Galaxy phenomenon in my coffee

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Dear Mr. Astronomer dude and SDC readers smarter than me,

Every morning I drink my coffee and swirl it with my spoon and something interesting always seems to happen. Try it yourself.

-Spin the coffee around with a spoon, so it makes a little whirlpool type thing.
-When it's at top speed, quickly take your spoon out and drop a few drops of milk in. The milk should form a spinning swirl that looks very much like a galaxy.
-Just watch it. What's strange to me is that the rotation of "milk galaxy" repeatedly slows down and speeds back up. My first guess is there's some kind of thermal or convective thing going on to make this happen.

My question is: could a similar type of scenario be going on with the rotation of galaxies in space? Maybe a similar thermal reaction is going on causing more complex physics of their rotations.

Or maybe not...


Hi, Welcome to

Two completely different things. The coffee cup0 is a bound system (constrained by the cup on the edges), you are adding rotation and turbulence, i.e. a whole bunch of energy that stays in that confined area. It stays there, speeding up and sloing down the swirls until the enrgy is dissapated by the friction from the viscosity of the liquid.

In a galxy, it is unbounded, there is no viscosity, only gravity pulling in, and galactic winds and radiation pushing out. Stars follow their intial motion following Newtons and Kepler's laws under the influence of the gravity. The swirls do not speed up and slow down just follow basic physics with the forces and scales involved (assuming of course, there is dark matter providing additional gravity).


The shapes seen in galaxies are due to the combined gravitational interactions of most of the stars in the galaxy. The spiral arms are density patterns with more stars in them than average. As stars enter an arm they slow down adding to the number of stars jammed up, while other stars leave the other side. So the spiral arm remains as the stars come and go. An earthly analogy is a traffic jam. Have you ever gone through a freeway jam with no apparent reason. In the past some cars were slowed down by some disturbance (say cars entering the flow of traffic badly; think RV = caravan). Cars coming to the same spot had to slow down because of the bunched up cars ...
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