You Are Observing a Binary Star System and Obtain a Series

question 32

Multiple Choice

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?

• A) there is a star in the system which is not moving at all: it is just sitting there
• B) there is a planet orbiting the stars in the system
• C) there are huge clouds of dust just behind this star system from our perspective
• D) the lines come from interstellar matter between us and the star, not from the stars themselves
• E) there is no explanation of this behavior: it is an unsolved mystery in science