How do spiral galaxies keep their shape?
A nearby spiral galaxy, M33. Atlas Image, obtained as part of the Two Micron All Sky Survey, a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
This is M33, a nearby spiral galaxy probably similar in appearance to our own Milky Way galaxy. It spins majestically in space. In that process, how does it keep its spiral shape?
Estimates generally range from about 220 million to about 250 million years for the galaxy’s rotation. Our sun is four to five billion years old. It’s made a couple of dozen revolutions around the galactic center already, according to the reckoning of astronomers.
Astronomers used to wonder why the spiral arms of a galaxy didn’t wind themselves up, around the galaxy’s core, as the galaxy spun on this majestic timescale in space. Then they figured out that a galaxy’s spiral arms must be temporary features: the result of a shock wave moving through the galaxy.
The idea is that the entire disk of a galaxy is filled with material. As this density wave passes through, it’s thought to trigger bursts of star formation. The spiral arms of a galaxy mark where in the galaxy the density wave recently passed, causing new stars to form and burn brightly.
Given the various estimates for the age of the galaxy as a whole, it seems likely that the whole galaxy has spun around perhaps 50 times.
A spiral galaxy’s arms are visible primarily because they’re made of hot, young stars, and these very luminous stars should live for less than one rotation of the galaxy (note, however, that the galaxy doesn’t rotate as a solid disk).
So modern astronomers see a galaxy’s spiral arms as temporary features that arise and disappear again and again, throughout the lifetime of the galaxy.
