# Andromeda wavelengths calculation

1. Sep 10, 2012

### Orion78

I have calculated the wavelength of a line that would be observed in a spectrum from Andromeda where speed and λ0 were known, using the formula z =∆λ/λ0 and z=v/c. I obtained a value smaller than λ0 because Andromeda display a blue shift. After I calculated the same wavelength pretending that Andromeda obeyed the Hubble relationship and I found a different value, greater than λ0. his is because the blueshift indicates a decrease in wavelengths while the redshift an increase in wavelengths? Thanks!

2. Sep 10, 2012

### TSny

Yes, that sounds right. We cannot expect relative velocities of galaxies withing a local group to obey Hubble's law because of the mutual gravitational attraction between the galaxies.