Protostar
Nebula is a cloud of interstellation gas and dust. The gas is mostly hydrogen and the dust is mostly carbon and silicon. The dust and gas in the cloud are pulled together by gravity and it begins to spin. As the gas spins faster, it heats up and becomes a protostar. More matter is attracted to the protostar and it continues to grow in size, mass, and heat.
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Main Sequence
When the temperature at the core of a protostar reaches 27,000,000°F, nuclear fusion starts. The nuclear fusion is taking the hydrogen atoms and smashing them together to form a helium atoms. This releases a tremendous amount of energy. The majority of stars in the universe are main sequence stars, including our Sun, and spend the majority of their lives in this stage. They have achieved a hydrostatic equilibrium, where the forces of gravity trying to collapse the star are balanced by the energy released from the fusion reactions trying to blow the star apart.
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Red Giant/Supergiant
Eventually, the hydrogen in the core will be used up. The core will contract but the outer layers, still mostly hydrogen, will expand, cool, and glow red. The expansion of the outer layers dramatically increases the size of the star. Thus they become a Red Giant or a Red Supergiant, depending on their initial starting mass. In the core of the red giant/supergiant, nuclear fusion will continue by converting the helium atoms into carbon. The amount of mass a star has determines which of the following life cycle paths it takes from here.
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A Red Giant low mass star moves on to
White Dwarf
After the helium in the core has all be converted into carbon, the core collapses again, and this time the outer layers are expelled into planetary nebula. The core remains as a white dwarf. No further nuclear reactions take place and eventually it will cool so that no light is seen. At that point it would be a black dwarf, but no star in the universe has reached that stage yet.
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A Red Supergiant high mass star moves on to
Neutron Star / Black Hole
The more massive stars will continue nuclear fusion converting the carbon into even heavier elements. Eventually it will stop. The force of the core collapsing in on itself will cause the electrical forces between the atoms to overcome the gravity and the star will explode into a supernova. If the star was 1.5 to 3 times the size of our Sun, any remaining mass will collapse into a small, dense neutron star. For stars larger than that, its remaining mass will collapse into a black hoe.
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