Critical to the understanding of several fundamental problems in astronomy (among which the determination of the primordial helium is of foremost importance), extremely metal-poor galaxies have been almost impossible to find. In the past few years we have been successful in discovering them.

We are embarked on a programme for obtaining with linear detectors, very high S/N spectra of these objects, in order to derive He abundances to better than the 5% per object needed to constrain the Big Bang model of the origin of the universe. We will discuss some results and problems encountered in this quest.

Introduction

Three observational findings sustain the Big Bang model of the origin of the Universe: 1) the relic 3-K microwave background radiation; 2) the expansion of the Universe; and 3) the relative abundances of the light elements (H, D, 3He, 4He, Li); for a review, see e.g. Walker et al. (1991). Accurate measurements of these observables provide better constraints on the details of the model.

Even though an accurate measurement of the primordial light-element abundance and in particular of He, is critical to our understanding of the origin of the Universe, it has not been the target of an observational effort comparable to that for the other cosmological observables: the microwave background and the expansion of the universe.