A process for estimating the temperature of the central star in a planetary nebula. The basic assumption is that the nebula absorbs all the ultraviolet light from the star which can cause ionization. For each ultraviolet photon absorbed a Ηα‎ photon is emitted when the ionized hydrogen subsequently recombines with an electron. Thus the strength of the Ηα‎ line is related to the ultraviolet magnitude of the star. The strength of the red continuum (broad-band) emission underlying the Ηα‎ line gives the red magnitude and hence a colour temperature for the star. This method is not affected by interstellar absorption, since both colours are effectively measured at the same wavelength. It is named after the Dutch astrophysicist Herman Zanstra (1894–1972), who first published it in 1927.