From the September 2010 issue

How do you distinguish between absorption lines that originate in a distant astronomical object and those from Earth’s atmosphere?

David Loh, Singapore
By | Published: September 27, 2010
Sorting out a spectrum
A spectrum obtained from a ground-based telescope incorporates spectral lines from both the studied object and Earth’s atmosphere. Astronomers have a way of separating the two, which is good news because a spectrum contains a lot of important information.
Astronomy: Roen Kelly, after Erin Wood

Various physical mechanisms broaden the intrinsic absorption lines. The most important is that the random motions of the absorbing atoms cause “Doppler broadening.” Larger motions within the object, such as turbulence and rotation, further broaden the lines. In contrast, telluric lines are sharp and narrow: Earth’s atmosphere is cooler, and thus Doppler broadening is typically smaller.