What will happen to the shape of a galaxy when a super massive black hole lying in its center dies(evaporates out)?

  • 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?


    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.

    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.

    @RossMillikan But the infall is coming from mass in the center of the galaxy, while the radiation is escaping from the galaxy. How much of "stuff near the center of the galaxy is in a black hole" is irrelevant to the gravity that the center of the galaxy produces, all that matters is the total mass.

    @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.

    @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).


  • 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).


    "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!

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Content dated before 7/24/2021 11:53 AM