Recent pieces of evidence have revealed that most , and possibly all , globular star clusters are composed of groups of stars that formed in multiple episodes with different chemical compositions .
In this sense , it has also been argued that variations in the initial helium abundance ( Y ) from one population to the next are also the rule , rather than the exception .
In the case of the metal-intermediate globular cluster M4 ( NGC 6121 ) , recent high-resolution spectroscopic observations of blue horizontal branch ( HB ) stars ( i.e. , HB stars hotter than the RR Lyrae instability strip ) suggest that a large fraction of blue HB stars are second-generation stars formed with high helium abundances .
In this paper , we test this scenario by using recent photometric and spectroscopic data together with theoretical evolutionary computations for different Y values .
Comparing the photometric data with the theoretically-derived color-magnitude diagrams , we find that the bulk of the blue HB stars in M4 have \Delta Y \lesssim 0.01 with respect to the cluster ’ s red HB stars ( i.e. , HB stars cooler than the RR Lyrae strip ) – a result which is corroborated by comparison with spectroscopically derived gravities and temperatures , which also favor little He enhancement .
However , the possible existence of a minority population on the blue HB of the cluster with a significant He enhancement level is also discussed .