Doppler shifted hydrogen absorption lines are seen in the spectrum of a star. The hydrogen line at 656.28 nm is seen to be shifted to 656.08 nm. Is the star moving towards or away from us, or can we not tell?

Question

Quizplus · Accepted Answer

The shift of the hydrogen absorption line towards the blue end of the spectrum (i.e. from 656.28 nm to 656.08 nm) indicates that the star is moving towards us. This is because the Doppler effect causes an apparent decrease in wavelength when a source of waves (such as light) is moving towards an observer.

Doppler Shifted Hydrogen Absorption Lines Are Seen in the Spectrum