We have made a detailed study of the color distribution of the main sequence of the Globular Cluster NGC 2808 , based on new deep HST-WFPC2 photometry of a field in the uncrowded outskirts of the cluster .
The observed color distribution of main sequence stars is not gaussian and is wider than expected for a single stellar population , given our ( carefully determined ) measurement errors .
About 20 % of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence .
Using synthetic CM diagrams based on new stellar models we estimate that the helium mass fraction of these stars is Y \sim 0.4 .
The newly found anomaly on the main sequence gives credit to the idea that GCs like NGC 2808 have suffered self–enrichment , and that different stellar populations were born from the ejecta of the intermediate mass asymptotic giant branch ( AGB ) stars of the first generation .

Enhancement and spread of helium among the stars in NGC 2808 have been recently suggested as a simple way to explain the very peculiar morphology of its horizontal branch .
We find that if in addition to the Y = 0.40 stars , roughly 30 % of the stars have Y distributed between 0.26–0.29 while 50 % have primordial Y , this leads to a horizontal branch morphology similar to that observed .
In this framework , three main stages of star formation are identified , the first with primordial helium content Y \simeq 0.24 , the second one born from the winds of the most massive AGBs of the first stellar generation ( \sim 6 - 7 M _ { \odot } ) , having Y \sim 0.4 , and a third one born from the matter ejected from less massive AGBs ( \sim 3.5 - 4.5 M _ { \odot } ) with Y \sim 0.26 - 0.29 .
There could have been a long hiatus ( several 10 ^ { 7 } yr ) , between the second and third generation , in which no star formed in the protocluster .
We suggest that , during this period , star formation has been inhibited by the explosion of late Supernovae II deriving from binary evolution .