Transcript: The best traditional test of cosmic nucleosynthesis and the predictions of the big bang model is the helium abundance in the universe. In the model of the big bang the predicted helium abundance is relatively insensitive to the cosmic density, so astronomers need a measurement accurate to three decimal places to make a good test of the model. This is a difficult observation. It’s best made by looking at unevolved stellar systems where the material has not been processed over the entire history since the big bang. Young, unevolved galaxies are looked at, their spectra observed, the helium line is measured, and its strength is interpreted in terms of a model of the emission line. The gas occurs in hot HII regions in the galaxy. Using these observations astronomers show that the helium abundance is consistent with the big bang model, one quarter by mass, and is inconsistent with having been produced in stars. Therefore, the helium abundance that’s being measured is primordial. The helium was present in the universe before the first stars and galaxies were formed.