<P> 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 M − m = − 5 log ⁡ (d / 10) (\ displaystyle M-m = - 5 \ log (d / 10)) (see distance modulus). The true distance to the star may be different than the one calculated due to interstellar extinction . </P> <P> 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 . </P> <P> The method ultimately derives from the spectroscopic studies of sunspots and stars by Walter Sydney Adams and Ernst Arnold Kohlschütter . </P>

Why does the spectroscopic parallax method only work for main sequence stars