We present BV photometry centered on the globular cluster M54 ( NGC 6715 ) .
The color–magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero age horizontal branch theoretical models .
These kinds of horizontal branch stars ( also called “ blue hook ” stars ) , which go beyond the lower limit of the envelope mass of canonical horizontal branch hot stars , have so far been known to exist in only a few globular clusters : NGC 2808 , \omega Centauri ( NGC 5139 ) , NGC 6273 , and NGC 6388 .
Those clusters , like M54 , are among the most luminous in our Galaxy , indicating a possible correlation between the existence of these types of horizontal branch stars and the total mass of the cluster .
A gap in the observed horizontal branch of M54 around T _ { eff } = 27 000 K could be interpreted within the late helium flash theoretical scenario , a possible explanation for the origin of those stars .