Astronomy Question: The APOD http://apod.nasa.gov/apod/ap110907.html for 2011 Se

Question

Astronomy Question: The APOD http://apod.nasa.gov/apod/ap110907.html for 2011 September 7 shows an image (well, just a point of light) of a star that apparently has fewer metals than predicted for any stars. I understand that hydrogen and helium and a trace amount of lithium was produced in the "epoch of primordial nucleosynthesis" about 3 minutes after the Big Bang. Why did primordial nucleosynthesis happen only at that time (3 minutes after the BB), not earlier and not later, and why was the primordial abundance set at about 75% of the mass in hydrogen, 25% in helium, a tiny amount of lithium, beryllium and boron and no elements heavier than that?

Explanation / Answer

As the universe cools, the matter content changes, new particles are formed out of the preexisting ones, such as protons and neutrons forming out of quarks. From about one second to a few minutes cosmic time, when the temperature has fallen below 10 billion K, the conditions are just right for protons and neutrons to combine and form certain species of atomic nuclei. This phase is called Big Bang Nucleosynthesis.

So it is the sudden drop in temperature is the reason for Nucleosynthesis.

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