Spreads in light element abundances among stars ( a.k.a .
multiple populations ) are observed in nearly all globular clusters .
One way to map such chemical variations using high-precision photometry is to employ a suitable combination of stellar magnitudes in the F275W , F336W , F438W , and F814W filters ( the so called “ chromosome map ” ) , to maximise the separation between the different multiple populations .
For each individual cluster its chromosome map separates the so-called first population –with metal abundance patterns typical of field halo stars– from the second population , that displays distinctive abundance variations among a specific group of light-elements .
Surprisingly , the distribution of first population stars in chromosome maps of several –but not all– clusters has been found to be more extended than expected from purely observational errors , suggesting a chemically inhomogeneous origin .
We consider here three clusters with similar metallicity ( [ Fe/H ] \sim - 1.3 ) and different chromosome maps , namely NGC 288 , M 3 and NGC 2808 , and argue that the first population extended distribution ( as observed in two of these clusters ) is due to spreads of the initial helium abundance and possibly a small range of nitrogen abundances as well .
The presence of a range of initial He and N abundances amongst stars traditionally thought to have homogeneous composition , plus the fact that these spreads appear only in some clusters , challenge the scenarios put forward so far to explain the multiple population phenomenon .