Two of the things I truly love about astronomy are that a) it changes rapidly sometimes, with new discoveries happening all the time, and 2) these force me to rethink things I thought I knew, and do a little mental housecleaning every now and again.
For example: Our Milky Way galaxy is an immense flat disk with spiral arms. Since long before I was born, astronomers have understood that these arms are very common in disk galaxies, and can't be due to the galaxy physically winding up like string on a spool. If that were the case, then the arms would eventually pull themselves apart. To counter this they came up with the idea that spirals are density waves, like traffic jams in space. This is what we've thought for decades, and been teaching students all this time.
However, a recent result is that in fact the arms might actually be physical structures after all! I wrote an article detailing all that, and it's really interesting, so I suggest you go read that.
But that's not why I'm writing today. That's a subtle argument dealing with spiral patterns and star motions and all that. What I want to show you ...