Question

Suppose we see the spectral lines to a distant star doppler shifted to smaller wavelengths. What does this tell us about the star's motion?

a. It is moving towards us
b. It is moving away from us
c. It is completely stationary
d. It is moving in the plane of the sky
e. There is a lot of dust between us and the star

Answer 1

A spectral shift to smaller wavelengths indicates that the star is moving towards us, as explained by the Doppler effect.

Step-by-step explanation:

If we observe the spectral lines of a distant star shifted to shorter (or smaller) wavelengths, this indicates that the star is moving towards us. This phenomenon is known as the Doppler effect, which affects waves, including light, when a source is moving relative to an observer.

In the case of light, if a source (such as a star) is approaching, the wavelengths of light become shorter, shifting towards the blue end of the spectrum (blueshift). Conversely, if a star is moving away, the wavelengths become longer, shifting to the red end (redshift). Therefore, a shift to smaller wavelengths implies that the star is approaching, answering option (a) It is moving towards us.