telluric star

A telluric star (or telluric standard star or standard star or just telluric) is a star with a known spectrum which is used for calibrating ground observations. The goal is to work around telluric contamination, the effects of Earth atmosphere on observation, particularly in spectroscopy, given that Earth atmosphere adds absorption lines. A challenge is that the degree and details of contamination depend strongly upon your location (e.g., your altitude), the angle of view, and weather conditions. The strategy is, in effect, to measure the contamination when/where you observe: when you observe an object (e.g., a star or galaxy), you also observe (at the same time or immediately before or after) a nearby star for which a spectrum is well documented, i.e., your telluric star. Using the difference between its known spectrum and your observation of it, you account for the effect of the atmosphere on your intended observation. The ideal telluric star is a short angular distance from your object-of-interest, has a well-documented, accurate spectrum, is bright (or at least not too dim), and has few particular spectral features, i.e., a minimum of spectral lines, continuous absorption regions, etc.

Note that the term standard star is a bit more general: it is also used for stars for similarly calibrating photometry even if telluric contamination is not the target of the calibration, as would be implied by the term telluric star. The term photometric standard star is used to explicitly indicate a star used in this manner. Catalogs are assembled of such stars, which would include magnitudes at different passbands, or at minimum, color indices. A star may well be both a photometric standard star and a telluric standard star.