Detection of helium-3 in a planetary nebula

HELIUM-3 is created by cosmological nucleosynthesis with an abundance ³He/H ~ 2 x 10^-5 (ref. 1), but then augmented by stellar nucleosynthesis. Stars comparable in mass to the Sun should contribute a large fraction of the present ³He abundance in interstellar material²: winds from these stars during their main-sequence lifetime, as well as planetary nebulae created by more rapid mass loss later in the stars' lives, are expected to have ³He/H about 100 times the cosmic value. These stars are also thought to be the principal source of new material to the interstellar medium³, and measurement of the present ³He abundance should therefore be an important diagnostic of chemical evolution in the Galaxy,as well as an essential prelude to determining the primordial cosmic abundance⁴. Over a decade ago we began a programme^5-7 to measure the galactic ³He abundance, but until recently it had been possible to detect it only in giant H II regions, where it is already well mixed into the interstellar medium. We report here its first detection in a ³He source, the planetary nebula NGC3242. We measure ³He/H >~10^-3, consistent with stellar models.