From the December 2017 issue

Do all spiral galaxies rotate in the same direction, and how can I tell the rotation from a photo?

Joe Mrusko King of Prussia, Pennsylvania
By | Published: December 4, 2017 | Last updated on May 18, 2023
The Pinwheel Galaxy (M101) is a stunning spiral with well-defined arms. Most spiral galaxies (though not all) rotate such that their arms trail the direction of rotation.
NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI; Canada-France-Hawaii Telescope/ J.-C. Cuillandre/Coelum; G. Jacoby, B. Bohannan, M. Hanna/ NOAO/AURA/NSF
About half of all spiral galaxies appear to be rotating clockwise and the other half counterclockwise. The direction a galaxy rotates depends on your perspective. As an analogy, consider a spinning bicycle wheel. When you look at a spinning wheel from one side, it looks like it’s spinning clockwise. But if viewed from the other side, it looks to be spinning counterclockwise.

Astronomers have long thought that about half of all galaxies should be rotating in one direction, and half in the other. This stems from the idea that we live in an “isotropic” universe, which means that the universe looks roughly the same in every direction. By extension, galaxies shouldn’t have a preferred direction of spin from our perspective. This finding was recently confirmed by a group of citizen scientists who looked at thousands of images of spiral galaxies and determined that half looked to be rotating clockwise and the other half counterclockwise.

In most cases, spiral galaxies spin with their arms trailing the direction of rotation. This is what we expect from our models. But there have been some recent observations that suggest not all galaxies rotate with their arms trailing. Galaxies that have recently merged with or been disturbed by another galaxy can sometimes rotate with their spiral arms leading. The only way to be sure of the rotational direction is to measure the motions of stars or gas in a galaxy, which requires more than just an image. But, when you see a photo of a spiral galaxy, a good guess is that the arms trail the direction of rotation.

Dan Weisz
Assistant Professor of Astronomy, University of California, Berkeley