Flat Rotation Curve Astronomy Question

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1. Nov 29, 2015

ktramsay

• You have to show your attempts at answering the question, this is a forum policy. Also, please use the template. The only reason your post is not deleted is that you already got some help.
I've been struggling with part (a) of this question. I'm not quite sure what the relation is. Any help is greatly appreciated! Thanks!

Consider a spiral galaxy with a “flat” rotation curve beyond the central 2 kpc.

a. Derive the general relation giving the orbital period, P, of a star (or other object) in the disc orbiting the centre of the galaxy, as a function of the radius, R (>2kpc), from the galactic centre.

b. If stars in the flat part of the rotation curve are orbiting at 250km/s, calculate the orbital period, P in years, for a star at 8kpc from the galactic centre and a hydrogen cloud at 30kpc.

2. Nov 29, 2015

phyzguy

Well, what does the 'flat' rotation curve mean? How fast are the stars moving? Given how fast they are moving, how long will it take them to complete an orbit of radius R?