# How is it possible to have red shift along one axis and blue shift on another in a spiral galaxy arm?

I don't understand this image: If the stars are moving in roughly circular orbits why would red shift occur in the direction of the orbit and how would it be possible to have blue shift in two opposing directions?

• I think the image is showing the galaxy being viewed from 2 perspectives... one along the red line the other alone the blue line to highlight the areas viewed that would be moving towards you but I could be wrong – Fuzzybear May 9 '18 at 8:12
• I don't know if this will help or not. I don't know anything about astronomy, but in an effort to try and come up with some sort of answer, your picture made me start thinking about Synchrotron radiation. When you accelerate a charged particle, it emits radiation. Then I vaguely remember something about a "beam of radiation" in the direction of motion. Then again, this might be totally unrelated – DWade64 May 9 '18 at 11:55
• @dwade64 interesting theory. I think that refers to charged particles on a smaller scale than galaxies, thus the polarized em is perpendicular to the motion. This photo appears to be the stars and their light. (I too am reading this as an amateur) – user5389726598465 May 9 '18 at 11:59
• @DWade64 Most move in the expected direction with the spiral, a few have been found that spin in the opposite direction. This model video seems to imply that they keep their spiral over time (see the video): en.wikipedia.org/wiki/Galaxy_rotation_curve – user5389726598465 May 9 '18 at 12:25
• Well, consider a (nearly) flat rotation curve; that means that the magnitude of the tangential velocity is about the same along the radius. If you are in a ref. frame fixed to a star, if you look at the stars closer to the galactic center then you are, they will overtake you in terms angular displacement (same speed, shorter distance to travel). So as the relative distance changes, you'll see them blueshifted as they approach, and redshifted as they zoom past you, going away. If you look radially, away from the center, you will overtake the other stars, and the pattern is reversed. – Filip Milovanović May 9 '18 at 14:11