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CaptainToonces
2011-Sep-17, 11:28 AM
When it is said that stars in a galaxy move at the same speed around the galactic center regardless of distance from the galactic center, are they referring to angular velocity, or velocity through space?

Hornblower
2011-Sep-17, 12:05 PM
When it is said that stars in a galaxy move at the same speed around the galactic center regardless of distance from the galactic center, are they referring to angular velocity, or velocity through space?
It is velocity through space, which for typical galaxies is nearly constant over a wide range of orbital radius. The angular velocity is reduced for outlying objects, but still is greater than what we would expect from the estimated total mass of what we can see.

Cougar
2011-Sep-17, 02:09 PM
It is velocity through space...

Well, minus the velocity of the galaxy 'through space.' The question is a bit ambiguous....

korjik
2011-Sep-17, 03:21 PM
Standard useage is to take the speed in the what you are going around frame. Moon goes around the Earth means Moon's orbital speed, Earth going around the Sun means the Earth's orbital speed, ect. So the question isnt really ambiguous. Especially since the angular velocity part fixes it to the galactic axis of rotation.

CaptainToonces
2011-Sep-18, 12:58 AM
It is velocity through space, which for typical galaxies is nearly constant over a wide range of orbital radius. The angular velocity is reduced for outlying objects, but still is greater than what we would expect from the estimated total mass of what we can see.

Thanks.

I do have a follow up question. If the stars that are more outlying from the center are moving faster than expected, wouldn't that imply that the missing or unseen mass is on the inside of this star's orbit?

Hornblower
2011-Sep-18, 03:12 AM
Thanks.

I do have a follow up question. If the stars that are more outlying from the center are moving faster than expected, wouldn't that imply that the missing or unseen mass is on the inside of this star's orbit?

Yes.

Jeff Root
2011-Sep-18, 05:44 AM
All the mass concentrated at the center gives Keplerian orbits
in which the speed is inversely proportional to the square of the
distance from the center.

Mass uniformly decreasing in density from the center outward
gives orbits with the same speed at all distances from the center.

Mass uniformly distributed throughout gives speed increasing
with distance from the center like a rotating solid body -- the
same angular speed at all distances.

-- Jeff, in Minneapolis

Hornblower
2011-Sep-18, 10:33 AM
All the mass concentrated at the center gives Keplerian orbits
in which the speed is inversely proportional to the square of the
distance from the center.

Mass uniformly decreasing in density from the center outward
gives orbits with the same speed at all distances from the center.

Mass uniformly distributed throughout gives speed increasing
with distance from the center like a rotating solid body -- the
same angular speed at all distances.

-- Jeff, in Minneapolis

My bold. If I am not mistaken, that should be square root.

grapes
2011-Sep-18, 04:54 PM
You are not mistaken! :)