A remarkable new information has recently emerged. Observations for the colour-magnitude diagrams on globular clusters Cen and NGC 2808 revealed the multiple nature of their stellar populations. The most massive globular cluster Cen for example is now known to have up to four different metallicities both for the main sequence and the red giant branch (Anderson 2002; Bedin et al. 2004). Most shockingly, the bluest main sequence is found spectroscopically to be more metal-rich (Ferraro et al. 2004) which implies an extremely high helium abundance of Y 0.4. Interestingly, Lee et al. (2005b) noted that such a helium-rich stellar population would evolve into extremely hot HB explaining the hitherto mysterious origin for the EHB stars of Cen. Lee et al. claims that the same phenomenon is seen in NGC 2808 as well. Such a high helium abundance could in fact be more mysterious than the origin of the EHB stars itself, hence became a hot topic. The high value of helium abundance (Y 0.4) seems particularly impossible when it is combined with its low metallicity empirically constrained (Z 0.002-0.003). This leads to Y / Z 70 which is extremely unlikely from the galactic chemical enrichment point of view unless some exotic situation is at work, such as the chemical inhomogeneity in the proto-galactic cloud enriched by first stars (Choi & Yi 2007).
No matter what the physical process may be, the CMD fits unanimously suggest that the high value of helium is the easiest solution. Figure 4 shows Lee et al. (2005b)'s comparison between the observed and model CMDs assuming that the hot end of the HB morphology is primarily governed by the variation in the helium abundance. The reproduction is impressive. If the EHB is indeed produced by helium variation, then, it almost seems that we are going back to two decades ago in terms of the debate on the second parameter for the HB morphology (see Lee et al. 1994). According to Lee et al. (2007), a pronounced EHB is more easily found among more massive globular clusters, which forces us to think deeply about the nature of globular clusters in general.
Figure 4. The observed and modeled colour-magnitude diagrams of the globular cluster NGC 2808. left: The cluster shows an exceptionally wide distribution of horizontal branch stars. right: It can be precisely reproduced by theory for example by assuming a large range of helium abundance. Excerpted from Lee et al. (2005b).
It is not yet clear whether the enhanced helium interpretation is physically plausible and whether it can be similarly significant to the galaxy scale where for example the primordial chemical fluctuation proposed should be hidden in the mean properties of the stellar populations of a galaxy (see Choi & Yi 2007).