In addition, astronomers often use infrared, rather than optical, luminosities in the Tully-Fisher relation to avoid absorption problems caused by dust, both in our Galaxy and in others. The Tully-Fisher relation is found using galaxies whose distances are known from Cepheid variables. Then, the relation may be used to get distances to more distant galaxies. The central stars of planetary nebulae are very bright in the ultraviolet part of the spectrum. The ultraviolet light emitted by the central star is absorbed in the surrounding nebula, and such of it is re-emitted in the form of emission lines in the optically visible part of the spectrum, where it can be easily observed.
Planetary nebulae are easy to find in other galaxies.
All one has to do is photograph the light of one of the emission lines. Ordinary stars will not emit strongly at this wavelength, and the planetary nebulae will therefore stand out from the background light of other stars. Observations how that the number of planetaries in a galaxy depends on their intrinsic luminosity. Suppose you were to count all the planetaries in a galaxy whose distance you already know from a study of the Cepheids in it. If you then plot the number of planetaries that fall within each interval of luminosity, you will find that there will be relatively few very bright planetaries.