It is common to think of using the brightest stars to determine extragalactic distances. Indeed, over 50 years ago, Hubble (1936a) first attempted to resolve stars in other galaxies to quantify the expansion of the universe, and since then, the use of blue and red supergiants for extragalactic distance determinations has been explored several times (e.g., Sandage and Tammann 1974, Humphreys 1983). However, only recently has it been appreciated that young planetary nebulae also fall into the ``brightest stars'' category and are therefore potentially useful as standard candles. As can be seen in the H-R diagram of Figure 14, the central stars of these objects are almost as luminous as the brightest red supergiants - the fact that their continuum emission emerges in the far ultraviolet, instead of the optical or near infrared, does not affect their detectability. On the contrary, since their surrounding nebulae reprocess the EUV radiation into discrete emission lines, planetaries can be viewed through interference filters which suppress the starlight from the host galaxy. As a result, observations made through a narrow band 5007 filter can detect ~ 15% of the energy emitted from these extremely luminous objects, with little contamination and confusion from continuum sources.
Figure 14. A schematic H-R diagram showing the evolutionary tracks of planetary nebula central stars. The tracks illustrate the strong dependence of magnitude and lifetime with core mass: the luminosity of a helium burning central star goes approximately as L ~ M3.5, but the lifetime of the object goes as ~ M-9.5. It is these two relationships, coupled with a sharply peaked core-mass distribution, which cause the abrupt truncation in planetary-nebula luminosity function.
Planetary nebulae have several advantages over other extragalactic distance indicators. Because PN are not associated with any one stellar population, they can be found in galaxies of all Hubble types, and hence are particularly valuable for probing the E and S0 galaxies which define the cores of large groups and clusters. Likewise, internal extinction is usually not a problem in extragalactic PN observations: unlike blue supergiants or Cepheids, PN can be found far away from star forming regions, in areas of the galaxy which are relatively dust free. Since PN are observed through narrow band filters which suppress the continuum, the identification and measurement of these objects does not require complex, crowded field photometric procedures, and, unlike variable star standard candles, PN observations are required only once. Perhaps most importantly, in a large galaxy there may be several hundred planetaries populating the brightest two magnitudes of the planetary nebula luminosity function (PNLF). With the luminosity function so well defined, distance derivations are straightforward, and the internal errors can be as small as 3% (cf. Jacoby et al. 1989).
Despite these facts, the use of PN to determine extragalactic distances is a relatively new phenomenon. The first suggestion that PN might be a useful standard candle can be found in the book Galaxies and Cosmology by Hodge (1966), where it is listed in Table 12.1 along with such well-known distance indicators as Cepheids, RR Lyrae, and novae. Actual PN distance measurements, however, were not not made until the 1970s, when Ford and Jenner (1978) used a 50 Å wide 5007 filter and the SIT Video Camera on the Kitt Peak 4-m telescope to find and measure the brightest PN in the bulge of M81. Ford (1978) had noticed that the absolute [O III] 5007 flux of the brightest PN in each of seven Local Group galaxies varied by less than 25%. Thus, by comparing the [O III] 5007 fluxes of M81's brightest PN with those of the brightest PN found in M31, Ford and Jenner (1978) estimated the distance ratio between these two galaxies to be ~ 4. A few years later, Jacoby and Lesser (1981) used a similar argument to place limits on the distances to five Local Group dwarfs, and Lawrie and Graham (1983) estimated the distance modulus of NGC 300.
None of the above results was exceptionally persuasive, however. Some of this skepticism arose from the analysis method which excluded all but the brightest objects from consideration. However, the main concern at the time was that little was known about the luminosity function of planetaries; hence the uncertainties associated with these distances were completely unknown. It is an irony of the subject that distances to Galactic planetary nebulae are extremely difficult to obtain, and that a single PN is definitely not a standard candle. (For instance, NGC 7027, one of the best studied Galactic PN has recent distance estimates that range from 178 pc [Daub 1982] to 1500 pc [Pottasch et al. 1982].) However, while a single PN may not be a standard candle, an ensemble of these objects can yield a well determined distance. The reason for this is the invariance of the [O III] 5007 planetary nebula luminosity function.