Figure 10-18 The Method of Spectroscopic Parallax If a star is too far away, its parallax angle is too small to allow a direct determination of its distance. This flowchart shows how astronomers deduce the properties of such a distant star. Note that the H-R diagram plays a central role in determining the star’s luminosity from its spectral type and luminosity class. Just as for nearby stars, the star’s chemical composition is determined from its spectrum, and the star’s radius is calculated from the luminosity and surface temperature.