Dark Matter: Is Gravity from a Central Mass or Cumulative Effect?

[jonnylockers]

hi, i don't know if this is in the correct place but I've got a question that has puzzled me.

i know that the stars on the outer edge of galaxies are moving too fast to be in a stable orbit without flying off away from the galaxy, does this mean that the gravity that the outer stars experience are from a central mass like a black hole, or is it the cumulative effect from every other star in the galaxy, so in effect instead of the star circling a central mass they are actually circling a much larger mass of every other star that is closer to the centre of the galaxy?

I know that gravity will effect every other star but will it effect it in the way i described or in other ways?

Thanks for any answers, its probably obvious but its something I've wondered about for a long time.

 

[Janus]

It is the cumulative effect.

Here's the thing, if you take the velocities of the stars (not just the outer ones but the inner ones also), and plot it against the stars' distances from the center of the galaxy and compare this to what you expect to get taking the mass and distribution of the visible matter into account, the two results do not agree.

Not only do the velocities we see indicate that there is more mass to the galaxy then what can be accounted for by visible matter, but that it is distributed differently than the visible matter.

 

[jonnylockers]

thankyou

 

[afennah]

I've been reading about the study of galaxy 'rotation curves' (the rotational velocity about the galactic centre). The Dark Matter Problem by Robert H Sanders. In 1941, Subrahmanyan Chandrasekhar demonstrated that gavitational attraction between pairs of individual stars was negligible (within a galaxy) when calculating rotation curves.