Q&A: Cosmology

Q:
"On their way to Earth, the X-rays from a distant quasar dim as they pass through a cloud of the intergalactic gas. By measuring the amount of dimming due to oxygen and other elements in the cloud, astronomers were able to estimate the temperature, density and mass of the absorbing gas." My question regards the presence of oxygen in the gas. Where did it come from? It seems that the gas is primordial and, therefore, should only contain hydrogen and trace amounts of helium. What am I missing? Oxygen is only produced inside of stars, and is spread only through supernova explosions. This gas hasn't been through this process?

A:
The primordial gas was enriched with oxygen and other elements by stars that formed when the universe was only few hundred million years to a few billion years old. A important by-product of the research on the intergalactic clouds is to set limits on the amount of oxygen, etc. produced in the early universe. At present this is poorly determined, but is thought to be about 10% of the present concentration of oxygen in the Sun.