface’ having this critical value of radius – the event horizon – marks the boundary inside which all information is trapped. Calculations, however, indicate that space and time (see spacetime ) become highly distorted inside the event horizon, and that the collapsing object's ultimate fate is to be compressed to an infinitely dense singularity at the center of the black hole. It has also been shown that the distortion of spacetime just outside the Schwarzschild radius causes the production of particles and radiation that gradually rob the black hole of energy and thus slowly diminish its mass. This Hawking radiation (Stephen Hawking, 1974) depends inversely on the black hole's mass, so the most massive black holes ‘evaporate’ most slowly. A black hole of the Sun's mass would last 1066 years. Once matter (or antimatter) has disappeared into a black hole, only three of its original properties can be ascertained: the total mass, the net electric charge, and the total angular momentum.

Because all black holes must have mass, there are four possible types of black hole: a Schwarzschild black hole (1916) has no charge and no angular momentum; a Reissner–Nordstrom black hole (1918) has charge but no angular momentum; a Kerr black hole (1963) has angular momentum but no charge; a Kerr-Newman black hole (1965) has charge and angular momentum. (The dates in brackets indicate when the named mathematician(s) solved the equations of general relativity for these particular cases.) In astrophysics the simple Schwarzschild solution is often used, but real black holes are almost certainly rotating and have very little electric charge, so that the Kerr solution should be the most applicable. The most promising candidates for black holes are massive stars that explode as supernovae , leaving a core in excess of 3 solar masses. This core must undergo complete gravitational collapse because it is above the stable limit for both white dwarfs and neutron stars . Once formed, a black hole can be detected only by its gravity.

Finding black holes only a few kilometers across (the size of the event horizon for a single-star black hole) is exceedingly difficult, but chances are increased if the black hole is a member of a close binary system. If the components of a binary system are close enough, mass transfer can occur between the primary star and its more compact companion (see equipotential surfaces ). Matter will not fall directly on to the companion, however, for it has too much angular momentum; instead it forms a rapidly spinning disk – an accretion disk – around the compact object. If the latter is a black hole considerable energy can be produced, predominantly at X-ray wavelengths, as matter in the accretion disk loses angular momentum and spirals in. When the accreting matter is unable to cool efficiently, most of the energy generated in the disk due to viscosity is not radiated away, but instead stored in the gas as internal energy and advected inward in an advection-dominated accretion flow onto the central object. Candidates for black holes in a binary system fall into two classes, massive and low-mass black-hole binaries, depending on the mass of the companion star.

The first massive black-hole candidate to be identified was the X-ray binary Cygnus X-1 , comprising a 20 solar mass B0 supergiant accompanied by an invisible companion with a mass about 10 times that of the Sun. This massive nonluminous object is probably a black hole emitting X-rays from its accretion disk. Another promising massive black-hole candidate is the X-ray binary LMC X-3 in the Large Magellanic Cloud. While the probable mass of the compact objects in this system is of the order of 10 solar masses, rigorous lower limits are as low as 3 solar masses (possibly even lower), marginally consistent with the maximum theoretically predicted mass of a neutron star . Even better evidence for a stellar-mass black hole comes from some of the low-mass black-hole candidates, in particular A0620-00 in Monoceros and V404 Cygni. Rigorous limits on the masses of the compact objects in A0620-00 and V404 Cygni are 3 and 6 solar masses, respectively. See also gamma-ray transients .

Supermassive black holes of 106 to 109 solar masses probably lie in the centers of some galaxies and give rise to the quasar phenomenon and the phenomena of other active galaxies . If a huge black hole is able to form and to capture sufficient gas and/or stars from its surroundings, the rest-mass energy of infalling material can be converted into radiation or energetic particles. There is now observational evidence to support this hypothesis. the dynamical motion of stars and ionized gas in the cores of nearby galaxies show that they are responding to a strong gravitational field, in excess of that expected from the number of stars accounting for the light at the galaxy center. These motions are thought to be due to the presence of the supermassive black hole (see Seyfert galaxy ; Virgo A ). Other strong evidence for the presence of supermassive black holes is found from recent observations of an extremely broad iron fluorescence line in the X-rays from several Seyfert galaxies ; this line has a very particular shape, revealing its origin in an accretion disk very close to the central black hole.

At the other mass extreme are the more speculative mini black holes, weighing only 1011 kg and with radii of 10–10 meters. These could have formed in the highly turbulent conditions existing after the Big Bang. They create such intense localized gravitational fields that their Hawking radiation makes them explode within the lifetime of the Universe; their final burst of gamma rays and microwaves should be detectable but has not yet been found. Black Hole a celestial object that is formed as a result of the relativistic gravitational collapse of a massive body. In particular, the evolution of a star whose mass at the moment of collapse exceeds some critical value may terminate in catastrophic gravitational collapse. The value of the critical mass is not precisely determined and, depending on the equation of state of matter used, ranges from 1.5 to 3 solar masses (Mʘ). For any equation of state of matter, the general theory of relativity predicts that no stable equilibrium exists for cold stars of several solar masses.

If, after a star becomes unstable, not enough energy is released to halt the collapse or to cause a partial explosion after which the remaining mass would be less than the critical mass, the central portions of the star collapse and, in a short time, reach the gravitational radius rg. No forces whatsoever can prevent the further collapse of a star if the radius of the star shrinks down to rg, which is also known as the Schwarzschild radius and is the radius of a sphere whose surface is called the event horizon. A fundamental property of the event horizon is that no signals emitted from the surface of the star and reaching the event horizon can escape from the region inside that horizon. Thus, as a result of the gravitational collapse of a massive star, a region in space-time is formed from which no information whatsoever about physical processes occurring within the region can emerge. A black hole has a gravitational field whose properties are determined by the hole’s mass, angular momentum, and—if the collapsing star was electrically charged—electric charge. At large distances, the gravitational field of a black hole is virtually indistinguishable from the gravitational field of a normal star.

In addition, the motion of other objects that interact with a black hole at large distances is governed by the laws of Newtonian mechanics. Calculations show that a region known as the ergosphere, which is bounded by a surface called the static, or stationary, limit, should exist outside the event horizon of a rotating black hole. The attractive force that a black hole exerts on a stationary object situated in the ergosphere tends to infinity. However, the attractive force is finite if the object has an angular momentum whose direction coincides with that of the black hole’s angular momentum. Therefore, any particles that happen to be in the ergosphere will revolve around the black hole. The presence of an ergosphere may lead to energy losses by a rotating black hole. In particular, energy losses are possible in the case where some object that has entered the ergosphere breaks up (for example, as a result of an explosion) into two fragments near the event horizon of the black hole. In this case, one of the fragments continues to fall into the black hole, but the other fragment escapes from the ergosphere. The parameters of the explosion may be such that the energy of the fragment that escapes from the ergosphere is higher than the energy of the original object.

The additional energy in this case is drawn from the rotational energy of the black hole. As the angular momentum of a rotating black hole decreases, the static limit comes closer to the event horizon; when the angular momentum is zero, the static limit and the event horizon coincide and the ergosphere disappears. Owing to the effects of the centrifugal force of rotation, the rapid rotation of a collapsing object prevents the formation of a black hole. Therefore, a black hole cannot have an angular momentum that is greater than some extreme value. Quantum-mechanical calculations show that particles—such as photons, neutrinos, gravitons, and electron-positron pairs—may be produced in the strong gravitational field of a black hole. As a result, a black hole radiates like a blackbody with an effective temperature of T = 10–6 (Mʘ/M) °K, where M is the mass of the black hole, even in cases where no matter whatsoever falls into the hole. The energy of the radiation is drawn from the energy of the black hole’s gravitational field; as a result, the mass of the black hole decreases with time.

However, owing to their low efficiency, the quantum radiation processes are unimportant for massive black holes, which are formed as a result of stellar collapse. In the early stages of the evolution of the universe, which were hot and ultradense, black holes with masses ranging from 10–5 g to a solar mass or higher may have been formed as a result of an inhomogeneous distribution of matter. In contrast to the black holes that are formed by collapsed stars, these objects are called primordial black holes. Since quantum radiation processes reduce the mass of a black hole, all primordial black holes with a mass of less than 1015 g should have evaporated by the present time. The intensity and effective temperature of black-hole radiation increase as the mass of a black hole decreases. Therefore, in the final stage, the evaporation of a black hole with a mass of the order of 3 × 109 g would be an explosion accompanied by an energy release of 1030 erg in 0.1 sec. Primordial black holes with a mass of greater than 1015 g have remained virtually unchanged.

The detection of primordial black holes on the basis of their radiation would make it possible to draw important conclusions about the physical processes that occurred in the early stages of the evolution of the universe. The search for black holes in the universe is a task of current interest in modern astronomy. Searches are carried out on the assumption that black holes may be the invisible components of certain binary star systems. However, this inference is not definite, since the normal star in a binary system may be invisible against the higher luminosity of the second component. Another method of identifying black holes in binary systems is based on the radiation emitted by matter flowing from the companion, which is a normal star, to the black hole. In this case, a disk consisting of matter flowing to the black hole is formed near the hole; the layers of the disk move around the hole with various velocities (see Figure 1). Owing to friction between adjacent layers, the matter in the disk is heated to tens of millions of degrees. The inner regions of the disk emit energy in the X-ray region of the electromagnetic spectrum. The same type of radiation is produced in the case where a binary system contains a neutron star rather than a black hole.

However, a neutron star cannot have a mass higher than some limiting value. As a result of space studies, a large number of X-ray sources in binary systems have been discovered. The X-ray source Cygnus X-l is the most likely candidate for a black hole. In this binary system, the mass of the X-ray source, which may be estimated from the observed orbital velocity of the optical star and from Kepler’s laws, exceeds 5 Mʘ, that is, is higher than the limiting mass for a neutron star. Figure 1. A black hole in a binary star system The hypothesis has also been advanced that supermassive black holes—that is, black holes with a mass M ≃ 106–108Mʘ—may be located in the nuclei of active galaxies and in quasars. In this case, the activity of the active galactic nuclei and quasars is attributed to the infall of ambient gas onto the black hole. REFERENCES What Are Black Holes and How Do They Form? By John P. Millis, Ph.D Updated December 16, 2016. What Is a Black Hole?

Chances are, you've come across the idea of a black hole in science fiction books and movies, but these objects exist in real life too, and are just as fascinating. Black holes are objects in the universe so massive and with such strong gravity that nothing can escape them once trapped in their gravitational fields. In fact, most black holes contain many times the mass of our Sun. But even though they are quite massive, an actual singularity that forms the core of the black hole has never been seen or imaged. We do know they are there by the effects they have on the matter surrounding them. And we are able to see the material that they suck in through their tremendous gravitational pull. The Structure of a Black Hole The basic "building block" of the black hole is that singularity. It's a pinpoint region of space that contains all the mass of the black hole. Then, there is the region of space surrounding the black hole from where light can not escape, giving the "black hole" its name. continue reading below our video Test Your General Science Knowledge The "edge" of this region is called the "event horizon". This is the invisible boundary where the pull of the gravitational field is equal to the speed of light .

It's where gravity and light speed are balanced. The event horizon's position depends on the gravitational pull of the black hole. You can calculate the location of an event horizon around a black hole using the equation Rs = 2GM/c2. R is the radius of the singularity. G is the force of gravity, M is the mass, c is the speed of light. How Do Black Holes Form? Since there are different types of black holes, the answer to how they form can be complicated. The most common type of black holes are known as stellar mass black holes, and 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 (SMBH) and micro black holes. A single SMBH can contain the mass of millions or billions of suns. Micro black holes are, as their name implies, very tiny. They might have perhaps only 20 micrograms of mass.

In both cases the mechanisms for their creation is not entirely clear. Micro black holes exist in theory, but have not been directly detected. Supermassive black holes are found to exist in the cores of most galaxies and their origins are still hotly debated. It's possible that supermassive black holes are the result of a merger between smaller, stellar mass black holes and other matter . Some astronomers suggest that they might be created when a single highly massive (hundreds of times the mass of the Sun) star collapses. What about those micro black holes? They could 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 affected by the event horizon , we really can't "see" a black hole. However, we can measure and characterize them by the effects they have on their surroundings. Black holes that are near other objects exert a gravitational effect on them.

In practice, astronomers deduce the presence of the black hole by studying how light behaves around it. 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. This is really apparent in galaxy clusters where the combined mass of the clusters, their dark matter AND their black holes create very oddly shaped arcs and rings by bending the light of more distant objects as it passes by. We can also see black holes by the radiation the heated material around them gives off, such as radio or x-rays. 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 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 loses energy, and therefore loses mass (by Einstein's famous equation, E=MC2, where E=energy, M=mass and C is the speed of light). Black Holes | Science Mission Directorate Science Mission Directorate Exoplanets Black Holes Don't let the name fool you: a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City.

The result is a gravitational field so strong that nothing, not even light, can escape. In recent years, NASA instruments have painted a new picture of these strange objects that are, to many, the most fascinating objects in space. Intense X-ray flares thought to be caused by a black hole devouring a star. (Video) Watch the Video Although the term was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole. A video about black holes. Watch the video Scientists can't directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby.

If a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole can tear the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others. One Star's End is a Black Hole's Beginning Most black holes form from the remnants of a large star that dies in a supernova explosion. (Smaller stars become dense neutron stars, which are not massive enough to trap light.) If the total mass of the star is large enough (about three times the mass of the Sun), it can be proven theoretically that no force can keep the star from collapsing under the influence of gravity. However, as the star collapses, a strange thing occurs.

As the surface of the star nears an imaginary surface called the "event horizon," time on the star slows relative to the time kept by observers far away. When the surface reaches the event horizon, time stands still, and the star can collapse no more - it is a frozen collapsing object. Astronomers have identified a candidate for the smallest-known black hole. (Video) Watch the Video Even bigger black holes can result from stellar collisions. Soon after its launch in December 2004, NASA's Swift telescope observed the powerful, fleeting flashes of light known as gamma ray bursts. Chandra and NASA's Hubble Space Telescope later collected data from the event's "afterglow," and together the observations led astronomers to conclude that the powerful explosions can result when a black hole and a neutron star collide, producing another black hole. Babies and Giants Although the basic formation process is understood, one perennial mystery in the science of black holes is that they appear to exist on two radically different size scales. On the one end, there are the countless black holes that are the remnants of massive stars. Peppered throughout the Universe, these "stellar mass" black holes are generally 10 to 24 times as massive as the Sun.

Astronomers spot them when another star draws near enough for some of the matter surrounding it to be snared by the black hole's gravity, churning out x-rays in the process. Most stellar black holes, however, lead isolated lives and are impossible to detect. Judging from the number of stars large enough to produce such black holes, however, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone. On the other end of the size spectrum are the giants known as "supermassive" black holes, which are millions, if not billions, of times as massive as the Sun. Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way. Astronomers can detect them by watching for their effects on nearby stars and gas. This chart shows the relative masses of super-dense cosmic objects. Read the full article Historically, astronomers have long believed that no mid-sized black holes exist. However, recent evidence from Chandra, XMM-Newton and Hubble strengthens the case that mid-size black holes do exist.

One possible mechanism for the formation of supermassive black holes involves a chain reaction of collisions of stars in compact star clusters that results in the buildup of extremely massive stars, which then collapse to form intermediate-mass black holes. The star clusters then sink to the center of the galaxy, where the intermediate-mass black holes merge to form a supermassive black hole.

Question: Which two word term is used to describe the gravitational boundary enclosing a 'Black Hole', from which nothing can escape?
Answer: EVENT HORIZON