Spectroscopic Parallax (2)

If all stars were on the “Main Sequence” in the HR diagram, then a measurement of the TEMPERATURE of the star would allow you to determine its LUMINOSITY from the H-R diagram
LUMINOSITY + APPARENT BRIGHTNESS + INVERSE SQUARE => DISTANCE
Notice that this only works AFTER we have found the luminosities of many stars other ways, and “calibrated” the H-R diagram

If all stars were on the “Main Sequence” in the HR diagram, then a measurement of the TEMPERATURE of the star would allow you to determine its LUMINOSITY from the H-R diagram