History UsefulNotes / BLACKholes

Black holes are collapsed stars, but not many people know ''why'' the stars have collapsed in such a way to create black holes. To find out we'll have to look at how stars evolve, for they can end in one of three ways: a white dwarf, a neutron star, or a black hole.

This means that other theories of relativity and gravity may or (more probably) may not allow similar effects. So all bets are off the moment 'verse have FasterThanLightTravel other than rather weird Alcubierre drive. Other signs that the universe is not GRT-compatible are mentions of either "gravitons" or "anti-gravitation": in GRT gravity isn't a proper field, but the space curvature, so have no quantum and it's rather obvious that the space can be at best "flat" -- the tradeoff being that a field theory does not only allow, but [[http://arxiv.org/abs/gr-qc/0412122 support]] the existence of [[DeflectorShields repulsion forces]].

If all that still is not weird enough for your taste, look up Einstein-Rosen bridges (think wormhole, but it's rather useless from a practical point of view due to its instability) or really big, (insanely fast) rotating, charged black holes (these enter areas where general relativity can get really bad, the worst This Troper has heard of so far proposes utilizing something like that for TimeTravel of all things, which is about the worst choice you could make if you absolutely want to open a can of worms around a physicist - lets just say, it isn't practical, and leave it at that).

However, as the star ages, more and more of the hydrogen at the core is fused into helium "ash", and less remains to counter the gravitational collapse of the outer layers. These layers of the star collapse in further, increasing the pressure and temperature at the core. Eventually, it becomes hot enough there that the helium ash starts to fuse into carbon and then oxygen in an even hotter reaction. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gain enough kinetic energy to "evaporate" away from the star, first as the outer layers of a red giant and then as a planetary nebula. No longer supported by fusion, the exposed core of the star condenses into a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs"]], which are substellar bodies that were never massive enough to sustain fusion to begin with]] only a fraction of a degree above absolute zero.

But that's only for mid-sized stars. For more massive stars, the core is bigger and thus this process starts much earlier. The thing which caused gravity to stop shrinkage in the white dwarf (electron degeneracy pressure) is unable to hold back the carbon and oxygen from fusing, heavier elements are created and added. Finally, the star (which from the outside is now a red supergiant as big as Jupiter's orbit) has an onion-like core of shells made up of hydrogen, helium, carbon, neon, oxygen, magnesium, sulfur, silicon, and some other elements. The star's core becomes so hot that those elements start to combine and ''iron'' is finally made -- but after iron it requires more energy to make heavier elements than this would output, so [[strike:it's the last thing the star can make before it dies]] even as the core starts making serious elements like gold, bismuth and uranium, it can't support its own weight and collapses further, taking in more mass. Once the super-dense core is massive enough that it passes the Chandrasekhar limit (1.4 solar masses), electron degeneracy pressure breaks down completely. This causes gravity to completely collapse the core into a neutron star (white dwarfs can never pass the Chandrasekhar limit unless they rotate fast enough to counter gravity with centrifugal force) -- outside the core, this usually expresses itself as your friendly neighborhood supernova. The neutron star is held up against its weight by the neutron degeneracy pressure (notice a theme here?). But if the core exceeds the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear), neutron degeneracy pressure will break down as well. Now, I hear you ask, what will keep the neutron star from simply collapsing in on itself at this point -- well, ''[[OhCrap nothing]]!'' (Note: there are weird theoretical things that could happen, but have never been observed, such as a quark star, which is supported by quark degeneracy pressure, or a strange star, which is when a quark star goes barking mad. But never mind that.)

That's where the black hole comes in. There's no more electron or neutron degeneracy pressure (which means that now electrons and neutrons can ''inhabit the exact same space at the same time''), so gravity condenses the core down to a point in space that is infinitely small, yet immensely massive. So small and so massive, not even light can escape.

Black holes are strange things. Besides the singularity at the center, there is the event horizon, the point of no return, that once you cross it...[[DepartmentOfRedundancyDepartment you can't return]]. Once inside the event horizon, you literally cannot go back: space is curved in such a way that anywhere you go, it all leads to the same place: the singularity.

However, you'd probably be long dead before that anyway as black holes come with some dangers attached due to the extremely intense gravity around them: First, you'll be spaghettified (this ''is'' the scientific term for it); the tidal forces of the black hole are so strong that, if you were going in feet first, your feet would feel a stronger attraction than your head and thus your body would stretch out. Black holes are so strong that they can even deform atoms. On the up side, the bigger a black hole is, the less drastic this effect becomes on its edge, in fact, for a ''really'' big one, you should survive at least past the event horizon. The second big danger is good old radiation, due to gravitational blueshifting, any radiation hitting you from the outside would be blueshifted and thus a lot more dangerous. Of course, if there is no incoming radiation, you could ''theoretically'' survive it all and get inside.

Black holes normally can't be seen (thus their moniker), but there are ways they are visible: if they are near another star and siphoning off mass, they can form accretion disks, which glow hot. There's gravitation lensing, in which black holes are detected by the image distortions of objects behind them (TheOtherWiki has a nice animation for that [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif here]]). And then there's Hawking radiation, which basically is a way for black holes to radiate stuff (by quantum mechanics), and is a whole other can of non-zero entropy worms. One of its more practically relevant attributes is that a black hole loses mass/energy this way -- the ''faster the smaller it is''! In other words, really small ones, like the ones that the Large Hadron Collider might produce, would just evaporate and be gone before you even notice them. A [[http://arxiv.org/abs/0908.1803v1 scientific paper]] proposes to use an artificial black hole's Hawking radiation as a means to convert mundane matter into energy and thrust to power a spaceship.

Another useful note is that black holes are one of predictions derived from General Relativity Theory -- and even in its context certain theorists saw the predictions of black holes in relativity and [[http://arxiv.org/abs/gr-qc/0412058 expressed doubts]] at least about the classical model. One such theorist was, initially, Einstein himself (although he did come around). Black holes just didn't make sense, especially how they muck up all the nice wonderful understanding of space and time (we think) we have.

However, you'd probably be long dead before that anyway as black holes come with some dangers attached due to the extremely intense gravity around them: First, you'll be spaghettified, the tidal forces of the black hole are so strong that, if you were going in feet first, your feet would feel a stronger attraction than your head and thus your body would stretch out. Black holes are so strong that they can even deform atoms. On the up side, the bigger a black hole is, the less drastic this effect becomes on its edge, in fact, for a ''really'' big one, you should survive at least past the event horizon. The second big danger is good old radiation, due to gravitational blueshifting, any radiation hitting you from the outside would be blueshifted and thus a lot more dangerous. Of course, if there is no incoming radiation, you could ''theoretically'' survive it all and get inside.

In short: black holes are really, ''really'' weird. It's speculated that there are supermassive black holes at the center of every galaxy and that they were there ''before'' the galaxies formed (rather than just have formed by a variety of small black holes merging into one -- yes, they can do that, and the simulations of that are pretty spectacular, but predict that the actual event is downright cataclysmic for anything too close).

-->''Inside the event horizon of a black hole, there ''is'' no way out. There are no directions of space that point away from the singularity. Due to the [[AlienGeometries Lovecraftian curvature]] of spacetime within the event horizon, all the trajectories that ''would'' carry you away from the black hole now point into the past.''
-->''In fact, this is the definition of the event horizon. It's the boundary separating points in space where there are trajectories that point away from the black hole from points in space where there ''are'' none.''
-->''Your magical infinitely-accelerating engine is of no use to you...because you cannot find a direction in which to point it. [[HighOctaneNightmareFuel The singularity is all around you, in every direction you look.]]''
-->''And it is getting closer.''

To learn more cool facts about black holes, please read [[IrregularWebcomic David Morgan-Mar's]] [[TheRant rant]] [[http://www.irregularwebcomic.net/2175.html here]]. Seriously, it's awesome.

Black holes normally can't be seen (thus their moniker), but there are ways they are visible: if they are near another star and siphoning off mass, they can form accretion disks, which glow hot. There's gravitation lensing, in which black holes are detected by the image distortions of objects behind them (TheOtherWiki has a nice animation for that [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif here]]. And then there's Hawking radiation, which basically is a way for black holes to radiate stuff (by quantum mechanics), and is a whole other can of non-zero entropy worms. One of its more practically relevant attributes is that a black hole loses mass/energy this way -- the ''faster the smaller it is''! In other words, really small ones, like the ones that the Large Hadron Collider might produce, would just evaporate and be gone before you even notice them. A [[http://arxiv.org/abs/0908.1803v1 scientific paper]] proposes to use an artificial black hole's Hawking radiation as a means to convert mundane matter into energy and thrust to power a spaceship.

Black holes normally can't be seen (thus their moniker), but there are ways they are visible: if they are near another star and siphoning off mass, they can form accretion disks, which glow hot. There's gravitation lensing, in which black holes are detected by the image distortions of objects behind them (TheOtherWiki has a nice animation for that [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif here]]. And then there's Hawking radiation, which basically is a way for black holes to radiate stuff (by quantum mechanics), and is a whole other can of non-zero entropy worms. One of its more practically relevant attributes is that a black hole loses mass/energy this way -- the ''faster the smaller it is''! In other words, really small ones, like the ones that the Large Hadron Collider might produce, would just evaporate and be gone before you even notice them.

Another useful note is that black holes are one of predictions derived from General Relativity Theory -- and even in its context certain theorists saw the predictions of black holes in relativity and [[http://arxiv.org/abs/gr-qc/0412058 expressed doubts]] at least about the classical model. One such theorist was, initially, Einstein himself (although he did come around). Black holes just didn't make sense, especially how they muck up all the nice wonderful understanding of space and time we (think we) have.

If all that still is not weird enough for your taste, look up Einstein-Rosen bridges (think wormhole, but it's rather harmless from a practical point of view due to its instability) or really big, (insanely fast) rotating, charged black holes (these enter areas where general relativity can get really bad, the worst This Troper has heard of so far proposes utilizing something like that for TimeTravel of all things, which is about the worst choice you could make if you absolutely want to open a can of worms around a physicist - lets just say, it isn't practical, and leave it at that).

However, you'd probably be long dead before that anyway as black holes come with some dangers attached due to the extremely intense gravity around them: First, you'll be spaghettified, the tidal forces of the black hole are so strong that, if you were going in feet first, your feet would feel a stronger attraction than your head and thus your body would stretch out. Black holes are so strong that they can even deform atoms. On the up side, the bigger a black hole is, the less drastic this effect becomes on it's edge, in fact, for a ''really'' big one, you should survive at least past the event horizon. The second big danger is good old radiation, due to gravitational blueshifting, any radiation hitting you from the outside would be blueshifted and thus a lot more dangerous. Of course, if there is no incoming radiation, you could ''theoretically'' survive it all and get inside.

If all that still is not weird enough for your taste, look up Einstein-Rosen bridges (think wormhole, but it's rather harmless from a practical point of view due to it's instability) or really big, (insanely fast) rotating, charged black holes (these enter areas where general relativity can get really bad, the worst This Troper has heard of so far proposes utilizing something like that for TimeTravel of all things, which is about the worst choice you could make if you absolutely want to open a can of worms around a physicist - lets just say, it isn't practical, and leave it at that).

Stars convert hydrogen into helium via fusion. Heat is the kinetic energy of moving particles, so make it hot enough and hydrogen nuclei will move at such speed that they have no time to repel each other with electromagnetic repulsion. When they collide, they are able to fuse together thanks to the "strong force", releasing an enormous amount of energy and creating helium. On Earth, this takes a Tokomak reactor and billions of dollars; in space, all it takes is a few hundred septillion tons of hydrogen. A main-sequence star like our Sun is in a constant balancing act, where the fusion at its core produces enough energy to support itself and the outer non-fusing layers from collapsing into "degenerate matter" (the stuff you hear about when someone starts talking about something weighing a mountain per teaspoonful or the like).

A black hole is, quite literally, a SwirlyEnergyThingy (although not all black holes rotate). A point of space so massive that even objects going at the speed of light (for example: ''light itself'') cannot escape its gravity (thus its name). This phenomenon has fascinated scientists and writers of fiction for many, many years.

-->''Consideration of black holes suggests, not only that God does play dice, but that He sometimes confuses us by throwing them where they can't be seen.''\\
-- Stephen Hawking, ''The Nature of Space and Time''

Stars convert hydrogen into helium via fusion. Heat is the kinetic energy of moving particles, so make it hot enough and hydrogen nuclei will move at such speed that they have no time to repel each other with electromagnetic repulsion. When they collide, they are able to fuse together thanks to the "strong force", releasing an enormous amount of energy and creating helium. On Earth, this takes a Tokomak reactor and billions of dollars; in space, all it takes is a few hundred septillion tons of hydrogen. A main-sequence star like our Sun is in a constant balancing act, where the fusion at its core produces enough energy to support itself and the outer non-fusing layers from collapsing into "degenerate matter" (the stuff you hear about when someone starts talking about something weighing a mountain per teaspoonful or the like).

However, as the star ages more and more of the hydrogen at the core is fused into helium "ash", and less remains to counter the gravitational collapse of the outer layers. These layers of the star collapse in further, increasing the pressure and temperature at the core. Eventually, it becomes hot enough there that the helium ash starts to fuse into carbon and then oxygen in an even hotter reaction. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gain enough kinetic energy to "evaporate" away from the star, first as the outer layers of a red giant and then as a planetary nebula. No longer supported by fusion, the exposed core of the star condenses into a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs"]], which are substellar bodies that were never massive enough to sustain fusion to begin with]] only a fraction of a degree above absolute zero.

But that's only for mid-sized stars. For more massive stars, the core is bigger and thus this process starts much earlier. The thing which caused gravity to stop shrinkage in the white dwarf (electron degeneracy pressure) is unable to hold back the carbon and oxygen from fusing, heavier elements are created and added. Finally, the star (which from the outside is now a red supergiant as big as Jupiter's orbit) has an onion-like core of shells made up of hydrogen, helium, carbon, neon, oxygen, magnesium, sulfur, silicon, and some other elements. The star's core becomes so hot that those elements start to combine and ''iron'' is finally made - but after iron it requires more energy to make heavier elements than this would output, so [[strike:it is the last thing the star can make before it dies]] even as the core starts making serious elements like gold and bismuth and uranium, it can't support its own weight and collapses further, taking in more mass. Once the super-dense core is massive enough that it passes the Chandrasekhar limit (1.4 solar masses), electron degeneracy pressure breaks down completely. This causes gravity to completely collapse the core into a neutron star (white dwarfs can never pass the Chandrasekhar limit unless they rotate fast enough to counter gravity with centrifugal force) - outside the core, this usually expresses itself as your friendly neighborhood supernova. The neutron star is held up against its weight by the neutron degeneracy pressure (notice a theme here?). But if the core exceeds the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear), neutron degeneracy pressure will break down as well. Now, I hear you ask, what will keep the neutron star from simply collapsing in on itself at this point - well, [[OhCrap "nothing]]!'' (Note: there are weird theoretical things that could happen, but have never been observed, such as a quark star, which is supported by quark degeneracy pressure, or a strange star, which is when a quark star goes barking mad. But never mind that.)

Black holes are strange things. Besides the singularity at the center, there is the event horizon, the point of no return, that once you cross it...[[DepartmentOfRedundancyDepartment you can't return]]. Once inside the event horizon, you literally cannot go back: space is curved in such a way that anywhere you go, it all leads to the same place: the singularity.

In fact, spacetime will become quite freaky around the event horizon: the closer you get to the event horizon, the [[YearInsideHourOutside slower time becomes]] (due to relativity, however, you won't notice it). In fact, if an observer outside the event horizon could see you, they would see as you get closer and closer (and get redder, due to gravitation red shift, while everything you see would be bluer), you would go slower and slower until you hit the edge of the event horizon at which point you would ''stop'' (Nobody would actually see you hit the event horizon, since you appear to slow down as you get closer, for an outside observer, you would take an infinite amount of time to reach it. You wouldn't actually stop, that's just what they'll see).

Black holes normally can't be seen (thus their moniker), but there are ways they are visible: if they are near another star and siphoning off mass, they can form accretion disks, which glow hot. There's gravitation lensing, in which black holes are detected by the image distortions of objects behind them (TheOtherWiki has a nice animation for that [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif here]]. And then there's Hawking radiation, which basically is a way for black holes to radiate stuff (by quantum mechanics), and is a whole other can of non-zero entropy worms. One of its more practically relevant attributes is that a black hole loses mass/energy this way - the ''faster the smaller it is''! In other words, really small ones, like the ones that the Large Hadron Collider might produce, would just evaporate and be gone before you even notice them.

If all that still is not weird enough for your taste, look up Einstein-Rosen bridges (think wormhole, but it's rather harmless from a practial point of view due to it's instability) or really big, (insanely fast) rotating, charged black holes (these enter areas where general relativity can get really bad, the worst This Troper has heard of so far proposes utilizing something like that for TimeTravel of all things, which is about the worst choice you could make if you absolutely want to open a can of worms around a physicist - lets just say, it isn't practical, and leave it at that).

Another useful note is that black holes are one of predictions derived from General Relativity Theory -- and even in its context certain theorists saw the predictions of black holes in relativity and [[http://arxiv.org/abs/gr-qc/0412058 expressed doubts]] at least about the classical model. One such theorist was, initially, Einstein himself (although he did come around). Black holes just didn't make sense, especially how they muck up all the nice wonderful understanding of space and time we (think we) have.\\

----

However, as the star ages more and more of the hydrogen at the core is fused into helium "ash", and less remains to counter the gravitational collapse of the outer layers. These layers of the star collapse in further, increasing the pressure and temperature at the core. Eventually, it becomes hot enough there that the helium ash starts to fuse into oxygen and then carbon in an even hotter reaction. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gain enough kinetic energy to "evaporate" away from the star, first as the outer layers of a red giant and then as a planetary nebula. No longer supported by fusion, the exposed core of the star condenses into a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs"]], which are substellar bodies that were never massive enough to sustain fusion to begin with]] only a fraction of a degree above absolute zero.

However, as the star ages more and more of the hydrogen at the core is fused into helium "ash", and less remains to counter the gravitational collapse of the outer layers. These layers of the star collapse in further, increasing the pressure and temperature at the core. Eventually, it becomes hot enough there that the helium ash starts to fuse into oxygen and then carbon in an even hotter reaction. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gain enough kinetic energy to "evaporate" away from the star, first as the outer layers of a red giant and then as a planetary nebula. No longer supported by fusion, the exposed core of the star condenses into a white dwarf which slowly cools over trillions of years into an inert black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs"]], which are substellar bodies that were never massive enough to sustain fusion to begin with]].

Black holes are collapsed stars, but not many people know ''why'' the stars have collapsed in such a way to create black holes. Stars can end in one of three ways: a white dwarf, a neutron star, or a black hole.

However, once the star becomes old enough, the helium ions start to vibrate fast enough as well, starting ''helium fusion'', which creates oxygen and then carbon. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gains enough kinetic energy to "evaporate" away from the star, first as the outer part of a red giant and then as a planetary nebula. No longer supported by fusion, the core of the star condenses into a white dwarf which slowly cools over trillions of years into an inert black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs"]], which are substellar bodies that were never massive enough to sustain fusion to begin with]].

Stars convert hydrogen into helium via fusion. Heat is the kinetic energy of moving particles, so make it hot enough and hydrogen nuclei will move at such speed that they have no time to repel each other and when they collide, they are able to fuse together, releasing an enormous amount of energy and creating helium for the core of the star. However, once the star (well, a mid-sized star) becomes hot enough, the helium ions start to vibrate fast enough as well, starting ''helium fusion'', which creates oxygen and then carbon. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gains enough kinetic energy to "evaporate" away from the star, first as the outer part of a red giant and then as a planetary nebula. The core of the star, at the center of the nebula, becomes a white dwarf, which slowly cools over trillions of years into an inert black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs"]], which are substellar bodies that are never massive enough to sustain fusion to begin with]].

(Incidentally, current theory indicates low-mass red dwarfs don't go through the giant stage because their outer layers are more efficiently mixed into the fusing core; it's believe they will become brighter "blue dwarfs" before settling into the inert white dwarf stage. This hasn't been verified by observation [[TimeAbyss as the universe hasn't been around for the hundreds of billions of years necessary for those stars to reach that state of evolution]].

But that's only for mid-sized stars. For more massive stars, the core is bigger and thus this process starts much earlier. The thing which caused gravity to stop shrinkage in the white dwarf (electron degeneracy pressure) doesn't have enough time to take hold, so instead of a shell of oxygen and carbon, heavier elements are made and added. Finally, the star has an onion-like core of shells made up of hydrogen, helium, carbon, neon, oxygen, magnesium, sulfur, silicon, and some other elements. The star's core becomes so hot that those elements start to combine and ''iron'' is finally made - but after iron it requires more energy to make heavier elements than this would output, so [[strike:it is the last thing the star can make before it dies]] even as the core starts making serious elements like gold and bismuth and uranium, it can't support its own weight and collapses further. Once the core becomes small enough that it passes the Chandrasekhar limit (1.4 solar masses), electron degeneracy pressure breaks down completely. This causes gravity to completely collapse the core into a neutron star (white dwarfs can never pass the Chandrasekhar limit) - outside the core, this usually expresses itself as a friendly neighborhood supernova. The neutron star is held up against its weight by the neutron degeneracy pressure (notice a theme here?). If the core actually becomes even smaller or the neutron star absorbs more mass from another star, it can pass the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear), at which point neutron degeneracy pressure will break down. Now, I hear you ask, what will keep the neutron star from simply collapsing in on itself? Well, ''nothing!'' (Note: there are weird theoretical things that could happen, but have never been observed, such as a quark star, which is supported by quark degeneracy pressure, or a strange star, which is when a quark star goes barking mad. But never mind that.)

Stars convert hydrogen into helium via fusion. Heat is the kinetic energy of moving particles, so make it hot enough and hydrogen nuclei will move at such speed that they have no time to repel each other and when they collide, they are able to fuse together, releasing an enormous amount of energy and creating helium for the core of the star. However, once the star (well, a mid-sized star) becomes hot enough, the helium ions start to vibrate fast enough as well, starting ''helium fusion'', which creates oxygen and then carbon. As the helium burns up and a shell of oxygen and carbon is created, the surface layers of the star gains enough kinetic energy to "evaporate" away from the star, first as the outer part of a red giant and then as a planetary nebula. The core of the star, at the center of the nebula, becomes a white dwarf, which slowly cools over trillions of years into an inert black dwarf[[hottip:*:[[InsistentTerminology Not to be confused with "brown dwarfs", which are substellar bodies that are never massive enough to sustain fusion to begin with]].

Stars convert hydrogen into helium via fusion. Heat is a kinetic energy of moving particles, so make it hot enough and hydrogen nuclei will move at such speed that they have no time to repel each other and when they collide, they are able to fuse together, releasing an enormous amount of energy and creating helium for the core of the star. However, once the star (well, a mid-sized star) becomes hot enough, the helium ions start to vibrate fast enough as well, starting ''helium fusion'', which creates oxygen and then carbon. As the helium burns up and a shell of oxygen and carbon is created, the surface of the star can get enough kinetic energy to fly away from the core and become a planetary nebula. The core of the star, at the center of the nebula, becomes a white dwarf.

If all that still is not weird enough for your taste, look up Einstein-Rosen bridges (think wormhole, but it's rather harmless from a practial point of view due to it's instability) or really big, (insanely fast) rotating, charged black holes (these enter areas where general relativity can get really bad, the worst ThisTroper has heard of so far proposes utilizing something like that for TimeTravel of all things, which is about the worst choice you could make if you absolutely want to open a can of worms around a physicist - lets just say, it isn't practical, and leave it at that).

<<|UsefulNotes|>>

But that's only for mid-sized stars. For more massive stars, the core is bigger and thus this process starts much earlier. The thing which caused gravity to stop shrinkage in the white dwarf (electron degeneracy pressure) doesn't have enough time to take hold, so instead of a shell of oxygen and carbon, heavier elements are made and added. Finally, the star has an onion-like core of shells made up of hydrogen, helium, carbon, neon, oxygen, magnesium, sulfur, silicon, and some other elements. The star's core becomes so hot that those elements start to breakdown and ''iron'' is finally made - but after iron it requires more energy to make heavier elements than this would output, so [[strike:it is the last thing the star can make before it dies]] even as the core starts making serious elements like gold and bismuth and uranium, it can't support its own weight and collapses further. Once the core becomes small enough that it passes the Chandrasekhar limit (1.4 solar masses), electron degeneracy pressure breaks down completely. This causes gravity to completely collapse the core into a neutron star (white dwarfs can never pass the Chandrasekhar limit) - outside the core, this usually expresses itself as a friendly neighborhood supernova. The neutron star is held up against its weight by the neutron degeneracy pressure (notice a theme here?). If the core actually becomes even smaller or the neutron star absorbs more mass from another star, it can pass the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear), at which point neutron degeneracy pressure will break down. Now, I hear you ask, what will keep the neutron star from simply collapsing in on itself? Well, ''nothing!'' (Note: there are weird theoretical things that could happen, but have never been observed, such as a quark star, which is supported by quark degeneracy pressure, or a strange star, which is when a quark star goes barking mad. But never mind that.)

In short: black holes are really, ''really'' weird. It's speculated that there are supermassive black holes at the center of every galaxy and that they were there ''before'' the galaxies formed (rather than just have formed by a variety of small black holes merging into one – yes, they can do that, and the simulations of that are pretty spectacular, but predict that the actual event is downright cataclysmic for anything too close).

But that's only for mid-sized stars. For more massive stars, the core is bigger and thus this process starts much earlier. The thing which caused gravity to stop shrinkage in the white dwarf (electron degeneracy pressure) doesn't have enough time to take hold, so instead of a shell of oxygen and carbon, heavier elements are made and added. Finally, the star has an onion-like core of shells made up of hydrogen, helium, carbon, neon, oxygen, magnesium, sulfur, silicon, and some other elements. The star's core becomes so hot that those elements start to breakdown and ''iron'' is finally made - but after iron it requires more energy to make heavier elements than this would output, so [[strike:it is the last thing the star can make before it dies]] even as the core starts making serious elements like gold and bismuth and uranium, it can't support its own weight and collapses further. Once the core becomes small enough that it passes the Chandrasekhar limit (1.4 solar masses), electron degeneracy pressure breaks down completely. This causes gravity to completely collapse the core into a neutron star (white dwarfs can never pass the Chandrasekhar limit) - outside the core, this usually expresses itself as a friendly neighborhood supernova. The neutron star is held up against its weight by the neutron degeneracy pressure (notice a theme here?). If the core actually becomes even smaller or the neutron star absorbs more mass from another star, it can pass the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear), at which point neutron degeneracy pressure will break down. Now, I hear you ask, what will keep the neutron star from simply collapsing in on itself? Well, ''nothing…'' (Note: there are weird theoretical things that could happen, but have never been observed, such as a quark star, which is supported by quark degeneracy pressure, or a strange star, which is when a quark star goes barking mad. But never mind that.)

Another useful note is that black holes are one of predictions derived from General Relativity Theory (and even in its context there are [[http://arxiv.org/abs/gr-qc/0412058 doubts]] at least about the classical model). Other theories of relativity and gravity may or may not allow similar effects. So all bets are off the moment 'verse have FasterThanLightTravel other than via rather weird Alcubierre drive. Right now there's no strict proof that such things exist: granted, there ''are'' heavy low-radiating objects ("black hole candidates"), but whether some low-emission star inside an enormous gas and dust cloud is really a black hole or not...

Still, this didn't stop certain theorists who saw the predictions of black holes in relativity and then try calling foul. Black holes just didn't make sense, especially how they muck up all the nice wonderful understanding of space and time we (think we) have. One such theorist was, initially, Einstein himself (although he did come around).

But that's only for mid-sized stars. For more massive stars, the core is bigger and thus this process starts much earlier. The thing which caused gravity to stop shrinkage in the white dwarf (electron degeneracy pressure) doesn't have enough time to take hold, so instead of a shell of oxygen and carbon, heavier elements are made and added. Finally, the star has an onion-like core of shells made up of hydrogen, helium, carbon, neon, oxygen, magnesium, sulfur, silicon, and some other elements. Finally, the star becomes so hot that those elements start to breakdown and ''iron'' is finally made. After Iron it requires more energy to make heavier elements than this would output, so it is the last thing the star can make before it dies. Once the core becomes small enough that it passes the Chandrasekhar limit (1.4 solar masses), electron degeneracy pressure breaks down completely. This causes gravity to completely collapse the core into a neutron star (white dwarfs can never pass the Chandrasekhar limit). The neutron star is held up against its weight by the neutron degeneracy pressure (notice a theme here?). If the core actually becomes even smaller or the neutron star absorbs more mass from another star, it can pass the Tolman-Oppenheimer-Volkoff limit (about two to three solar masses, and definitely no more than five, but it's still unclear), at which point neutron degeneracy pressure will break down. Now, I hear you ask, what will keep the neutron star from simply collapsing in on itself? Well, ''nothing…'' (Note: there are weird theoretical things that could happen, but have never been observed, such as a quark star, which is supported by quark degeneracy pressure, or a strange star, which is when a quark star goes barking mad. But never mind that.)