Comment by physix
13 hours ago
This reminds me of when I was a physics undergrad way back in the mid 80s. We used to spend nights drinking beer and hacking some simulations from the Computer Recreations section of Scientific American.
Once we wanted to simulate the dynamics of galaxies. I don'it think it was an SA article, but we did it the slow way by calculating the force on every star individually from each other star. It was excruciatingly slow and boring.
Then some time later, I don't recall where I picked that up, I updated the simulation to just model the force on each star coming from the galaxy's centre of mass.
I could simulate many more stars, have galaxies collide and see them spin off with their stars scattering around.
What struck me was that they looked like real galaxies we see out there.
I wasn't aware of the postulations made in the 60s/70s about there being supermassive black holes at the centre of galaxies, but to me, this simplified simulation was kind of like a smoking gun for that... from an 80286 IBM PC AT.
Even the largest SMBHs mass is a minute fraction of their host galaxies' total mass so it is not the case that everything is just orbiting the SMBH.
Ah, yes, of course! Thanks.
If we're assuming that the galaxy is radially symmetrical, doesn't it immediately follow that the combined gravitational force on a given star is the same as if we applied the force from a combined mass at the center?
This wouldn't work for something like the Solar system with a very sparse distribution of mass, but at the galaxy level it seems right even without the presence of a black hole.
Even when the distance between the centres of mass of two colliding galaxies become comparable to their size?
It's a long time ago, but what I remember was being fascinated by the shapes of the galaxies emerging from a collision under this centre-of-mass approximation, and that it created shapes we see out there. It was as if the main effect were a central mass in each galaxy dominating the dynamics.