Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: why do galaxies spin?

Learn more physics!

Most recent answer: 06/09/2013
Q:
Dear Van, All I can find is the statement that in space matter contracts into a spinning disk due to gravity (to form galaxies, solar systems, etc.), with no explanation as to why the spin. I see a lot about the conservation of angular momentum, but these discussions all presume that the 'spin' already exist. What caused the spin in the first place. Shouldn’t gravity simply attract particles of matter together along a straight path till they collide, as a magnet does to a paper clip? The magnet does not make the paper clip revolve around it, and if I fall off of a building, I don’t spin around the earth. I fall in a straight path till I collide with the earth. What am I missing? Thanks, Tally
- Tally Heulett (age 67)
Palm Bay, FL, USA
A:

Nice question. Let's start with one a little simpler. Why do galaxies form at all? If the matter were distributed completely uniformly, there would be nothing to pick one spot over another for the galaxy to start clumping. Each bit of matter would have a uniform surrounding and not be pulled any direction. However, there's some randomness (apparently ultimately from quantum mechanics) giving some lumpiness to get the process started.

The same random lumpiness means that as a galaxy starts to form there will be random lumps of stuff outside the part forming the galaxy. So different parts of the galaxy will be pulled different directions by gravity from the neighbors.  Unless there's a special accident, the torques won't perfectly balance and the galaxy will pick up some "spin"- angular momentum. A quick web search suggests that the details of the process are still being worked out:

Mike W.


(published on 06/09/2013)

Follow-up on this answer.