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Q & A: Why do galaxies look like draining water?

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Most recent answer: 04/17/2013
Why does the milky way look like water going down a plug hole?
- christine morgan (age 52)
United Kingdom

Hi Christine,

Good question. Ultimately, both galaxies and draining water rotate because of conservation of angular momentum. That rotation has something to do with hopw they form the spirally streaks, but we're not sure about quyite how that works. First let's look at water, and then at galaxies.

Imagine you just pulled the plug on a sink full of water. At first, the water doesn't rotate at all. As the water drains, it begins to rotate, faster and faster, until a small whirlpool has formed.

Initially, there are small random motions in the water, which add up to a small but nonzero total angular momentum. As the water drains from the outside of the sink towards the plug hole, its moment of inertia decreases. By conservation of angular momentum, it must begin to rotate faster. The classic analogy for this effect is a spinning ice skater; when she pulls her arms in towards her body, her moment of inertia goes down, and she spins faster.

Now let's think about how galaxies form. For a galaxy to form, you need a lot of gases, like hydrogen. Usually, an astronomically-sized cloud of gas collapses in on itself by the force of gravity, and the gases begin to condense into stars. During this process, as in the case of the draining water, the galaxies moment of inertia decreases. By conservation of angular momentum, we know that the galaxy rotation speed must increase. This gravitational collapse and corresponding increased rotation increases until the centrifugal force from rotation equals the inward force from gravity, and then you get a brilliant, rotating spiral galaxy. The galaxy is usually disk-shaped, since the matter is thrown outward only around its axis of rotation.

In both cases, conservation of angular momentum governs the specific form of the vortex or galaxy that we observe.

Hope that makes sense!
David Schmid

p.s. There are several much more complicated theories about how vortexes form around plug holes, but the experimental consensus* agrees with my description above: conservation of angular momentum magnifies the small net rotation left over from when the sink/tub was filled.

*See Shapiro (1962), Binnie (1964), etc.


But the problem of how the spiral streaks form probably has different solutions for the galaxies, in which gravity is the only important force, and the draining wtarer. /mw

(published on 04/17/2013)

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