Dotter, AaronSarajedini, AAnderson, JayAparicio, ABedin, L RChaboyer, BrianMajewski, SMarin-Franch, AMilone, AntoninoPaust, NathanielPiotto, GReid, I Neill2015-12-132041-8205http://hdl.handle.net/1885/79058The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon, first described in the 1960s, acknowledges that metallicity alone is not enough to describe the HB morphology ofKeywords: Globular clusters: generalThe ACS survey of galactic globular clusters. IX. Horizontal branch morphology and the second parameter phenomenon201010.1088/0004-637X/708/1/6982016-02-24