Astronomers are still trying to understand how a galaxy's spiral arms form

The Whirlpool galaxy (M51) is a classic grand spiral galaxy.

It was the first object identified as a spiral, way back in 1845.

At the time, it and other galaxies were considered spiral nebulae, since astronomers didn't know other galaxies existed.

Edwin Hubble finally proved that there were other galaxies in the early 1920s. ...