History UsefulNotes / BLACKholes

What's left behind is dependent on the mass of the star, especially its core. A lighter star, such as the Sun, becomes a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf.[[note]]Not to be confused with "brown dwarfs", which are sub-stellar bodies, like large planets, that were never massive enough to sustain fusion to begin with.[[/note]] According to current estimates, no black dwarfs yet exist, as a star cooling to that level would take longer than the universe has existed. The Sun is expected to become a black dwarf in approximately 1 quadrillion years. Stars with cores weighing in at more than 1.4 times the mass of the Sun have exceeded the [[https://en.wikipedia.org/wiki/Chandrasekhar_limit Chandrasekhar limit]] and gravity combines electrons and protons to form neutrons, resulting in a neutron star. Stars whose core mass exceeds the [[https://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limit Tolman-Oppenheimer-Volkoff limit]] (about two to three solar masses, and definitely no more than five, but it's still unclear) are so massive that even the neutrons can't resist further collapse;[[note]]Neutron stars are prevented from collapsing further by a pressure called neutron degeneracy pressure. This is caused by the Pauli exclusion principle, and the degeneracy pressure is insufficient to prevent collapse over the Tolman-Oppenheimer-Volkoff. However, it is possible that there are other forms of degenerate matter, which may be capable of preventing further collapse until the object's mass reaches a new limit.[[/note]] it can be assumed that the star collapses down to the event horizon, and past it to a singularity (a single point, or a ring for a rotating black hole).

Black holes are strange things. Besides the singularity at the center[[note]]Assuming it exists, since it's believed it's an artifact caused by breakdown of relativity under those conditions, that would disappear with a--still not existent--theory of quantum gravity[[/note]], there is the event horizon, the point of no return. Once inside the event horizon, you literally cannot go back: spacetime is curved in such a way by the black hole's mass that any path you take leads to the same place: the singularity. In three-dimensional space the black hole is not a disc, just like the Sun is not a big yellow circle. The Sun is a sphere, an event horizon is a smaller sphere, and the singularity is an infinitely tiny ball so small and tightly packed that it has basically turned itself ''inside out'' — so when you are inside the singularity, you are inside the ball. Rotating black holes also have an ergosphere, a region near the event horizon where space-time spins around the black hole at speeds so great that you'd need to move faster than light just to stay still, let alone move in a direction counter to the black hole's rotation.

# You appear outside the black hole with your memories and the recordings of your ship intact. Congratulations! You just rewound the entire history of the universe ''except yourself!'' So you don't go in again. Then a collection of paradoxes gangs up on you asking unpleasant questions like how you can have information about the black hole if you didn't go in. Your impossible presence causes the universe to disintegrate, implode, or just [[CessationOfExistence switch off instantly like a light-bulb.]]

# You appear outside the black hole with your memories and the recordings of your ship intact. Congratulations! You just rewound the entire history of the universe ''except yourself!'' So you don't go in again. Then a collection of paradoxes gangs up on you asking unpleasant questions like how you can have information about the black hole if you didn't go in. Your impossible presence causes the universe to disintegrate, implode, or just switch off instantly like a light-bulb.

* Black: At least, not the matter around it. Black holes themselves are definitely black (no light can escape from inside the event horizon), but matter falling into a black hole glows brightly, and a "feeding" black hole can be one of the brightest objects in the universe due to in-falling matter. [[http://www.wired.com/wp-content/uploads/2014/10/ut_interstellarOpener_f.png A real black hole might look more like this]] if enough matter has gone in.

* Black: At least, not the matter around it. Black holes themselves are definitely black (no light can escape from inside the event horizon), but matter falling into a black hole glows brightly, and a "feeding" black hole can be one of the brightest objects in the universe due to in-falling matter glowing so brightly. [[http://www.wired.com/wp-content/uploads/2014/10/ut_interstellarOpener_f.png A real black hole might look more like this]] if enough matter has gone in.

Stars convert hydrogen into helium in their cores via fusion, which produces enormous amounts of energy; this balances out the inward pull of gravity and keeps the star stable. However, as the star ages, the hydrogen in the core starts to run out, the fusion reactions slow down, and gravity begins to collapse the star. At this point, a star of sufficient mass can begin to fuse helium, starting a cycle of fusions that can produce several heavier elements and turns the star into a red giant or supergiant, but even that can't go on forever. Stars that are massive enough can eventually fuse silicon into iron, but producing elements heavier than iron by fusion ''costs'' energy instead of producing it, so once the star builds up enough iron, fusion stops and its core completely collapses in on itself; what happens from there depends entirely on the star's mass.

A lighter star, such as the Sun, becomes a degenerate-matter white dwarf which slowly cools over trillions of years into a black dwarf.[[note]]Not to be confused with "brown dwarfs", which are sub-stellar bodies, like large planets, that were never massive enough to sustain fusion to begin with.[[/note]] According to current estimates, no black dwarfs yet exist, as a star cooling to that level would take longer than the universe has existed. The Sun is expected to become a black dwarf in approximately 1 quadrillion years. Stars with cores weighing in at more than 1.4 times the mass of the Sun have exceeded the [[https://en.wikipedia.org/wiki/Chandrasekhar_limit Chandrasekhar limit]] and gravity combines electrons and protons to form neutrons, resulting in a neutron star. Stars whose core mass exceeds the [[https://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limit Tolman-Oppenheimer-Volkoff limit]] (about two to three solar masses, and definitely no more than five, but it's still unclear) are so massive that even the neutrons can't resist further collapse;[[note]]Neutron stars are prevented from collapsing further by a pressure called neutron degeneracy pressure. This is caused by the Pauli exclusion principle, and the degeneracy pressure is insufficient to prevent collapse over the Tolman-Oppenheimer-Volkoff. However, it is possible that there are other forms of degenerate matter, which may be capable of preventing further collapse until the object's mass reaches a new limit.[[/note]] it can be assumed that the star collapses down to the event horizon, and past it to a singularity (a single point, or a ring for a rotating black hole).

Stars convert hydrogen into helium in their cores via fusion, which produces enormous amounts of energy; this balances out the inward pull of gravity and keeps the star stable. However, as the star ages, the hydrogen eventually runs out and gravity begins to collapse the star. At this point, a star of sufficient mass can begin to fuse helium, starting a cycle of fusions that produces several heavier elements and turns the star into a red giant or supergiant, but even that can't go on forever. Producing elements heavier than iron ''costs'' energy instead of producing it, so once the star hits this point, fusion stops and its core completely collapses in on itself; what happens from there depends entirely on the star's mass.

Black holes normally can't be seen (thus their moniker), but there are ways they are detectable. If they're [[StarKilling siphoning off matter from a nearby star]], they can form accretion disks, which glow hot. There's gravitational lensing, in which black holes are detected by the image distortions of objects behind them (Wiki/TheOtherWiki has [[http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif a nice animation]] for that). 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 ''smaller it is, the faster it goes''! 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 (although the immense release of energy from the Hawking radiation would be noticeable). A solar-mass black hole, on the other hand, would lose about a milligram of its mass-energy every 3.1 x 10[[superscript:31]] (31 nonillion) years, which they'd more than make up for by consuming the cosmic microwave background. A [[http://arxiv.org/abs/0908.1803v1 scientific paper]] proposes to use a small artificial black hole's Hawking radiation as a means to convert mundane matter into energy and thrust to power a spaceship.

In April 2019, a black hole was actually observed for the first time. The Event Horizon Telescope, a network of 8 interlinked telescopes, was able to photograph a black hole in the center of the distant giant galaxy [[https://en.wikipedia.org/wiki/Messier_87 M87]], 500 million trillion km away (see the image that illustrates this article). This is roughly equivalent, in terms of scale, to reading the date on a coin in New York... from Los Angeles. The black hole looks like... [[ShapedLikeItself a black hole]], illuminated by the glowing ring of superheated gas around its event horizon. The internet immediately [[MoeAnthropomorphism anthropomorphized]] it. Three years later, in May 2022, [[https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy Sagittarius A*]] (the aforementioned supermassive black hole thought to exist at the heart of the Milky Way) was imaged with the same telescope.

However, this is all a moot point. FTL travel is impossible (at least, based on all the numbers and laws and theories and quantum we currently possess) and so you can't have yourself a magical FTL-speed ship anyway.[[note]]Fortunately, since FTL ships don't exist you're not going to be ''able'' to go anywhere near a black hole anyway.[[/note]] Because if you could, you might as well click your heels together three times and wish your way out.

# You reverse the history of the universe to before you entered the black hole. Everything, including your ship's instruments and your memories of everything that happened before that point, has reversed to that point. Not remembering that you have been into the black hole before, you enter it again like an idiot... and then again... and again. You have just ''doomed the entire universe'' to exist as a broken record, but nobody ever notices. Nice going.

# You only manage to kill yourself faster because space beyond the event horizon has warped by the gravitational forces to the point the only direction you can move is further in. At best you can manage to stay in place long enough for the black hole to dissipate on its own and you might live long enough to do so since time is moving extra slow for you. A lot of time will have passed for everyone not breaking light speed barriers beyond event horizons. [[note]]See page quote for an alternate possibility.[[/note]]

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

Black holes are strange things. Besides the singularity at the center[[note]]Assuming it exists, since it's believed it's an artifact caused by breakdown of relativity under those conditions, that would disappear with a--still not existent--theory of quantum gravity[[/note]], there is the event horizon, the point of no return--once you cross it, [[DepartmentOfRedundancyDepartment there is no going back]]. Once inside the event horizon, you literally cannot go back: spacetime is curved in such a way by the black hole's mass that any path you take leads to the same place: the singularity. In three-dimensional space the black hole is not a disc, just like the Sun is not a big yellow circle. The Sun is a sphere, an event horizon is a smaller sphere, and the singularity is an infinitely tiny ball so small and tightly packed that it has basically turned itself ''inside out'' — so when you are inside the singularity, you are inside the ball. Rotating black holes also have an ergosphere, a region near the event horizon where space-time spins around the black hole at speeds so great that you'd need to move faster than light just to stay still, let alone move in a direction counter to the black hole's rotation.

In fact, space-time will become quite freaky around the event horizon: the closer you get to it, 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 appear to ''stop''. You won't ''actually'' stop, that's just what they'll see. This is because space-time around the black hole's event horizon is so warped that light would take a progressively longer time to reach a distant observer as you approach the event horizon — ''ad infinitum''. They'd never see you actually touch the horizon, and the light you emitted would slowly be red-shifted to the point of invisibility. This prediction, however, assumes a zero-mass incoming object and neglects quantum effects, so reality may be more tricky.

Black holes are strange things. Besides the singularity at the center[[note]]Assuming it exists, since it's believed it's an artifact caused by breakdown of relativity under those conditions, that would disappear with a--still not existent--theory of quantum gravity[[/note]], there is the event horizon, the point of no return--once you cross it, [[DepartmentOfRedundancyDepartment you can't return]]. Once inside the event horizon, you literally cannot go back: spacetime is curved in such a way by the black hole's mass that any path you take leads to the same place: the singularity. In three-dimensional space the black hole is not a disc, just like the Sun is not a big yellow circle. The Sun is a sphere, an event horizon is a smaller sphere, and the singularity is an infinitely tiny ball so small and tightly packed that it has basically turned itself ''inside out'' — so when you are inside the singularity, you are inside the ball. Rotating black holes also have an ergosphere, a region near the event horizon where space-time spins around the black hole at speeds so great that you'd need to move faster than light just to stay still, let alone move in a direction counter to the black hole's rotation.

Until February 2016, with the [[https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves first detection of gravitational waves]] with the LIGO instrument and other similar detections in the following years[[note]]As most of the detected signals exactly match the theoretical predictions of a black hole merger[[/note]], there was 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... Yet there is one [[http://arxiv.org/abs/0903.1105 article]], that states: Sagittarius A* (a source of radio waves, associated with a supermassive object in the center of the Milky Way) must have an event horizon because, given the amount of superhot infalling matter we've detected around it, its surface luminosity is too low to be explained ''without'' something that traps radiation.[[note]]That's part of the reason its name includes an asterisk, actually: We ''think'' there's a black hole there, but aren't completely sure yet (photographs ''would'' help, but there's currently too many stars in the way, explaining why it took longer to picture it).[[/note]]

* Black: At least, not always. Black holes themselves are definitely black (no light can escape from inside the event horizon). However, matter falling into a black hole glows brightly, and a "feeding" black hole can be one of the brightest objects in the universe due to in-falling matter glowing so brightly. [[http://www.wired.com/wp-content/uploads/2014/10/ut_interstellarOpener_f.png A real black hole might look more like this]] if enough matter has gone in.

* Whirlpools: Black holes don't gain any powers of suction when they become black holes. Their mass exerts the same gravitational force as a star, planet, or any other object of the same mass. If our Sun were suddenly turned into a black hole of the same mass, the Solar system would not get "sucked in." In fact, all the orbits would stay exactly as they are now. Nothing would happen to us apart from freezing to death. If we wanted to study a black hole, we could put a probe in orbit around it the same way we put probes around other astronomical bodies. It's not going to instantly spiral to its doom (at least not any faster than it would around anything else).

* Flat: Staying away from the more [[TimeyWimeyBall wibbly-wobbly]] stuff, it's convenient to think of a black hole as a tiny sphere and the event horizon as a shell around it. Once you hit the shell, you're stuck. It's also going to look pretty much exactly the same as you circle it, regardless of the direction you chose[[note]]Black holes that rotated very fast are expected to look ellipsoidal due to the centrifuge force caused by said fast rotation[[/note]]. The objects orbiting it do so due to spin.

Until February 2016, with the [[https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves first detection of gravitational waves]] with the LIGO instrument and other similar detections in the following years[[note]]As most of the detected signals exactly match the theoretical predictions of a black hole merger[[/note]], there was 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... Yet there is one [[http://arxiv.org/abs/0903.1105 article]], that states: Sagittarius A* (a source of radio waves, associated with a supermassive object in the center of the Milky Way) must have an event horizon because, given the amount of superhot infalling matter we've detected around it, its surface luminosity is too low to be explained ''without'' something that traps radiation.[[note]]That's part of the reason its name includes an asterisk, actually: We ''think'' there's a black hole there, but aren't completely sure yet (photographs ''would'' help, but there's currently too many stars in the way).[[/note]]

In April 2019, a black hole was actually observed for the first time. The Event Horizon Telescope, a network of 8 interlinked telescopes, was able to photograph a black hole in the center of the distant giant galaxy [[https://en.wikipedia.org/wiki/Messier_87 M87]], 500 million trillion km away (see the image that illustrates this article). This is roughly equivalent, in terms of scale, to reading the date on a coin in New York... from Los Angeles. The black hole looks like... [[ShapedLikeItself a black hole]], illuminated by the glowing ring of superheated gas around its event horizon. The internet immediately [[MoeAnthropomorphism anthropomorphized]] it. Three years later, in May 2022, [[https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy Sagittarius A*]] (the supermassive black hole thought to exist at the heart of the Milky Way) was imaged with the same telescope.

In April 2019, a black hole was actually observed for the first time. The Event Horizon Telescope, a network of 8 interlinked telescopes, was able to photograph a black hole in the distant giant galaxy [[https://en.wikipedia.org/wiki/Messier_87 M87]], 500 million trillion km away (see the image that illustrates this article). This is roughly equivalent, in terms of scale, to reading the date on a coin in New York... from Los Angeles. The black hole looks like... [[ShapedLikeItself a black hole]], illuminated by the glowing ring of superheated gas around its event horizon. The internet immediately [[MoeAnthropomorphism anthropomorphized]] it.

Black holes don't actually, technically, emit anything, despite what people say about them emitting radiation.[[note]]This may not be true. Due to quantum effects at the horizon, a black hole emits a form of black body radiation and '''''VERY''''' slowly loses mass until it eventually evaporates. However, the energy of the radiation "emitted" by a black hole is inversely proportional to its mass. This means that lighter black holes can quickly evaporate in a burst of radiation; this also means that larger black holes won't be able to evaporate, or even lose mass, for quite some time, due to the cosmic microwave background. This form of radiation is called "Hawking radiation", named after its discoverer (well, "predictor" wold be more accurate, no one has ever actually been able to observe this radiation yet — due to obvious reasons) Stephen Hawking. You can read more about this in [[https://en.wikipedia.org/wiki/Hawking_radiation the other wiki]]. However, until irrefutable evidence is found, it's just a hypothesis despite the fact that it's mathematically all working out.[[/note]] That's the extreme conditions the matter ''entering'' the black hole is subject to. If you heat something up, it gives off radiation. And the matter entering a black hole gets very, VERY hot with all the spinning and stretching and friction and gravity it's going through. Nothing leaves once it's past the event horizon itself. However, because a black hole is so unique in how it operates, the matter wouldn't achieve those conditions in any other way, so it kind of is, except it's not.

* Black: Oh sure, it's black beyond what looks like the horizon, but it may well glow brightly from heat generated by whatever it is that they're sucking in, and from accretion. [[http://www.wired.com/wp-content/uploads/2014/10/ut_interstellarOpener_f.png A real black hole might look more like this]] if enough matter has gone in. And even then, the black isn't anywhere you can fall into; you won't even know when you've fallen through the real event horizon, assuming you haven't already been torn apart by tidal forces.

Stars convert hydrogen into helium via fusion, which produces enormous amounts of energy; this balances out the inward pull of gravity and keeps the star stable. However, as the star ages, the hydrogen eventually runs out and gravity begins to collapse the star. At this point, a star of sufficient mass can begin to fuse helium, starting a cycle of fusions that produces several heavier elements and turns the star into a red giant, but even that can't go on forever. Producing elements heavier than iron ''costs'' energy instead of producing it, so once the star hits this point, fusion stops and its core completely collapses in on itself; what happens from there depends entirely on the star's mass.

Black holes normally can't be seen (thus their moniker), but there are ways they are detectable. If they're near another [[StarKilling star and siphoning off mass]], they can form accretion disks, which glow hot. There's gravitational lensing, in which black holes are detected by the image distortions of objects behind them (Wiki/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 ''smaller it is, the faster it goes''! 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 (although the immense release of energy from the Hawking radiation would be noticeable). A solar-mass black hole, on the other hand, would lose about a milligram of its mass-energy every 3.1 x 10[[superscript:31]] (31 nonillion) years, which they'd more than make up for by consuming the cosmic microwave background. A [[http://arxiv.org/abs/0908.1803v1 scientific paper]] proposes to use a small artificial black hole's Hawking radiation as a means to convert mundane matter into energy and thrust to power a spaceship.

In a scenario somewhat related to the information paradox, [[https://en.wikipedia.org/wiki/Black_hole_cosmology Black hole cosmology]] is arguably just as controversial, yet particularly interesting when you think about it. In this model of the Universe's creation, the 'nothing' outside the universe just so happens to be the inside of a black hole. Since time and space are infinitely vast at a black hole's singularity, this means the universe has an infinite amount of space to grow. This theory gets weirder still when you consider that if we're inside a black hole, then what about the black holes inside ''our'' universe? The result comes out looking something like a matryoshka doll, only that it's impossible to tell where it begins and where, if ever, it will end. Our universe could be just one of many, nested inside an infinite number of black holes containing other universes.

Unless you're watching ''extremely'' hard SciFi ([[MohsScaleOfScienceFictionHardness like 5.5 or 6]]), a black hole is probably nothing like you've generally seen in fiction. Black holes rank up there with FTL and TimeTravel as one of the most frequently exploited bits of science.

Another common misconception is that all the stars in the galaxy orbit the supermassive black hole at the center, the same way all the objects in the solar system orbit the Sun. However, the Sun makes up 99.8 percent of the mass of the solar system, while the supermassive black hole at the center of our galaxy (Sagittarius A*), despite having the equivalent of 4.5 ''million'' solar masses, is only 0.0001 percent of the total mass of the galaxy. Sagittarius A* therefore cannot be solely or even mainly responsible for the orbits of all the stars in the galaxy. If it were removed, almost nothing would change. Strictly speaking, everything in our galaxy does not orbit the supermassive black hole at the center, but rather orbits the ''center of mass'' of the galaxy, which ''includes'' the supermassive black hole (and which happens to be at the center), but also includes the tens of millions of stars also clustered around the middle. The galactic orbital paths of all objects in the galaxy are caused by the ''total'' mass of the galaxy, not solely the mass of the black hole at the center. [[note]]This is actually true of ''all'' orbits. Smaller objects do not actually orbit larger objects, but rather both orbit their combined center of mass, called the barycenter. If one object is significantly larger than the other, that barycenter may be near or below the surface of the larger object, but it will not be located at its exact center. For example, the center of mass of the solar system is not actually the very center of the Sun, but is a point which is constantly changing - sometimes even being located outside the surface of the Sun. And yes, this means the orbital paths of all the planets are also constantly changing. Not by much, but it's measurable. Likewise, the supermassive black hole Sagittarius A* is located ''very near'' the galactic rotational center, but is not ''exactly'' at the center.[[/note]]

Another common misconception is that all the stars in the galaxy orbit the supermassive black hole at the center, the same way all the objects in the solar system orbit the Sun. However, the Sun makes up 99.8 percent of the mass of the solar system, while the supermassive black hole at the center of our galaxy (Sagittarius A*), despite having the equivalent of 4.5 ''million'' solar masses, is only 0.0001 percent of the total mass of the galaxy. Sagittarius A* therefore cannot be solely or even mainly responsible for the orbits of all the stars in the galaxy. If it were removed, the orbits of almost all objects in the galaxy would not change (except for those few objects that are very close to it). Strictly speaking, everything in our galaxy does not orbit the supermassive black hole at the center, but rather orbits the ''center of mass'' of the galaxy, which ''includes'' the supermassive black hole (and which happens to be at the center), but also includes the tens of millions of stars also clustered around the middle. The galactic orbital paths of all objects in the galaxy are caused by the ''total'' mass of the galaxy, not solely the mass of the black hole at the center. [[note]]This is actually true of ''all'' orbits. Smaller objects do not actually orbit larger objects, but rather both orbit their combined center of mass, called the barycenter. If one object is significantly larger than the other, that barycenter may be near or below the surface of the larger object, but it will not be located at its exact center. For example, the center of mass of the solar system is not actually the very center of the Sun, but is a point which is constantly changing - sometimes even being located outside the surface of the Sun. And yes, this means the orbital paths of all the planets are also constantly changing. Not by much, but it's measurable. Likewise, the supermassive black hole Sagittarius A* is located ''very near'' the galactic rotational center, but is not ''exactly'' at the center.[[/note]]

Another common misconception is that all the stars in the galaxy orbit the supermassive black hole at the center, the same way all the objects in the solar system orbit the Sun. However, the Sun makes up 99.8 percent of the mass of the solar system, while the supermassive black hole at the center of our galaxy (Sagittarius A*), despite having the equivalent of 4.5 ''million'' solar masses, is only 0.0001 percent of the total mass of the galaxy. Sagittarius A* therefore cannot be solely or even mainly responsible for the orbits of all the stars in the galaxy. If it were removed, the orbits of almost all objects in the galaxy would not change (except for those few objects that are very close to it). Strictly speaking, everything in our galaxy does not orbit the supermassive black hole at the center, but rather orbits the ''center of mass'' of the galaxy, which ''includes'' the supermassive black hole (and which happens to be at the center), but also includes the tens of millions of stars also clustered around the middle. The galactic orbital paths of all objects in the galaxy are caused by the ''total'' mass of the galaxy, not solely the mass of the black hole at the center. [[note]]This is actually true of ''all'' orbits. Smaller objects do not actually orbit larger objects, but rather both orbit their combined center of mass, called the barycenter. If one object is significantly larger than the other, that barycenter may be near or below the surface of the larger object, but it will not be located at its exact center. For example, the center of mass of the solar system is not actually the very center of the Sun, but is a point which is constantly changing - sometimes even being located outside the surface of the Sun. And yes, this means the orbital paths of all the planets are also constantly changing. Not by much, but it's enough to be measurable. Likewise, the supermassive black hole Sagittarius A* is located ''very near'' the galactic rotational center, but is not ''exactly'' the center.[[/note]]

Another common misconception is that all the stars in the galaxy orbit the supermassive black hole at the center, the same way all the objects in the solar system orbit the Sun. However, the Sun makes up 99.8 percent of the mass of the solar system, while the supermassive black hole at the center of our galaxy (Sagittarius A*), despite having the equivalent of 4.5 ''million'' solar masses, is only 0.0001 percent of the total mass of the galaxy. Sagittarius A* therefore cannot be solely or even mainly responsible for the orbits of all the stars in the galaxy. If it were removed, the orbits of almost all objects in the galaxy would not change (except for those few objects that are very close to it). Strictly speaking, everything in our galaxy does not orbit the supermassive black hole at the center, but rather orbits the ''center of mass'' of the galaxy, which ''includes'' the supermassive black hole (and which happens to be at the center), but also includes the tens of millions of stars also clustered around the middle. The galactic orbital paths of all objects in the galaxy are caused by the ''total'' mass of the galaxy, not solely the mass of the black hole at the center. [[note]]This is actually true of ''all'' orbits. Smaller objects do not actually orbit larger objects, but rather both orbit their combined center of mass, called the barycenter. If one object is significantly larger than the other, that barycenter may be near or below the surface of the larger object, but it will not be located at its exact center. For example, the center of mass of the solar system is not actually the very center of the Sun, but a point which is constantly changing - sometimes even outside the surface of the Sun. And yes, this means the orbital paths of all the planets are also constantly changing. Not by much, but it's measurable. Likewise, the supermassive black hole Sagittarius A* is located ''very near'' the galactic rotational center, but is not ''exactly'' at the center.[[/note]]