Summary of the sections on 07-11-96
Review
In our discussion of the early Universe, we have seen how protons and neutrons form (7 protons for every neutron), and how they combine into the atomic nuclei of
He4 and traces of D, He3, and Li7. We understand now why only these light elements formed and why we ended up with
a Universe consisting of 75% H1, 25% He4 and only small amounts of the other nuclei. We have also seen that the atomic nuclei of hydrogen (protons) combine with electrons to form atomic hydrogen when the Universe was about 100,000 years old and had a temperature of about 3000 K. This is referred to as the era of recombination. Radiation and matter decoupled and the left over radiation which could escape freely is observed today as the Cosmic Microwave Background Radiation with a characteristic temperature of 2.7 K. We have then seen how slight initial fluctuations in the extremely smooth early Universe, which can be observed as slight differences in the temperature of the CMBR, were the seeds for the gravitational collapse of clusters of galaxies. The processes that govern this collapse are very similar for clusters of galaxies, individual galaxies, and stars: A gas cloud that reaches a certain critical density starts to collapse. It is transparent to infrared radiation, so the heat which is produced by the collapsing gas can escape freely and does not slow it down. At some point, the gas cloud becomes opaque and traps the radiation. This causes it to heat up and slow down the collapse. In a star, the gravitational force that tries to make the star smaller is exactly balanced by the pressure force generated by the high temperature in the stars interior. Galaxies and clusters of galaxies are supported against further collapse by the
orbital motion of the stars in the galaxies and the galaxies in the clusters of galaxies.
Nuclear Fusion in Stellar Interiors
In a week or so we'll be starting to talk about the formation and evolution of life on Earth and possibly other planets. But so far, we haven't even made the atoms crucial for life and the planet on which the only lifeform we know of has evolved, such as carbon, nitrogen, oxygen, silicon, and iron. These elements are
not made in primordial nucleosynthesis - they are solely made by stars.
Low-Mass Stars
After a star has been formed out of a cloud of gas and dust, its evolution strongly depends on its mass. A lightweight stars like our Sun will spend most of its life on the main sequence (about 10 billion years for the sun) which is defined by the star burning hydrogen in its core and turning it into helium (see handout). After the star has exhausted the hydrogen in its core, it will turn into a red giant star. It will eventually eject its atmosphere and become a white dwarf. Whit dwarfs don't generate any nuclear energy and simply cool off. These low mass stars are not important for the enrichment of the Universe with the elements crucial for life, because 1. they usually don't make any atoms heavier than carbon, and 2. they don't recycle the elements made in their core back into the interstellar medium.
High-Mass Stars
Heavyweight stars exhaust their hydrogen much quicker and therefore have a much shorter main sequence lifetime. They also turn into giant stars after they leave the main sequence, but they generate much higher temperatures in their cores because the gravitational pressure on the core from the material around the core is much larger. To support this pressure, the stars have to generate much higher core temperatures. This allows the stars to contiue their fusion processes and produce all elements up to iron in their core (elements heavier than iron can't be produced because to make them, one has to actually add energy).
The key for the enrichment of the interstellar medium with these heavier elements is the fact that when the heavyweight star has made all the iron it can in its core, there is no other fusion process it can use to generate the energy it needs to support itself agains gravitational collapse. It therefore collapses violently in a so-called type II supernova explosion and recycles all the elements it has made back into the interstellar medium, where it can be used to form the next generation of stars.
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