You are observing a binary star system and obtain a series of spectra of the light from the two stars. In this spectrum, most of the absorption lines shift back and forth as expected from the Doppler Effect. A few lines, however, do not shift at all, but remain at the same wavelength. How can we explain the behavior of the non-shifting lines?

Question

Quizplus · Accepted Answer

The fact that only a few lines are not shifting suggests that they are not coming from the stars themselves, but from interstellar matter between us and the stars. This matter is not affected by the motion of the stars and therefore its absorption lines will not shift.

You Are Observing a Binary Star System and Obtain a Series