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 `v lambda=c, (Deltav)/(v)=-(Deltalambda)/(lambda)" "("for small changes in v and "lambda)` . For `Delta lambda=589.6-589.0=+0.6nm`
We get `["using Eq. "(10.9)]`
`(Deltav)/(v)=-(Delta lambda)/(lambda)=-(v_("radial"))/(c)`
or, `v_("radial")~=+c((0.6)/(589.0))=+3.06xx10^(5)ms^(-1)`
`=306km//s`
Therefore, the galaxy is moving away from us