$A$ star emits light of $5500 \ \mathring{A}$ wavelength. If it appears blue to an observer on the Earth,it means:

What speed (in $km/s$) should a galaxy move with respect to us so that the sodium line at $589.0 \;nm$ is observed at $589.6 \;nm$?

Light coming from a star is observed to have a wavelength of $3737 \ \mathring{A}$,while its real wavelength is $3700 \ \mathring{A}$. The speed of the star relative to the earth is (Speed of light $c = 3 \times 10^8 \ m/s$).

$A$ star is moving away from the Earth. An observer on the Earth will see the wavelength of light coming from the star as:

The $6563\,\mathring{A}$ $H_{\alpha}$ line emitted by hydrogen in a star is found to be redshifted by $15\,\mathring{A}$. Estimate the speed with which the star is receding from the Earth.

If a star emitting yellow light is accelerated towards the Earth,then to an observer on Earth,it will appear: