- #1
Einstein
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Hydrogen and Helium were the main constituents formed soon (thousands of years?) after the big bang. Why did they not burn (as they are flammable), I am sure the universe was hot enough then to ignite it?
After Big Bang: Hydrogen & Helium Formed, But Why Didn't They Burn?
- Thread starter Einstein
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In summary, the elements that have lasted the longest are hydrogen, helium, and lithium. They were created from the fusion of atomic nuclei in stars, and before stars existed, these were the only elements that existed. Oxygen would never form until stars came into being.
- #2
marcus
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hydrogen and helium are the main constituents of the sun at the present time
and the sun also is hot
so one way you could think about the problem you have raised
is to ask why the sun does not immediately burn up
Wouldn't you say the sun was remarkably stable?
It seems so to me anyway.
Whatever it is doing it seems to be doing it very very slowly,
only a little bit (percentagewise) at a time.
Perhaps whatever regulates the sun and prevents it from
immediately consuming itself in some reaction
also (by some analogous mechanism) regulated the
universe a long time ago
- #3
quantum
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Also, helium isn't flammable.
- #4
russ_watters
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Hydrogen burns if you combine it with oxygen. There was not then nor is there now a large amount of oxygen (percentagewise) in the universe.
- #5
Tyger
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When Hydrogen fuses in the Sun the process involves the release of neutrinos, which are very rare events, so the Sun uses its Hydrogen very slowly. The process is slow enough that the Universe could cool down before significant quantitites of Hydrogen were converted after the Big Bang.
- #6
Simfish
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Hydrogen, Helium, and small quantites of Lithium were the only elements that lasted from the Big Bang to until stars first formed. All elements with the exception of these three elements were formed from the fusion of atomic nuclei in stars, and before stars existed, these were the only elements that existed.
Additionally, burning is a chemical reaction that requires the exchange of electrons to occur. Up until 300,000 years after the big bang, electrons were free particles, and there was too much energy for electrons to be brought into orbits around hydrogen, helium, and lithium nuclei. If an electron encountered a nuclei, it would simply collide but would not be placed in orbit around the nuclei of the hydrogen/helium/lithium atom. However, when the universe cooled 300,000 years after the Big Bang, electrons then started orbiting hydrogen, helim, and lithium nuclei; and the universe became transparent. However, oxygen would never form until stars came into being.
Additionally; helium is a noble gas; it does not combine naturally.
Additionally, burning is a chemical reaction that requires the exchange of electrons to occur. Up until 300,000 years after the big bang, electrons were free particles, and there was too much energy for electrons to be brought into orbits around hydrogen, helium, and lithium nuclei. If an electron encountered a nuclei, it would simply collide but would not be placed in orbit around the nuclei of the hydrogen/helium/lithium atom. However, when the universe cooled 300,000 years after the Big Bang, electrons then started orbiting hydrogen, helim, and lithium nuclei; and the universe became transparent. However, oxygen would never form until stars came into being.
Additionally; helium is a noble gas; it does not combine naturally.
- #7
Phobos
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Welcome to Physics Forums, Einstein & Simfishy! 
Good topic, everyone.
Good topic, everyone.
1. How did hydrogen and helium form after the Big Bang?
After the Big Bang, the universe was extremely hot and dense. As it expanded and cooled, protons and neutrons combined to form the nuclei of hydrogen and helium atoms. These atoms then combined to form the first stars and galaxies.
2. Why didn't the hydrogen and helium atoms immediately start burning?
At the time of their formation, the universe was still expanding and cooling. The temperature and pressure were not high enough for nuclear fusion to occur, which is necessary for the burning of hydrogen and helium atoms.
3. How did the universe become hot enough for nuclear fusion to occur?
As the universe continued to expand and cool, gravity caused the gas and dust to clump together, forming dense clouds. These clouds exerted enough pressure and temperature to trigger nuclear fusion, leading to the formation of stars.
4. What role did the ratio of hydrogen to helium play in the formation of the universe?
The ratio of hydrogen to helium is crucial in the formation of the universe. If the ratio had been slightly different, the universe would have either expanded too quickly, preventing the formation of stars, or collapsed too quickly, preventing the formation of galaxies. The precise ratio of 75% hydrogen and 25% helium was necessary for the universe to evolve as it has.
5. How do we know that hydrogen and helium were the first elements to form after the Big Bang?
Scientists have observed the chemical composition of stars and interstellar gas, which shows a high percentage of hydrogen and helium. This confirms that these elements were the first to form after the Big Bang, and their abundance has remained relatively unchanged over time.
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