The Rise of the Heavy Elements. We will soon see that the elements hydrogen and helium were created in the early universe - in fact, during the first seconds of the Big Bang, but the early universe created no heavier elements ("metals") at all. On the other hand, at its birth, the solar system did contain heavy elements, though still mostly hydrogen and helium. The table below summarizes the compositions of both the early universe and the solar system at birth, giving percentages of ordinary matter in each form.
Element Abundance in Early Universe Abundance at Solar Birth
hydrogen (H) 75% 70%
helium (He) 25% 28%
heavier elements ("metals") 0% 2%
As discussed, elements heavier than helium are made deep inside stars. When stars end their lives, they eject some of their insides into space. The solar system was born after many generations of stars lived and died.
(a) Explain how we can understand how the abundances (percentages) of hydrogen, helium, and heavy elements each have changed between the Big Bang and the birth of the solar system. That is, why would we expect each of these to go up or down as they do?
(b) For each of hydrogen, helium, and heavy elements, sketch a graph of how its abundance has changed over time from the Big Bang to the birth of the solar system. That is, make three plots of abundance versus time, and for example, the heavy element plot should have a time axis going from the Big Bang to solar birth, and the abundance axis should go from 0% to 2% heavy elements. All you have been given are the beginning and ending abundances for each element, so sketch your best guess as to how the abundances changed over the times in between.
(c) Briefly explain how the composition of a star can be deduced from the star's spectrum. What part of the star has the composition we measure? Note that nuclear reactions in stars only occur in their cores and do not affect the star's outer layers. Thus the star's outer layers retain the original composition of the star until the star dies. In light of these facts, what do we learn when we measure a star's composition?
