= Solution
TODO: assign the physical differences to the actual two panels once the missing evolved-star sequences are recovered.
The main intrinsic parameters for old <globular clusters> are age, <metallicity>, helium abundance and the distribution of envelope <masses> after <red giant branch> mass loss. Distance and foreground <interstellar extinction> alter the appearance and must first be removed before assigning intrinsic differences.
Age controls the <main-sequence turnoff>: a more massive <star> has a shorter <main-sequence lifetime>, approximately fuel divided by <luminosity>. An older coeval population therefore has a smaller turnoff <mass> and a fainter, generally redder <main-sequence turnoff>. The separation in <astronomical magnitude> between the <main-sequence turnoff> and <horizontal branch> is useful because a common <distance modulus> cancels.
At comparable age, larger <metallicity> increases stellar-envelope <opacity> and atmospheric line blanketing. This generally makes the <red giant branch> and <main sequence> redder; metal-poor populations tend to have hotter, bluer <red giant branches>. <Metallicity> also affects <hydrogen burning> through the <CNO cycle>, so a quantitative age inference needs composition-dependent stellar models.
The <horizontal branch> is especially sensitive to envelope <mass>: at similar helium-core <mass>, a thinner hydrogen envelope gives a higher <effective temperature> and hence a bluer location. <Red giant branch> mass loss, age and helium abundance all influence that envelope and produce the familiar “second parameter” complexity. Enhanced helium also changes mean molecular weight, <luminosity> and evolutionary lifetime, and can shift <main sequences> and <horizontal branches>.
<Interstellar extinction> dims the diagram, while wavelength-dependent <interstellar extinction> reddens $B-V$; distance primarily supplies a vertical <distance modulus> shift. A systematic bend or different <horizontal branch> colour distribution cannot be explained by distance alone. Because the source hides the evolved branches, it is not possible to assign “older”, “more metal rich”, or “bluer horizontal branch” to a particular panel reliably. The preceding mechanisms give the warranted physical interpretation without inventing the lost observational differences.
Back to article page
