# Why is Andromeda only partially blue-shifted?

The light from the center of andromeda isn't blue-shifted, according to images from the Hubble space telescope, but the light from the areas around the center is blueshifted. Is there a reason for this in general relativity?

• How do "images from the Hubble space telescope" tell you about the redshift/blueshift? – Rob Jeffries Aug 19 '16 at 17:36
• Well they show it for one thing. – Sam Cottle Aug 19 '16 at 17:39
• That is simply a colour image of the Andromeda galaxy, it tells you nothing about the doppler shift of various components. If it did, you would be able to tell from the image that Andromeda was rotating... – Rob Jeffries Aug 19 '16 at 17:45
• It does not mean its blue shifted because it appears blue; blue shifted means it will appear bluer than it is (when seen at rest). There are many reasons thus photo may look blue: color/wavelength filters, false coloring (if image was taken in infrared/UV)... other image processing to make it look nice. – anon01 Aug 19 '16 at 17:55
• Light is a spectrum. Blue shift means the whole spectrum is shifted towards smaller wavelengths. So for the visible part this would mean that (when shifting much more than in Andromeda's case) red becomes orange, orange becomes yellow, which becomes green, which turns into blue. And blue itself becomes violet. Violet moves out of the part our eyes can see. But then again some (normally invisible) infrared light would come in from the other side as red. So the amount of blue light tells you nothing at all about the shifting. Read about absorption spectroscopy to learn how we can measure it. – linac Aug 20 '16 at 19:22

These colours have nothing to do with redshift. The average motion of Andromeda towards us is about 300 km/s. This produces a blueshift in visible light (550nm) of a mere 0.55nm, which will not affect the perceived visible colour at all. In fact if such small shifts were perceptible in the colour of an image then the rotation of Andromeda, of around $\pm 250$ km/s, would be even more readily seen, in that one side of the galaxy would be red and one blue! Clearly that is not the case.