What is a Black hole?

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starialove

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hello and here you can say anything you know about blackholes goodluck
 
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csmyth3025

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starialove":19rkm17y said:
hello and here you can say anything you know about blackholes goodluck
If your looking for information about black holes a good place to start is the Wikipedia article on them, which can be found here:

http://en.wikipedia.org/wiki/Black_holes

If your looking for a debate about black holes, I think there are several other threads in this forum that contain a wide variety of opinions on the subject.

Chris
 
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Technetium

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Generally... Black holes are holes in space that have an extremly high gravitational pull that pulls everything including light into it.

But obviously, I'm not going to type everything I understand about Black holes.
So the wiki page csmyth gave you will be everything you need.
 
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Boris_Badenov

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Space.com has many years of excellant articles on Black Holes. Here you'll find the search page for SDC. Just type in Black Hole & have fun in your studies.
 
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neilsox

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Here are some bits of information that often don't get mentioned. We can't see black holes, but we have enough inferences to be quite sure about some of the black holes at the center of galaxies, plus a few that are only a few solar mass, elsewhere in our galaxy. Since several percent of the super nova, the past 13.6 billion years produced a black hole and most of them have likely gained more mass than they lost by Hawking's radiation (if Hawking's radiation occurs) there likely are many trillions of black holes in the universe, billions of black holes cruising our galaxy. A fast black hole could pass through our solar system with only a few years warning. Non rotating black holes are easier to analyze, but likely nearly all black holes rotate rapidly/even faster than typical neutron stars, which may also be very numerous. Black holes with low mass, such as the mass of a large mountain, have been hotly debated, but none have been detected, possibly because their event horizon would have microscopic radius. Neil
 
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CalliArcale

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Technetium":30ts1vlw said:
Generally... Black holes are holes in space that have an extremly high gravitational pull that pulls everything including light into it.
Well, they're not exactly holes in the conventional sense. They're super-dense objects. There may be a ball of stupendously dense stuff in the middle, or it may be compressed into a singularity, or it may have winked itself out of existence entirely leaving only its gravity well behind -- beyond the event horizon, you can't see and the difference may be academic anyway.

They don't all have an extremely high gravitational pull, actually. The famous ones certainly do, but did you know that you could turn the Earth into a black hole? You could -- by compressing it down very tightly. You see, gravitational attraction depends on two things: the mass of the object (and technically your own mass too, unless the difference in mass is so enormous that your mass is negligible) and your distance from its center of gravity. This has some interesting effects; did you know that you weigh less in Brazil than you would on the North Pole? It's true! The Earth's rotation causes it to bulge at the equator, and this means at the equator, you are higher up than you are at the poles. Mountains cause this effect too; you weigh less on Mount Everest than you do in Death Valley.

Now, this effect only holds true as long as you are on or above the surface of the object. If you tunnel down a mile, you will not weigh that much more because now there is Earth above you, pulling you that way. If you could somehow drill all the way to the core, you could theoretically end up suspended in the middle, weightless, pulled equally in all directions. So the maximum attraction you can feel from an object is at the lowest point on its surface, right? (Assuming a mostly spherical object, anyway.)

Now imagine compressing that object down. Now you are reducing the distance from its center of gravity but remaining above its surface, so now if you stand on the surface, the gravitational force you feel has gone up. If you compress it a lot, the gravity you feel at the surface will go up a lot. Compress it enough and the force will be so great that your escape velocity from the surface will exceed the speed of light. At this point, you have made a black hole. (You can keep compressing it, but what will happen is that the event horizon will stay at the same place, so observers on the outside won't really be aware of it. It could theoretically compress forever.)

Weird thing -- the object's mass hasn't increased at all during this, and as we know, gravitational force depends on mass, not density. If we're compressing the Earth into a black hole via some sort of magic, the Moon will stay right where it is, in its current orbit, mostly unaffected. (I say mostly because this will affect the tides, but it would take millions of years to notice any change.) What's more, you have to compress the object a LOT. It turns out, in order to turn the Earth into a black hole, you'd have to compress the entire thing down to less than a centimeter. A black hole that is smaller, like the size of the black holes some have speculated could be formed in the Large Hadron Collider, would be so tiny that it could pass completely through the Earth without anybody noticing. (They'd be the size of neutrinos.)

Want to have your mind blown more? If you can think about compressing the Earth down, you can think about compressing the entire universe. Taking the best estimates for the mass of the Universe and working out its Schwartzchild radius (radius of the event horizon of a non-rotating spherical black hole), you get a number pretty close to the estimated size of the actual known Universe . . . .
 
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ramparts

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Actually, as best we know the size of the visible Universe is actually nearly exactly equal to its Schwarzschild radius. Assuming the Universe has critical density, try solving for both. You'll find they're exactly the same (and in fact, they're the same for any radius of cosmological scales, where the density is roughly homogeneous and isotropic). This isn't anything too surprising: if the Universe is at the critical density, then it's spatially flat. If the density were just higher, it would be spatially closed, but the Universe's radius would also lie within its Schwarzschild radius. Both of these imply the Universe would eventually collapse on itself.

It certainly doesn't mean we're living inside a black hole! Nothing outside our "black hole" can get sucked in, because it won't know about the mass at the far end. A black hole of the size of the visible Universe can be considered to be existing at any point in the Universe, because what's visible changes depending on where you're located, meaning there's no point which can be considered the "singularity", and thus no single point to collapse on, so we're not in danger of getting sucked into the center of our cosmic black hole any time soon ;)
 
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Agelesslink

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neilsox":3r3fbf8d said:
there likely are many trillionSince several percent of the super nova, the past 13.6 billion years produced a black hole and most of them have likely s of black holes in the universe, billions of black holes cruising our galaxy. A fast black hole could pass through our solar system with only a few years warning. Neil
i'm not in any sense doubting you as my knowlege on anything outer space comes from the series "The Universe" lol, but it was my understanding the black holes didnt move. explain if i'm wrong. i'm curious! lol
 
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Agelesslink

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lol. i feel like an absolute moron for even thinking that something in space WOULDN'T move
 
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Boris_Badenov

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neilsox":16fm5zl2 said:
Here are some bits of information that often don't get mentioned. We can't see black holes, but we have enough inferences to be quite sure about some of the black holes at the center of galaxies, plus a few that are only a few solar mass, elsewhere in our galaxy. Since several percent of the super nova, the past 13.6 billion years produced a black hole and most of them have likely gained more mass than they lost by Hawking's radiation (if Hawking's radiation occurs) there likely are many trillions of black holes in the universe, billions of black holes cruising our galaxy. A fast black hole could pass through our solar system with only a few years warning. Non rotating black holes are easier to analyze, but likely nearly all black holes rotate rapidly/even faster than typical neutron stars, which may also be very numerous. Black holes with low mass, such as the mass of a large mountain, have been hotly debated, but none have been detected, possibly because their event horizon would have microscopic radius. Neil
The population of identified Neutron Stars of the Milky Way & the Magellanic Clouds combined is in the neighborhood of 2000. I'd like to know where you got this "billions of BH's" in the Milky Way from?
 
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neilsox

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Hi Boris: Perhaps we have identified most of the young neutron stars in the local group of galaxies. The newer one's have much hotter photospheres than the sun, but they are only about 10 kilometers in diameter, so they are dim even as viewed from a few light years away. They cool very slowly, but perhaps those over five billion years old have cooled to about 1000 degrees k and are thus are almost invisible, unless significant matter is falling onto their surface. Black holes (up to about ten solar mass) with thin accretion disks are almost invisible and their gravity is detectable, only if there are visible objects within a few billion kilometers of the black hole.
 
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