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I understand that the Milky way is a spiral galaxy with a supermassive black hole in the center.

I understand that because of the stress-energy and the gravitational field of the black hole, the center is rotating faster then the outer parts, and that gives the galaxy a spiral shape.

Question: 1. Let's say the black hole in the middle would suddenly disappear. How long would it take for the Earth and other parts of the galaxy to stop moving in spirals? So how long would all the planets follow the geodesics created by the black hole's gravity, and when would all the Solar systems start to fly away?

  1. What shape would that galaxy be?
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closed as off-topic by Qmechanic May 2 '18 at 20:28

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "We deal with mainstream physics here. Questions about the general correctness of unpublished personal theories are off topic, although specific questions evaluating new theories in the context of established science are usually allowed. For more information, see Is non mainstream physics appropriate for this site?." – Qmechanic
If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ I'm closing this post (v1) as off-topic as a black hole cannot suddenly disappear. To reopen this post (v1), consider rewriting it in terms of a mainstream physics scenario. $\endgroup$ – Qmechanic May 2 '18 at 20:31
  • $\begingroup$ As the OP seems interested in the cause(s) of the spiral shape of the Milky Way, perhaps a simpler and more direct question asking just that would be better, either here or on Astronomy SE. $\endgroup$ – StephenG May 2 '18 at 21:25
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the center is rotating faster then the outer parts, and that gives the galaxy a spiral shape.

It's more complex than that. The spirals are density waves that pass through the galaxy. They are not explained simply by the presence of more mass at the center of the galaxy.

The black hole at the center is enormous in terms of a single object, but almost insignificant at the scale of the galaxy (a bit more than a millionth of the mass of the galaxy). Removing it would affect the orbits of stars very near the center, but not much other than that. The overall shape and structure of the galaxy would be unchanged.

The center of the galaxy is very dense with massive objects. Many other stars and black holes would remain.

How long would it take for the Earth and other parts of the galaxy to stop moving in spirals?

The solar system and other objects don't move in spirals. The spiral is a shape that appears because some regions of the galaxy have more stars than others. But most of the stars are moving in roughly circular orbits. They don't spiral to the center at any time.

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  • $\begingroup$ wow thank you. I know it might sound silly, but if we take it like our solar system, removing the sun would make all the planets fly away I think. But in the case of the galaxy, the central gravity force is not even a single object, but a lot of objects, that would still make the inner parts rotate faster then the outer parts? $\endgroup$ – Árpád Szendrei May 2 '18 at 19:50
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    $\begingroup$ Correct. Almost all the mass of our solar system is in the sun,. It completely dominates the gravity of the system. But the galaxy has billions of stars spread everywhere, plus dark matter. That makes for a lot of differences. $\endgroup$ – BowlOfRed May 2 '18 at 19:54
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    $\begingroup$ @ÁrpádSzendrei The rest of the solar system amounts to about 0.14% of the solar mass. In comparison the rest of the galaxy is million times more massive than Saggitarius A* $\endgroup$ – OON May 2 '18 at 19:55

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