What speed should a galaxy move with respect to us so that the sodium line at `589.0 nm` is observed at `589.6 nm` ?

Explanation

Since `vlambda=c,(Deltav)/(b)=(Deltalambda)/( lambda)` (for small changes in v and `lambda`.) for
`Deltalambda=589.6=589.0=+0.6 nm`
we get [using Eq. (10.9)]
`(Deltavv)/(v)=-(Deltalambda)/(lambda)=-(U_("radial"))/(c)`

or, `v_("radial")~=+c((0.6)/(589.0))=+3.06xx10^(5) ms^(-1)`
`" " =306 km//s`
Therefore, the galaxy is moving away from us.