When the density and temperature are high enough, Fusion in the core of stars is reached. Different fusion cycles that happen in different phases of the life of a star. The fusion of hydrogen into Helium is the first fusion cycle. This is the stage that our Sun is in. When there are stars with a extremely high temperature there are other fusion cycles that occur like the CNO cycle. Helium burning causes Carbon at higher temperatures. The burning of helium generates Carbon. The heavier elements such as Iron are formed, at even higher temperatures. Neutrinos are caused by fusion reactions happening in Stars that reach Earth. Scientists learn a lot about fusion inside the stars by discovering these neutrinos.
Stars do not stay the same, but change as they get older. A star goes through different stages during its lifespan. Stars form from giant clouds of gas and dust in space. Secondly, Gravity pulls the dust and gas together, so it gets really hot. Third, A star forms when it is hot enough for a nuclear reaction to begin. Hydrogen Nuclei combine to become helium nuclei. This emits a ton of energy and the star '' burns'' vibrantly. A nuclear fusion is when there's any nuclear reaction. The star stays stable in this phase for billions of years. The star burns hot from the nuclear reaction. The stars gravity balances the outward pressure from the hot gases that expand. Gravity pulls tinier amounts of outer dust and gas together, which make planets in orbit around the Star. The Yellow dwarf is the sun. The Sun is a kind of star, and has been shining for over 5 billion years. It has a plentiful amount of hydrogen fuel to last another five billion years. The Sun expands to turn into a red giant, when the Sun runs out of the Hydrogen nuclear fuel in its core. Super giants are very huge red giants. There's many conditions that create each type of star. A Prostar is what you have before a star actually forms. It's a assortment of gas that has collapsed down from a huge molecular cloud. The Protostar phase of stellar evolution lasts over 100,000 years. As the years go on, pressure and gravity increase. This forces the protosrar to collapse down. All of the energy emitted by the prostar comes from just the heating caused by the energy from gravity. A T Tauri star is a time frame in a star's formation and evolution just prior to becoming a main sequence star. This phase happens at the end of the protostar phase. This is when the gravitational pressure keeping the stars united is the source of all its energy. T Tauri stars lack the pressure and temperature at their cores to make nuclear fusion. They all look like main sequence stars. They are all the same temperature but brighter |