1 INTRODUCTION

2 DATA AND DATA ANALYSIS

3 THE IMPACT OF HELIUM AND LIGHT ELEMENTS ON THE STELLAR COLORS

4 HOW TO READ CHROMOSOME MAPS

5 DETERMINATION OF THE HELIUM ABUNDANCE OF THE MULTIPLE POPULATIONS

6 RESULTS

7 COMPARISON WITH THE LITERATURE

8 A POSSIBLE PURE-HELIUM SPREAD AMONG FIRST-GENERATION STARS

9 SUMMARY AND CONCLUSIONS