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About Stars: Cosmology

Cosmology is the lore of the origin and the evolution of the universe. I try to resume this naturally very extensive subject in a generally understandable way. The statements refer to the momentary state of knowledge and aren't completely indisputable. But they are the probably most correct available models of this universe in which we live.

The universe forms the frontier of our possible knowledge. Outside or before the universe is and was nothing which we could detect or which could influence us in any way. We live in a part of the universe which isn't significantly different from other parts. The laws of nature are the same everywhere in the universe. The universe is finite in space, but has no borders, like the surface of a sphere, but with one more dimension. Therefore it doesn't have a center.

It emerged in the Big Bang 13.7 billion years ago. In the beginning it was infinitely small, infinitely hot and infinitely dense. Since then it expands.

After the Planck time, 5.391*10-44 seconds, the shortest possible time span, the universe had the tiny extent of 1.6 * 10-33 cm and a temperature of 1032 kelvin. Gravity separated from the unification of the other forces. From here on it makes sense to speak of time. Space, time and matter exist now separately, the particles weren't distinguishable then.

After 10-37 seconds cosmic strings emerged, one-dimensional structures, which today because of their gravitational force influence the motion of galaxies and galaxy clusters and order them. Supposedly they are responsible for the formations visible in the picture to the right.

From 10-35 seconds to 10-33 seconds there was the inflationary epoch, in which the universe expanded from a radius of a hundred billionth of an atomic core to one billion light-years. Why the universe did something like that we don't know, but there is much evidence that this phase really happened.

After 10-33 seconds at 1027 kelvin the strong nuclear force separated. This force is responsible for the cohesion of elementary particles and atoms. From there on matter isn't unified any more. It can be distinguished now between quarks, leptons (e.g. electrons) and their antiparticles.

After 10-12 seconds at 1015 kelvin the weak nuclear force separated. Charged and uncharged particles from now on differ.

Until 10-7 seconds and 1013 kelvin was the quark epoch. This was followed by the hadron epoch, during which the quarks combined to hadrons (protons, neutrons and their antiparticles). Furthermore there emerged myones, electrons, positrons, neutrinos and photons. Particles and antiparticles destroyed themselves after their appearance at once and nearly completely and so created more photons. But because there was slightly more matter than antimatter we today still have matter in the universe, but (nearly) no antimatter.
The hadrons were complete after about 10-4 seconds, the leptons not until 10 seconds. All elementary particles that still exist in the universe were formed then and the antimatter was destroyed completely.

Until 200 seconds the primordial nucleosynthesis (nucleon epoch) took place. Protons and neutrons fusioned to bigger cores. Deuterium, tritium and helium 3 for the most part fusioned to helium 4. In only very, very small amounts lithium formed. Heavier elements weren't produced at all. At the end of this phase for every 12 hydrogen cores (protons) was one helium 4 core. The amounts of deuterium, helium 3 and lithium were minuscule. Neutrons and tritium are radioactive and decay in a short time.

For the next 300 000 years (photon epoch) the universe was intransparent, because electrons and atomic cores were separated from each other (plasma) and interacted with the light particles, the photons. At the end of this era the universe had cooled down from 1 billion to 3000 kelvin.

Then began the epoch of matter. Atomic cores and electrons joined to become atoms, the universe became transparent. This is the earliest stage which we theoretically could observe with our instruments.
Because matter wasn't distributed evenly in the early universe there were chunks of matter, which grew due to their gravitation. Cosmic strings probably played a role there. These chunks have been the progenitors of the superclusters, which again inside formed smaller chunks, galaxy clusters, galaxies and finally stars.

The first galaxies appeared not sooner than several million years after the Big Bang. Again later the first stars formed, population III stars. These were (at least mostly) all tremendous stars with up to 1000 solar masses. The lack of heavy elements prevented the formation of smaller stars like these we have today. Those early giants had an accordingly short span of life. They fusioned with an enormous speed and an unimaginable luminosity elements up to iron and soon exploded. Thereby they spread the first heavy elements into the interstellar medium.

So the universe evolved in which we find ourselves today. Space still expands. The further a galaxy is, the faster it moves away from us.
If the universe will expand forever or if the expansion will invert some time we don't know yet. This depends on the mass the universe contains. Here dark matter and dark energy play a leading role. Those make about 90% of our universe. Sadly we know almost nothing about both. It is supposed that the mass of the universe is exactly on the frontier between what is needed for an eternal expansion and a future collaps.

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    Distribution of visible matter in the universe
Computed distribution of visible matter in the universe (extract).
Graphic: MPA Garching

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