Submitted by Sabre-Tooth-Monkey t3_zyesvt in askscience
swampshark19 t1_j26jh4h wrote
Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey
How about the effects of dark matter? I heard from somewhere that most stars revolve around GC with the same or similar orbital period due to the influence of dark matter leveling the angular momentum drop off and causing further stars to have a greater velocity. Is this at all true? If so how do I integrate this with what you said?
Aseyhe t1_j26oxu4 wrote
You might have misheard or been misinformed -- the impact of the extended galactic mass distribution (including dark matter) is that the orbital velocity remains approximately uniform over a wide range of radii (see again figure 16 of this review article). The orbital period does not.
Orbital periods are only uniform near the very centers of some galaxies (not ours, and mostly dwarf galaxies). That's actually a challenge to the standard dark matter picture (the core-cusp problem) because it requires that the system's density be uniform in the relevant region, which is not what dark matter simulations predict. But there are lots of proposed solutions to this.
swampshark19 t1_j26t33z wrote
Thank you for educating me.
canineraytube t1_j277ik4 wrote
To what extent could it be said that the distribution of dark matter changes the effective dimensionality of our galaxy? I ask this because, in contrast to typical circular orbits in our 3+1 dimensional spacetime, which slow with increasing radius, all circular orbits around a given mass in 2+1 dimensional gravity (attenuating at 1/d^1) share the same velocity. Is this coincidental?
Aseyhe t1_j284skm wrote
That's correct that the orbits within the extended galactic mass distribution resemble orbits about a point mass in two dimensions (or an infinite line mass in 3D). In both cases the gravitational potential is logarithmic with respect to distance. That's coincidental, and I'm not familiar with any mathematical tricks that take advantage of the correspondence.
e_j_white t1_j26ocwx wrote
Yes, it's true.
The outer stars are moving too rapidly to stay in orbit, and under normal calculations they should be thrown out of the galaxy. Additional gravitational forces must be keeping them in orbit at those faster speeds, and that's where the theory of dark matter comes in.
For example, Venus is moving at 78K mph, while Saturn is moving at 22K mph (because it is much farther away from the sun). If Saturn were moving at the same speed as Venus, it would be thrown farther out from its current orbit.
bitwaba t1_j26vh0u wrote
I thought you have to speed up to go to a higher orbit? I remember seeing something that said a mission to the sun is really difficult because you have to essentially slow your orbit down to 0 to move to an orbit that brings you closer to the center of the solar system.
Aseyhe t1_j26wt7e wrote
Yes, you have to speed up to get to a higher orbit -- and paradoxically, that still results in you moving slower, on average! This is an extremely interesting feature of gravitational systems; for example, it means they have a negative heat capacity (adding energy cools them).
desepticon t1_j271xta wrote
Learning this in Kerbal space program was a big "aha". You aren't so much controlling your speed as you are just altering your orbit on the opposite side of the planet.
swampshark19 t1_j2brjyx wrote
Is this related to gravitational potential being understood as negative energy?
e_j_white t1_j26wo0k wrote
Nope, orbital speed goes down as you get farther away. The equation is:
v = sqrt(G * M / R)
Larger R, smaller v.
In order to REACH a higher orbit, you need to do work to move the mass to a higher gravitation potential. That type of work requires thrust, but once you're at the larger orbit, the speed is slower.
Conversely, to move CLOSER to the sun, you need "anti-thrust" to move lower in the gravitational potential.
bitwaba t1_j26xukm wrote
Ah, thank you. That is much clearer.
adm_akbar t1_j279juf wrote
Yes you have to speed up to get to a higher orbit but then your orbital speed is slower.
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