SSCs have earned their superlative title largely because of their very high luminosities, which in many cases surpass that of the R136 cluster in the 30 Doradus complex in the Large Magellanic Cloud (LMC). Some of the clusters, for instance, have absolute visual magnitudes exceeding -14 to -15, whereas MV = -11.3 mag for R136 (O'Connell et al. 1994). The clusters will fade, of course, as they age, by an amount depending on their current age. According to models of evolving stellar populations (e.g., Bruzual & Charlot 1993), a 10-Myr old cluster will fade by 6 to 7 mag in V after 10-15 Gyr.
Meurer et al. (1995) define SSCs as having an absolute magnitude at 2200 Å -14 (roughly corresponding to MV -13 mag for a cluster with an age of 10 Myr); this criterion, however, is clearly arbitrary, for there exists a continuum of luminosities among clusters. The luminosity function at the bright end can be approximated by a power law of the form dN LdL, where -2 (Whitmore et al. 1993; Meurer et al. 1995; Meurer 1995; Vacca 1996). Although the luminosity function at the faint end is increasingly affected by detection incompleteness, it evidently preserves the same functional form and slope (Maoz et al. 1996a). The luminosity function, however, as pointed out by van den Bergh (1995), does not resemble that of evolved globular clusters, which obey a nearly universal Gaussian function, but rather that of open clusters (e.g., van den Bergh & Lafontaine 1984). Van den Bergh cites this as evidence that the young clusters are more closely related to open clusters instead of globular clusters. Several factors, however, weaken van den Bergh's argument. As noted by Larson (1993), a direct comparison between the luminosity function of young clusters and globular clusters is inappropriate since the two populations are viewed at very different stages: the former most likely have a heterogeneous mix of ages while the latter represent an evolved, much more uniform group. Dynamical evolution undoubtedly will substantially modify the luminosity function of the young population, especially at the faint, low-mass end. Meurer (1995) has shown that the luminosity function of the clusters in the Antennae system (Whitmore et al. 1993) can be modeled as a combination of a globular cluster mass function and continuous cluster formation.