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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. This technique can be applied to any main sequence star for which a spectrum can be recorded. The method depends on the star being sufficiently bright to provide a measurable spectrum, which as of 2013 limits its range to about 10,000 parsecs.
To apply this method, one must measure the apparent magnitude of the star and know the spectral type of the star. The spectral type can be determined by observing the star's spectrum. If the star lies on the main sequence, as determined by its luminosity class, 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, in parsecs) of the star using (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 relationship between absolute magnitude and spectral type.