We observe a glowing cloud of gas in space with a spectroscope. We note that many of the familiar lines of hydrogen that we know on Earth seem to be in a different place. They are shifted toward the blue or violet end of the spectrum compared to their positions in the spectrum of glowing hydrogen gas on Earth. From this we can conclude that:
A) the cloud is much hotter than hydrogen on Earth
B) the cloud is much cooler than hydrogen on Earth
C) the cloud is moving toward us
D) the cloud is moving away from us
E) none of the above can be concluded from this observation.

Question

Quizplus · Accepted Answer

The shift of spectral lines toward the blue or violet end of the spectrum is known as a blue shift, which indicates that the object (in this case, the cloud of gas) is moving toward the observer.

We Observe a Glowing Cloud of Gas in Space with a Spectroscope