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View Full Version : Galactic orbital period - is it really the same everywhere?



Cheap Astronomy
2019-May-05, 11:53 AM
I understand the general idea underlying the need for dark matter is that the outer stars revolve around a galaxy's center with much the same orbital period as stars closer to the center?

But surely this general relationship must break down somewhere. For example, in the Milky Way galaxy, it takes the Sun a quarter of a million years to orbit the galaxy? Close-in stars orbiting Sagittarius A* must have quicker orbital periods than that?

Thanks.

Strange
2019-May-05, 12:11 PM
The relationship is not completely flat, just a lot flatter than one would expect from the amount and distribution of "normal" matter.
http://ircamera.as.arizona.edu/NatSci102/NatSci/images/galaxrota.jpg

From here: http://ircamera.as.arizona.edu/NatSci102/NatSci/lectures/darkmatter.htm

Hornblower
2019-May-05, 01:10 PM
I understand the general idea underlying the need for dark matter is that the outer stars revolve around a galaxy's center with much the same orbital period as stars closer to the center?

But surely this general relationship must break down somewhere. For example, in the Milky Way galaxy, it takes the Sun a quarter of a million years to orbit the galaxy? Close-in stars orbiting Sagittarius A* must have quicker orbital periods than that?

Thanks.

Many of us misinterpret that long flat stretch of the curve as meaning the disk rotates like a wagon wheel over that range when it does nothing of the sort. The orbital velocity is nearly the same over that range, so the orbital period gets longer as we go out, just not as quickly as with a Keplerian system without the dark matter halo.

cjameshuff
2019-May-06, 01:19 AM
In addition to what others have said, Sagittarius A* is an object orbiting near the center of the Milky Way. It's not the center of mass of the Milky Way or the rotational center, and its vicinity is certainly not representative of the overall galactic gravitational field.

Also, that overall field is the net influence of all the masses in the galaxy, but is far from smooth when you're looking at a region of space containing a small number of objects, as you're doing when talking about very small orbits relative to the size of the galaxy. The stars near the center are going to move far more due to the mutual gravitational influence of their immediate neighbors than due to the gravitation of the rest of the galaxy, the latter will only become apparent at a larger scale in the motions of those stars as a group.