# How would a Supermassive-black-hole-less galaxy behave?

The Supermassive black hole in the center of the galaxy only contains a tiny fraction of the mass of our Galaxy, so it has minimal direct effect on the orbits of most stars. However I have sort of one potential issue. Newton's shell equations shows that when inside of a spherical planet, you can ignore the effect of all mass further from the center then you are. And while the galaxy is shaped like a disk instead of a sphere, that doesn't help, because the net force on a object "inside" of a ring or hollow disk is outward. Gravitational field of thin 2D ring - numerical simulation

As a result this means that if you were to take a galaxy and remove all matter within x light years from the center, star sufficiently far out would be initially be affected only very slightly, but stars only slightly further would experience minimal gravitational attraction the the center, and possibly even experience a net repulsion.

I've concluded from this you simply can't have a galaxy where EVERY star has a very low eccentricty orbit without having somekind of large black hole in the center.

So considering this, what would the net result of the removal of Sagittarius A* be on the core regions of the galaxy in the long term.

• It's worse for a disk physics.stackexchange.com/q/224299 Commented Jul 19, 2023 at 11:15
• The core regions of the galaxy aren't shaped like a disk. Commented Jul 19, 2023 at 21:32
• @ProfRob What fraction of the total galaxy mass is in the core region? Is it a small part of the total or the majority of the total mass? Commented Jul 20, 2023 at 3:14
• @safesphere I see now how it can be correctly read, and deleted my comment. Commented Jul 20, 2023 at 9:01
• The SMBH sits inside a nuclear star cluster with a radius of $\sim 4$ pc and a mass (in stars) about four times the SMBH. So you don't have to go very far away at all before the SMBH's gravitational influence is pretty minimal. Commented Jul 23, 2023 at 13:13

## 1 Answer

What would the net result of the removal of Sagittarius A* be on the galaxy in the long term ?

The black hole Sagittarius A* has a mass of about $$4 \times 10^6$$ solar masses. The mass of our galaxy as a whole is about $$10^{12}$$ solar masses. So the removal of Sagittarius A* would have an insignificant effect on the dynamics of our galaxy as a whole.

However, supermassive black holes at the centre of a galaxies are believed to have a significant effect on galactic evolution. Specifically, the high-energy jets formed from the energy released in the accretion disk around the black hole can eject cold gas from the galaxy and slow down star formation. So if the Sagittarius A* black hole were removed it is possible that our galaxy could see the birth of more young stars over the next several million years.

• ...would have an insignificant effect on the dynamics of our galaxy as a whole. OP seems to be interested in the inner regions near Sag A*, so while the remark is accurate for the whole, it's missing details on the important part OP's looking for. Commented Jul 19, 2023 at 15:57
• @KyleKanos I answered the question as originally asked, which I quoted at the top of my answer. The OP has since amended their post, adding "core regions" to the final sentence. Commented Jul 19, 2023 at 16:31
• Okay, and? If they're able to edit their question to be more clear, surely you can edit your answer to address the clarification, no? Commented Jul 19, 2023 at 19:00
• @KyleKanos No, zero interest in playing catch up while the OP moves their goalposts. But do feel free to post your own answer if this upsets you. Commented Jul 19, 2023 at 20:54