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What's at the Center of the Milky Way? In this article it is said that a supermassive black hole lies in the center of milky way galaxy.

At its center, surrounded by 200-400 billion stars and undetectable to the human eye and by direct measurements, lies a supermassive black hole called Sagittarius A*, or Sgr A* for short. The Milky Way has the shape of a spiral and rotates around its center, with long curling arms surrounding a slightly bulging disk. It's on one of these arms close to the center that the sun and Earth are located. Scientists estimate that the galactic center and Sgr A* are around 25,000 to 28,000 light-years away from us. The entire galaxy is around 100,000 light-years across.

We revolve around the center every 250 million years.Presumbably we rotate beacuse of the BH.

When the black hole dies in our galaxy will we be thrown out of the revolving orbit?

The shape of the galaxy is expected to change right?It will be some irregular shape not spherical?

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    $\begingroup$ The black hole evaporation is so slow that as long as there is a tiny bit of gas in the vicinity the infall will exceed the evaporation and the mass will increase. $\endgroup$ – Ross Millikan May 29 '18 at 21:52
  • $\begingroup$ And BHs will continue to absorb CMB and stellar radiation, further adding to their mass even if they've cleared their neighbourhood of gas and dust. $\endgroup$ – Chappo May 30 '18 at 4:23
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    $\begingroup$ This question is pretty much the same: physics.stackexchange.com/questions/98186/… $\endgroup$ – Steve Jessop May 30 '18 at 9:45
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    $\begingroup$ @Acccumulation: I agree but it seemed OP was thinking about the black hole evaporating while there were stars and a normal galaxy around. My point is that the black hole will not evaporate until long after the region is bereft of material to feed it. $\endgroup$ – Ross Millikan May 30 '18 at 21:15
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    $\begingroup$ @Acccumulation Even just the radiation from anywhere (CMB, other galaxies) is strong enough to more than outweigh the losses through Hawking radiation. Remember, a black hole is pretty, well, black. It's a shadow in front of the CMB (what we see in X ray flares etc.comes from its environment, not the hole proper). $\endgroup$ – Peter A. Schneider May 30 '18 at 22:29
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Presumbably we rotate beacuse of the BH.

No. The galaxy is being held in one piece due to its own total gravity. The black hole is only a small fraction of that. Basically, the BH doesn't matter.

When the black hole dies in our galaxy

The BH will probably be the last thing left of our galaxy at the end. And even then it will take some incredibly long time for it to evaporate. BH evaporation for very large BHs is basically the slowest process you could imagine.

It will be some irregular shape not spherical?

The galaxy is not spherical. Its shape is rather more like a round disk (with some irregularities and some features like arms, etc).

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    $\begingroup$ "BH evaporation for very large BHs is basically the slowest process you could imagine." Several factors slower even than the (predicted) rate of proton decay! $\endgroup$ – curiousdannii May 31 '18 at 9:19
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Answer: Not much

The Milky Way's central black hole (BH) masses about 5 million suns, while the galaxy masses 100 billion to a trillion suns. Consequently, the central BH is pretty much irrelevant to the dynamics of stellar orbits except very close to the center.

But what do you mean by "the black hole dies"? Do you mean evaporates through Hawking radiation? (That's the only process we know of that can do away with a BH, and it is so slow that the galaxy will long since have disappeared before the central black hole evaporates.)

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    $\begingroup$ Don't forget that Hawking radiation is still a theory. Nobody has every actually seen it. IMHO it's worth reading Hawking's 1975 paper particle creation by black holes. $\endgroup$ – John Duffield May 29 '18 at 18:40
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    $\begingroup$ @John Duffield: That looks interesting. Note, though, that he's criticizing the thermodynamic interpretation of BHs, whereas Hawking radiation arises from applying quantum field theory in a GR context. If I understand things correctly, in principal, Hawking radiation doesn't actually need a BH, though it's hard to imagine it being observable anywhere else. Hawking radiation is taken as support for the thermodynamic interpretation/analogy/metaphor/whatever and not a consequence of it. $\endgroup$ – Mark Olson May 29 '18 at 20:47
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    $\begingroup$ People claim they have, but when you look into say nature.com/articles/nphys3863 you find it's the waterfall analogy, which is wrong. Einstein rejected Gulstrand-Painlevé coordinates for good reason - we do not live in some Chicken Little world where space is falling down. $\endgroup$ – John Duffield May 29 '18 at 21:10
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    $\begingroup$ Note that the paper claiming the theory of Hawking radiation to be flawed is neither published nor cited by anyone. In fact, the author has no publications, but does have another paper out on arXiv starting with "Our universe is probably a huge black hole". Red flag! $\endgroup$ – pela May 30 '18 at 11:59
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    $\begingroup$ @JohnDuffield Please say "hypothesis" instead of "theory" when preceded with "Just a ___" $\endgroup$ – wedstrom May 30 '18 at 22:14
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Absolutely nothing left.

The time for stellar black holes to evaporate is said to exceed the proton half life. How much more the galactic black holes. And by the way, this time is currently increasing as even stellar black holes are currently growing from the cosmic background radiation alone.

The universe must pass through the intermediate phase of black holes and empty space before this happens.

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    $\begingroup$ The point about CMB infall is crucial, I think. The universe must be old enough that background is colder than the Hawking radiation from the black hole, and we should consider what the galaxy will look like by then. Furthermore, the galaxy must be dark enough that the bit surrounding the black hole is colder the Hawking radiation, which kind of tells you what it looks like. Then the black hole starts to evaporate. It's just thermodynamics, and a black hole that size is incredibly cold. $\endgroup$ – Steve Jessop May 30 '18 at 9:42
  • $\begingroup$ I think that because Hawking radiation is so small the black hole will grow because the net radiation flux is inward as long as there are any radiation sources around, even if we ignore the CMB. Now if we assume for the fun of it that the universe gets old enough that only black holes are left, and if we assume that some are still within each other's event horizon, will they not at some point be in thermodynamic equilibrium, i.e. exchange equal amounts of radiation with each other? $\endgroup$ – Peter A. Schneider May 30 '18 at 21:28
  • $\begingroup$ If the proton decays, that is - all we know is that if it does, the half-life must be absurdly long. And of course, assuming there's no other process that would eventually "destroy" everything but the black holes. $\endgroup$ – Luaan May 31 '18 at 8:17
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    $\begingroup$ @PeterA.Schneider Don't forget the universe is expanding. Even if black holes happened to be in thermal equilibrium at that point, the expansion changes that. In the extreme case, eventually (if spacetime continues to expand at least at present rates), each black hole not gravitationally bound to another black hole will be alone in its observable universe. $\endgroup$ – Luaan May 31 '18 at 8:22
  • $\begingroup$ @Luaan True. Our local group is gravitationally bound though so that Sagittarius A* will not be alone until all black holes have fallen into the last one left. Newer research indicates that Sagittarius A* itself already has a penumbra of thousands of black holes. $\endgroup$ – Peter A. Schneider May 31 '18 at 9:41
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To answer this, let's look at the next few billion/trillion/quadrillion/? years, and get a sense of the size of our galaxy and its central black hole.

The first thing that happens related to your question, is that our galaxy and Andromeda collide and merge. This happens in some billions of years. When galaxies merge, the combined galaxy exists, but may have a different form, merged central black holes, and stars (or in some cases even one or both black holes) may be flung out of the combined galaxy. But the galaxy will endure, in some form or other.

That's because a galaxy isn't held together by its central black hole.

A sense of scale: mass

In our galaxy, the central BH has a mass of about 4 -4.5 million suns.

A bigger part is the stars, gas, and other ordinary baryonic matter (some hundreds of billions of stars, although many are red dwarfs and smaller than our sun). The ordinary matter is estimated to be about 600 billion suns,or about 150,000 times the mass of the central black hole.

But the biggest part is dark matter. Explained simply, even taking into account all of the above mass, the galaxy still wouldn't be massive enough to rotate as it does. Calculations show that about 85% of all matter in our galaxy is "dark matter" - a type of matter that isn't made of ordinary atoms, but is suspected to be made of particles that can't interact much except through gravity (so we can't detect it through radiation, it doesn't form planets, stars or black holes, etc). Dark matter would be about 3.5 trillion suns, or about 850,000 times the mass of the central BH.

So the total mass (ordinary + dark matter) is about 4 trillion suns or about a million times the mass of the central black hole.

A sense of scale: diameter

Considering size rather than mass, the central BH is perhaps the size of Uranus' orbit (about 12 light hours diameter).

The visible galaxy is about 100,000 light years diameter, or about 70 million times the BH size.

The extent of the dark matter halo is less certain (and has less of a defined edge), but depending on which research is right, may extend between 500,000 and 1 million light years diameter, or something along those lines (from memory), or a little under half a billion times the BH size.

Summary

The central BH contains about a millionth (0.0001%) of the galaxy's mass, and about 2 billionths (0.0000002%) of its diameter.

So the central black hole is actually, and oddly, almost insignificant in terms of our galaxy's present-day structure. It might have been crucial for the formation of the galaxy, but that was long, long ago. It's not the current reason we rotate, and its not the reason we stay in galactic orbit. If it vanished or was ejected tomorrow, nothing at all would change except for a comparatively few stars in the galactic centre that directly orbit the BH. We're nowhere near there. We are in a spiral arm.

The bottom line is, if the central BH vanished or left our galaxy, we and our descendants wouldn't ever notice, except for a change in X-ray emissions from that region (as detected by radio telescopes), and a few very faint stars in that region moving slightly differently over the millennia. That's all.

But as other answers explain, a black hole takes an immense time to evaporate, so in reality, two things will happen:

  • On a timescale of billions to trillions of years At some point the merging Milky Way/Andromeda galaxy (or a successor galaxy) will keep, merge or eject its central BH. This event won't be an 'end' to the galaxy or the stars in them, although the combined galaxy probably won't be a spiral shape; merged galaxies are common. The combined galaxy will settle down and things will continue.

  • On a timescale beyond human comprehension (quadrillions upon quadrillions of years) If our universe still exists in its present structure and the standard model and standard cosmology are about right, the central BH will eventually evaporate. But the galaxy (and all galaxies, and most matter) will have decomposed long, long, long before that can happen.

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  • $\begingroup$ Detail: The baryonic matter of the galaxy is ~150,000 times the mass of Sagittarius A*, not 100,000,000. Not that that changes much ;-). $\endgroup$ – Peter A. Schneider May 31 '18 at 9:59
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    $\begingroup$ The sources I found were that the ordinary mass was about 6 x 10^11 solar masses and Sagittarius A* is about 4 to 5 million solar masses. 6x10^11/4x10^6 = 1.5x10^5 ..... and apparently I can't do basic mental arithmetic. Fixed, thank you! $\endgroup$ – Stilez May 31 '18 at 14:12
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A macroscopic black hole cannot shrink as long as any radiation source (like other galaxies) is within sight. The Hawking radiation is very weak; black holes are called black for a reason. In fact the Hawking radiation is already outweighed by the cosmic microwave background radiation alone for black holes heavier than the moon. This is just a function of temperature: The universe background has a temperature of 2.72 K — in order to emit more radiation than it absorbs the black hole must be hotter, which requires a mass smaller than the moon's. Solar mass black holes have a low temperature in the 6E-8 K order of magnitude. That means that even absent any matter it could absorb and absent any specific source of radiation a massive black hole would still grow, not shrink.

In the case of Sagittarius A* there is a lot of matter and radiation around, namely our galaxy, which will eventually fall into the black hole, if left undisturbed for a sufficiently long time. The resulting super duper massive black hole would be super duper cold (around E-19K, give or take a few orders of magnitude) and could feed even from an ever-cooler microwave background for a long time. Only when everything has been absorbed or disappeared beyond the event horizon can it even start shrinking at all. And because it is very very cold it will shrink very very slowly.

It is more likely that other events will precede this evaporation though. This paper describes how in the far future — say, 100 billion years — the accelerating expansion of the universe will leave us stranded on the gravitationally bound island of our local group, because everything else "expands away".

At some point the black holes in this island will have absorbed all surrounding matter until only orbiting black holes are left. They will eventually fall into each other because they lose kinetic energy through gravitational waves. The end scenario is a single giant black hole which rotates enormously fast (making temperature estimates more difficult). It is conceivable that at some point in this process the background radiation will become colder than the black hole(s) so that the ever more massive black holes indeed, finally, start evaporating. Very very very slowly though.

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  • $\begingroup$ Does this perhaps describe the birth of another universe? Obviously with less mass than our "current" universe, which may have been "born" in the same way - as a gravitational island from an earlier universe, and etc. $\endgroup$ – Bob Jarvis May 31 '18 at 3:12
  • $\begingroup$ @BobJarvis It's compatible. Lee Smolin elaborated on and popularized an idea of John Wheeler and Bryce DeWitt in his book The Life of the Cosmos. The basic idea is a multiverse whose diverse population of universes evolves over "time": Some reproduce and some less well equipped die, or at least do not reproduce. Reproduction happens through black holes; any universe whose laws of nature -- especially the power of the various forces -- are such that matter does not condense to form black holes are evolutionary dead ends. (Ctd.) $\endgroup$ – Peter A. Schneider May 31 '18 at 6:40
  • $\begingroup$ ... This elegant argument provides an underlying rationale why we live in a universe like ours: It's the descendant of an evolutionary line able to produce black holes and thus procreate. The argument expands the anthropic principle to a "universic" principle: The universe we observe is the way it is not only because the way it is supports intelligent life but also because the way it is supports universes. As an aside it is also a holistic Gaia paradigm (the nurturing environment is a living entity). $\endgroup$ – Peter A. Schneider May 31 '18 at 6:51
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    $\begingroup$ I suspect the CMB will cool enough to let SMBs evaporate BEFORE they colesce due to gravitational radiation. It is at least, not obvious what order these events happen in $\endgroup$ – Steve Linton May 31 '18 at 14:45
  • $\begingroup$ @SteveLinton True... Although it is also a race because as long as matter falls in (including the occasional black hole) the temperature of the remaining black holes decreases significantly. I mean, 1E-8 K is fairly cold already, and that's just a normal hole. $\endgroup$ – Peter A. Schneider May 31 '18 at 15:05
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The evaporation of the super massive black holes will take a billions of years and thus the gravitational attraction will get weaken in a very long course of time . This will result in expansion of the the galaxy and all the stars system and gases will spread in the universe . But Hawking radiation is very slow process , even it is possible that till that time all the fuel of the stars will get burnt up (hydrogen) resulting into total darkness .

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    $\begingroup$ -1. This is mostly incorrect. As noted in other answers, the time taken for evaporation of black holes hugely exceeds "billions of years", and galactic black holes are a relatively tiny fraction of the mass of the galaxy and are not what's holding a galaxy together. $\endgroup$ – Chappo May 30 '18 at 4:19
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    $\begingroup$ @Chappo Not even mentioning that macroscopic black holes are not shrinking at all and never will as long as anything is inside the event horizon -- the Hawking radiation is too weak to make up for absorbed radiation from the cosmic environment. $\endgroup$ – Peter A. Schneider May 30 '18 at 22:26
  • $\begingroup$ Indeed. Nice to support another SE user who uses a Mandelbrot image in their icon/avatar :-) $\endgroup$ – Chappo May 31 '18 at 6:50
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    $\begingroup$ @Chappo True! They are a metaphor on many levels, so to speak ;-). For example, not shying away from complications of complications $\endgroup$ – Peter A. Schneider May 31 '18 at 7:21

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