<p>The same way rotation does.  If you look at each star, it is following some kind of orbit, so it has a motion.  The only difference between a spiral and an elliptical is that the motions in a spiral are all in the same plane and the same direction, but in the elliptical, they are all over the place.  So "random velocity" just means you look at a bunch of stars-- each one is still in an orbit, so it's no surprise it stays in orbit.</p>

CC BY-SA 3.0
2016-09-17T16:09:25.523Z
2016-09-17T16:09:25.523Z
Ken G
<p>As opposed to spiral galaxies, which are rotationally supported, elliptical galaxies are said to be supported by random motions of their stars. I don't really have an intuitive understanding of this. How do random motions prevent a collapse?</p>

CC BY-SA 3.0
2016-09-17T21:31:32.893Z
The phrase rotationally supported is a poor one. There is no outward force in spirals or ellipticals, only an inward one that accelerates starsintheir orbits.
ProfRob
CC BY-SA 3.0
2016-09-19T12:34:31.850Z
Nonetheless, both "rotationally supported" and "pressure supported" are used fairly often. (Though "pressure supported" is ambiguous, since sometimes it's used as in the OP and sometimes it refers to actual *gas pressure*, as in a rotating gas disk.)
Peter Erwin
<galaxy><galactic-dynamics><stellar-dynamics>
Ellipticals supported by random motions
user1991
# Ellipticals supported by random motions

(Asked by: username_0 on 2016-09-17)

As opposed to spiral galaxies, which are rotationally supported, elliptical galaxies are said to be supported by random motions of their stars. I don't really have an intuitive understanding of this. How do random motions prevent a collapse?

**Tags:** galaxy, galactic-dynamics, stellar-dynamics
> **Comments:**
> **username_1**: The phrase rotationally supported is a poor one. There is no outward force in spirals or ellipticals, only an inward one that accelerates starsintheir orbits.  
> **username_2**: Nonetheless, both "rotationally supported" and "pressure supported" are used fairly often. (Though "pressure supported" is ambiguous, since sometimes it's used as in the OP and sometimes it refers to actual *gas pressure*, as in a rotating gas disk.)  

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## 1 answers

### Answer by username_3 on 2016-09-17. Score: 3

The same way rotation does. If you look at each star, it is following some kind of orbit, so it has a motion. The only difference between a spiral and an elliptical is that the motions in a spiral are all in the same plane and the same direction, but in the elliptical, they are all over the place. So "random velocity" just means you look at a bunch of stars-- each one is still in an orbit, so it's no surprise it stays in orbit.


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