We can therefore calculate a stars luminosity from its apparent brightness and distance. We can measure the latter through stellar parallax. We measure a stars surface temperature from its color or spectrum, and we classify spectra according to the sequence of spectral types obafgkm, which runs from hottest to coolest. Cool, red stars of spectral type m are much more common than hot, blue stars of spectral type o. We can measure the masses of stars in binary star systems using newton"s version of kepler"s third law if we can measure the orbital period and separation of the two stars. An h-r diagram plots stars according to their surface temperatures (or spectral types) and luminosities. Stars spend most of their lives fusing hydrogen into helium in their cores, and stars in this stage of life are found in the h-r diagram in a narrow band known as the main sequence.