Figure 17-19: 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 (see Figure 17-14), the star’s chemical composition is determined from its spectrum, and the star’s radius is calculated from the luminosity and surface temperature.