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Post by Shan on Jun 26, 2007 1:14:19 GMT
What is a black hole?A black hole is an object that is so compact (in other words, has enough mass in a small enough volume) that its gravitational force is strong enough to prevent light or anything else from escaping. The existence of black holes was first proposed in the 18th century, based on the known laws of gravity. The more massive an object, or the smaller its size, the larger the gravitational force felt on its surface. John Michell and Pierre-Simon Laplace both independently argued that if an object were either extremely massive or extremely small, it might not be possible at all to escape its gravity. Even light could be forever captured. The name "black hole" was introduced by John Archibald Wheeler in 1967. It stuck, and has even become a common term for any type of mysterious bottomless pit. Physicists and mathematicians have found that space and time near black holes have many unusual properties. Because of this, black holes have become a favorite topic for science fiction writers. However, black holes are not fiction. They form whenever massive but otherwise normal stars die. We cannot see black holes, but we can detect material falling into black holes and being attracted by black holes. In this way, astronomers have identified and measured the mass of many black holes in the Universe through careful observations of the sky. We now know that our Universe is quite literally filled with billions of black holes.
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Post by Shan on Jun 26, 2007 1:18:23 GMT
How big is a black holeAll matter in a black hole is squeezed into a region of infinitely small volume, called the central singularity. The event horizon is an imaginary sphere that measures how close to the singularity you can safely get. Once you have passed the event horizon, it becomes impossible to escape: you will be drawn in by the black hole's gravitational pull and squashed into the singularity. The size of the event horizon (called the Schwarzschild radius, after the German physicist who discovered it while fighting in the first World War) is proportional to the mass of the black hole. Astronomers have found black holes with event horizons ranging from 6 miles to the size of our solar system. But in principle, black holes can exist with even smaller or larger horizons. By comparison, the Schwarzschild radius of the Earth is about the size of a marble. This is how much you would have to compress the Earth to turn it into a black hole. A black hole doesn't have to be very massive, but it does need to be very compact! Some black holes spin around an axis, and their situation is more complicated. The surrounding space is then dragged around, creating a cosmic whirlpool. The singularity is an infinitely thin ring instead of a point. The event horizon is composed of two, instead of one, imaginary spheres. And there is a region called the ergosphere, bounded by the static limit, where you are forced to rotate in the same sense as the black hole although you can still escape. In a non-rotating black hole, a central point singularity is surrounded by an imaginary sphere called the event horizon. Its size is called the Schwarzschild radius. In a spinning black hole, a central ring singularity is surrounded by two event horizons, the ergosphere and the static limit.
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Post by janggut on Jun 26, 2007 2:33:02 GMT
every time i think of black holes, i shudder. kinda scary.
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Post by killerzzz on Jun 26, 2007 4:09:20 GMT
I love the subject of black holes, and I've done some for-fun research in the past myself. I'll pull out a few notes. How they are formed-This is the common way it works. When a really big star (about 6 times the mass of our Sun) dies, it can go out in many ways. One way, is that it will explode, supernova (in other words, a really big boom that can wipe out many solar systems). -Next, the remains might then compress into itself from the remenants of the dead star's gravity (sucked into the middle), forming a nutron star (a compressed mass; one that weighs as much as the Sun would only be about 10km across). -Now, if the nutron star collects enough remains to become 3 times the mass of the Sun, it will nolonger be able to control its own gravity, and continuously collapse into itself, thereby creating a black hole. But the 'supergiant' star might not explode first. -If the combustible material (stuff that burns like fuel) in the core of the supergiant star runs out, and if its gravity becomes so massive that its own rays of light will begin to bend back, it will create it's own event horizon (as already explained by Shan). -A star in this condition will simply collapse into itself, and become a black hole. If any of that stuff is wrong, correct me. Its been a while. Killerzzz
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Post by killerzzz on Jun 26, 2007 4:22:08 GMT
every time i think of black holes, i shudder. kinda scary. Well, I have a few things that might make you shudder further. Our Black HoleAs Shan said, there are many many black holes in the universe. But what about in our galaxy. What about good ole Milky Way? Its hard to tell, but there are probably a few. But most importantly, scientists are very sure of at least one: right smack in the center. Thats right, there is a particularily massive black hole, right in the middle of our galaxy. In fact, the entire Milky Way is in orbit around it. Now, want to know something even more usettling? The Death of a Black HoleYes, even a black hole can meet it's end. Sometimes, (I don't even know if we have a recorded occurance, or if its just theory, but...) sometimes, somehow, a black hole will lose just a bit of its mass. When that happens, the event horizon changes and some light escapes. In this way, more mass and more light is continuously able to escape, and the black hole begins to break down. This continues until it completely "evaporates" into nothing. But thats not the bad part. What happens when all this light and mass suddenly bursts out of a tiny space? Lets just say, things get a little explosive. Except, not a little. The result would be a extremely powerful flash of light and a massive drop in temperature. The heat has to go somewhere, doesn't it? The explosion would be immense and devistating. So, the bad: a giant explosion will wipe a truckload of space, right out. Planets and solar systems all say bye-bye. But wait, thats not all. Part 2 of the bad: that black hole that just blew up was an important source of gravity, and alot of stuff revolved around it, even really far away. Remember what I said about our entire galaxy? What happens when that source of gravity fails? The galaxy goes to hell, thats what. Everything changes. If there was life on a planet, good chance you can kiss it goodbye. There, try to sleep at night knowing that. ;D Killerzzz
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Post by ss on Jun 27, 2007 0:32:26 GMT
I fell into a black hole once, when I had to much to drink...but it let me go in the morning... ;D
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Post by janggut on Jun 27, 2007 1:36:12 GMT
ss, that is the kind of black hole i'm worried about. ;D
thanks, killlerzzz. now i can sleep peacefully. ;D
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Post by killerzzz on Jun 27, 2007 4:00:02 GMT
thanks, killlerzzz. now i can sleep peacefully. ;D Well, I wouldn't be satisfied unless I gave you some sweet dreams to go along with that pleasant sleep. ;D First off, I'll say that I won't go onto the topic of white holes and worm holes, because they are so theoretical and unsure that they anger me. Grrr... Instead... What It Would Feel Like, Falling Into A Black HoleYou are in a spaceship (a very special spaceship to have gotten this far ) not too far from a black hole. Well, you're pretty far (over five times the length of its event horizon from the center), but in space thats not alot. Everything is normal, because this far out, its just like being close to any big object in space: the gravity doesn't effect you much. So you get closer. When you reach a distance of five times the length of its event horizon, things start to get wierd. Because at that point, there will be a difference of 1-g in gravity on your head and your feet. That means that you're feeling more gravity on your feet than on your head (or vice-versa). This is called Tidal Force. No worries, you'll only feel a little discomfort. The human body can take up to 15-g difference in gravity between the head and feet without beginning to get really nasty. You're okay. For now. So then, you're curiosity gets the better of you and you go closer. You'll reach that 15-g at two times the length of the event horizon. Now you're really feeling crappy. And no wonder; you're being stretched!! Like spaghetti, in fact. Actually, the technical word for it (and this is no joke) is spaghettification. So, you're getting stretched. This sucks. And things get worse faster as you continue to approach (you're probably in too much pain to change the ship's course). Here, things get a little fuzzy with theories. But it all revolves around this: what happens when you hit the event horizon? Well, lets just go with mathimatical theory number 1: you are dead. In fact, you might never actually reach the event horizon, not just because you get stretched more and more as you get closer, but because time actually slows down as you get closer. According to this theory, time stops at the horizon, and so do you (at that point, an invisible spaghetti noodle). I don't like that theory. The other suggests that you will survive the journey through the event horizon (that is, if you have a ship that can make it that far, if all kinds of radiation doesn't kill you, among other things). Don't worry, you'll get torn apart not long afterwards, maybe seven seconds. But until then... What would you see? What is it like in there? Well, if you look back out from the event horizon, everything will seem normal enough, since light is making its way in. But, if you were to look around you, you'd probably see an impressive display of distortered shapes and colours, because light does not escape the black hole, and is bent awkwardly by it. Don't even think of escaping. First of all, you're way too screwed up for that by now, being stretched and all. And also (just a little problem) you'd have to travel faster than the speed of light (phyisically impossible, btw). So, you're dead. All you can do is wait that short time until you're torn to shreds and crushed into a singularity. Well, that was pretty grim, wasn't it? Nighty-night. Killerzzz
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Post by Shan on Jun 27, 2007 13:17:33 GMT
How are black holes born?A black hole is born when an object becomes unable to withstand the compressing force of its own gravity. Many objects (including our Earth and Sun) will never become black holes. Their gravity is not sufficient to overpower the atomic and nuclear forces of their interiors, which resist compression. But in more massive objects, gravity ultimately wins. Stellar-mass black holes are born with a bang. They form when a very massive star (at least 25 times heavier than our Sun) runs out of nuclear fuel. The star then explodes as a supernova. What remains is a black hole, usually only a few times heavier than our Sun since the explosion has blown much of the stellar material away. We know less about the birth of supermassive black holes, which are much heavier than stellar-mass black holes and live in the centers of galaxies. One possibility is that supernova explosions of massive stars in the early Universe formed stellar-mass black holes that, over billions of years, grew supermassive. A single stellar-mass black hole can grow rapidly by consuming nearby stars and gas, often in plentiful supply near the galaxy center. The black hole may also grow through mergers with other black holes that drift to the galactic center during collisions with other galaxies. Astronomers are actively investigating these and other scenarios through observations and computer simulations. Stellar-mass black holes are created when massive stars explode as supernovae. The image shows a supernova remnant in the constellation Cassiopeia. Collisions of galaxies contribute to the growth of supermassive black holes. The image show the ''Antennae,'' a pair of colliding galaxies.
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Post by Shan on Jun 27, 2007 13:26:25 GMT
How do black holes grow?Black holes grow in mass by capturing nearby material. Anything that enters the event horizon cannot escape the black hole's gravity. So objects that do not keep a safe distance get swallowed. Despite their reputation, black holes will not actually suck in objects from large distances. A black hole can only capture objects that come very close to it. They're more like Venus' Flytraps than cosmic vacuum cleaners. For example, imagine replacing the Sun by a black hole of the same mass. Permanent darkness would fall on Earth, but the planets would continue to revolve around the black hole at the same distance and speed as they do now. None of the planets would be sucked into the black hole. Our Earth would be in danger only if it came within some 10 miles of the black hole, much less than the actual distance of Earth from the Sun (a comforting 93 million miles). The diet of known black holes consists mostly of gas and dust, which fill the otherwise empty space throughout the Universe. Black holes can also consume material torn from nearby stars. In fact, the most massive black holes can swallow stars whole. Black holes can also grow by colliding and merging with other black holes. This growth process is what can reveal the presence of a black hole. As gas falls toward a black hole, it is heated to high temperatures, generating powerful radio waves and X-rays that can be studied by astronomers. Stellar-mass black holes can grow by pulling gas of a companion star that orbits around it. A disk of dust and gas in the galaxy NGC 7052. Supermassive black holes can grow by consuming gas and dust from such disks. Gas that falls into a black hole settles into a so-called accretion disk. Friction and magnetic fields in the disk cause the gas to heat and emit X-rays. Magnetic fields near a spinning black hole can propel electrons outward in a jet along the rotation axis. The electrons produce bright radio waves.
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Post by ss on Jun 27, 2007 16:07:13 GMT
ss, that is the kind of black hole i'm worried about. ;D thanks, killlerzzz. now i can sleep peacefully. ;D Absolutely.... ;D Be wary of such things Jang... ;D
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Post by Shan on Jun 28, 2007 14:28:29 GMT
I fell into a black hole once, when I had to much to drink...but it let me go in the morning... ;D Hey Jags, if ss falls into another black hole, i appoint you to go rescue him. ;D
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