Viewing a single comment thread. View all comments

etherified t1_j14t9nz wrote

Reply to comment by DoctorWho984 in What assumptions are made about gravity when calculating the gravity within a galaxy? by bizzehdee

Thank you,

"... majority of mass in the galaxy does not orbit the central black hole but orbits the mass interior to it"

a little confused because, isn't orbiting the mass interior to the black hole the same as orbiting the black hole...?

Ah, your second point clears something up for me. I think I had read some time ago that some galaxies are found to actually lack dark matter, and by coincidence or not, also don't have black hole centers (were just start clusters?). But if galaxies exist without black holes and yet still have the velocity anomalies, then that's pretty definitive indeed.

2

DoctorWho984 t1_j14vtko wrote

Sorry, that's bad use of pronouns on my part. What I was trying to get across is that the effects of gravity a star, or gas cloud, or whatever feels is mostly not due to the central black hole. The star feels forces of gravity from all the mass interior to its position in the galaxy. For example, our Sun is 8 kpc away from the central black hole, SagA*. Even though SagA* is massive, we're just so ridiculously far away that the force of gravity exerted by SagA* on the Sun is tiny. Instead, what keeps the Sun orbiting is the force of gravity applied from stars and gas closer to the Sun. So increasing the mass of the central black hole would have almost no effect on the Sun's orbit, and the same goes for most the stars in the galaxy.

3