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- https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/29%3A_The_Big_Bang/29.03%3A_The_Beginning_of_the_UniverseThe universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided wi...The universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided with one another, they could produce material particles. As the universe expanded and cooled, protons and neutrons formed first, then came electrons and positrons. Next, fusion reactions produced deuterium, helium, and lithium nuclei.
- https://phys.libretexts.org/Courses/Grossmont_College/ASTR_110%3A_Astronomy_(Fitzgerald)/14%3A_The_Big_Bang/14.03%3A_The_Beginning_of_the_UniverseThe universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided wi...The universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided with one another, they could produce material particles. As the universe expanded and cooled, protons and neutrons formed first, then came electrons and positrons. Next, fusion reactions produced deuterium, helium, and lithium nuclei.
- https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_2e_(OpenStax)/29%3A_The_Big_Bang/29.04%3A_The_Beginning_of_the_UniverseThe universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided wi...The universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided with one another, they could produce material particles. As the universe expanded and cooled, protons and neutrons formed first, then came electrons and positrons. Next, fusion reactions produced deuterium, helium, and lithium nuclei.
- https://phys.libretexts.org/Courses/Prince_Georges_Community_College/General_Physics_I%3A_Classical_Mechanics/07%3A_Density/7.03%3A_Density_TriviaThe least dense planet is Saturn, with a density of 0.687 g/cm 3 . Saturn is the only planet in the Solar System that would float on water (given a large enough ocean). If the star is large enough, gr...The least dense planet is Saturn, with a density of 0.687 g/cm 3 . Saturn is the only planet in the Solar System that would float on water (given a large enough ocean). If the star is large enough, gravity can be strong enough to push the electrons of the atoms into the nucleus, forming a “neutron star”, which is essentially a giant ball of neutrons.
