I usually try to add some interesting commentary to my questions whenever I post to GQ… but, nah, this time I just want to know how long (many millions of years, I’d estimate) for the Milky Way to spin 360°?
I know it’d take a light particle over 300,000 years to travel around the whole circumference, but beyond that, I have no idea how to extrapolate from there.
I just happened to have been watching Cosmos last night. According to Carl Sagan (c. 1980) the Milky Way rotates every quarter billion years. Of course, as implied above, that is just some sort of “average”: the individual stars in different locations may orbit at different rates.
Rereading this, I notice that the thread title asks a different question than the OP itself. That is, the title is asking about the orbital velocity whereas the OP is asking about orbital period. You already have your answer for orbital period, at least for the sun. In the case of orbital velocity, according to this page: