From laboratory measurements, we know that a particular spectral line formed by hydrogen appears at a wavelength of 121.6 nanometers (nm). The spectrum of a particular star shows the same hydrogen line appearing at a wavelength of 121.8 nm. What can we conclude?
A)The star is getting hotter.
B)The star is moving toward us.
C)The "star" actually is a planet.
D)The star is moving away from us.
E)The star is getting colder.

Question

Quizplus · Accepted Answer

The fact that the hydrogen spectral line appears at a longer wavelength in the star's spectrum means that the star is moving away from us. This is because of the Doppler effect - the wavelength of light emitted by an object moving away from us is shifted towards the red end of the spectrum (i.e. a longer wavelength).

From Laboratory Measurements, We Know That a Particular Spectral Line