D5 Astronomical Spectroscopy

Topic

Spectroscopy is astronomy's most powerful tool: it measures line-of-sight velocities based on the Doppler shift experienced by an object's spectral lines—a shift that increases in direct proportion to speed at low velocities but requires an additional relativistic correction as speed approaches a significant fraction of the speed of light, as occurs, for instance, with quasars.

This same technique also identifies the chemical elements present in an object based on the specific wavelengths associated with each atomic transition and determines the object's temperature, surface gravity, and chemical abundances from those very lines.

Within this analysis, the equivalent width of a spectral line quantifies the line's intensity relative to the background continuum against which it stands out; it is precisely this equivalent width that serves as the basis for measuring the aforementioned chemical abundances.

A spectrograph's resolving power—the instrument's ability to distinguish between very closely spaced wavelengths—determines the minimum Doppler velocity it can detect. Spectrographs built specifically to search for exoplanets via radial velocity push this resolving power—combined with rigorous control of the instrument's temperature and vacuum—to the point where they can detect velocity variations of just a few centimeters per second. Thus, from the Doppler shift with which this text begins to the detection of minute velocity variations with which it concludes—encompassing the identification of chemical elements and the spectrograph's resolving power—spectroscopy enables the light from an object, once broken down into its spectral lines, to reveal both what the object is made of and how fast it is moving.

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