The Second Parameter Problem(s)

TL;DR

This paper reviews the historical development of the Second Parameter Problem in globular clusters, discusses recent advances, and proposes that two distinct problems are often conflated, each requiring different solutions.

Contribution

It clarifies the dual nature of the Second Parameter Problem and synthesizes recent progress towards understanding the distinct issues involved.

Findings

Identification of two separate problems within the Second Parameter Issue

Recent advances are providing insights into each problem

Reconciliation of controversies by distinguishing the two problems

Abstract

The Second Parameter (2ndP) Problem recognizes the remarkable role played by horizontal branch (HB) morphology in the development of our understanding of globular clusters, and the Galaxy, over the last 50 years. I will describe the historical development of the 2ndP and discuss recent advances that are finally providing some answers. I will discuss how the controversies surrounding the nature of the 2ndP can be reconciled if we acknowledge that there are actually two distinct problems with entirely different solutions.