Bars are common features of galactic structure but we ain’t talking whiskey chasers. 56 per cent of galaxies have strong bars and a large fraction of these have spiral arms emanating from the ends of the bars. The question: is how did these structures form and why are they so common?
Now Merce Romero-Gomez at the Laboratoire d’Astrophysique de Marseille in France and some buddies have worked it out saying that bars form naturally because of instabilities that occur in the orbits of large structures like galaxies.
The pictures say it all.
Ref: arxiv.org/abs/0712.4391: The Formation of Spiral Arms and Rings in Barred Galaxies