Spectroscopic parallax
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Spectroscopic parallax is an astronomical method for measuring the distances to stars. Despite its name, it does not rely on the apparent change in the position of the star (see parallax). This technique can be applied to any main sequence star for which a spectrum can be recorded. Today, a practical limit is 10 megaparsecs.
To apply this method, one must measure the apparent magnitude of the star and know the spectral type of the star. If the star lies on the main sequence, the spectral type of the star provides a good estimate of the star's absolute magnitude. Knowing the apparent magnitude (m) and absolute magnitude (M) of the star, one can calculate the distance (d) of the star using m − M = 5log(d / 10pc) (see distance modulus). The true distance to the star may be different than the one calculated due to interstellar extinction.
While the method of trigonometric parallax provides accurate distances without underlying astrophysical assumptions, it is limited to distances of less than 500-1000 parsecs by current technology. Within the volume measurable by trigonometry are numerous main sequence stars that can be used to calibrate the absolute magnitude - spectral type relationship.
