A star is a huge ball of burning plasma that is held together by gravity.
The key to a star's existence is a phenomenon known as hydrostatic equilibrium. The inward gravitational pressure created by the star's mass is balanced by the outward radiation pressure created by the nuclear fusion taking place in the core.
The mechanism driving the outward radiation pressure in a star's core is the nuclear fusion process where hydrogen is fused into helium via the proton-proton chain. This reaction is exothermic, that is it produces more energy than it takes to initiate the reaction.
fusion is a natural process, but it is a difficult one to achieve. It takes a tremendous amount of energy in order to initiate enough fusion reactions to actually balance the force of gravity in a star.
Specifically, a star's core needs to reach temperatures in excess of about 10 million kelvin to energize the hydrogen enough to fuse. Our Sun, for instance has a core temperature around 15 million kelvin.
Therefore a star isn't said to have actually formed until the core temperature reaches this level and fusion begins. Prior to this the object is said to be a protostar.
This process, though, causes the core temperature to rise allowing helium to fuse into carbon. At this point the star has expanded and become a red giant.
The next phase in the star's evolution is completely dependent on the mass of the star. If it is a low mass star, like our Sun, it will eventually blow off its outer layers, creating a planetary nebula with a white dwarf in the middle.
High mass stars, however, will explode in a supernova. The core of the original star is left behind as either a neutron star or a black hole.
The key to a star's existence is a phenomenon known as hydrostatic equilibrium. The inward gravitational pressure created by the star's mass is balanced by the outward radiation pressure created by the nuclear fusion taking place in the core.
The mechanism driving the outward radiation pressure in a star's core is the nuclear fusion process where hydrogen is fused into helium via the proton-proton chain. This reaction is exothermic, that is it produces more energy than it takes to initiate the reaction.
fusion is a natural process, but it is a difficult one to achieve. It takes a tremendous amount of energy in order to initiate enough fusion reactions to actually balance the force of gravity in a star.
Specifically, a star's core needs to reach temperatures in excess of about 10 million kelvin to energize the hydrogen enough to fuse. Our Sun, for instance has a core temperature around 15 million kelvin.
Therefore a star isn't said to have actually formed until the core temperature reaches this level and fusion begins. Prior to this the object is said to be a protostar.
Stellar Death
A star will continue existing on this primary part of its life, known as the main sequence, until it has used virtually all of the hydrogen fuel in its core. At this point the core will contract because the outward radiation pressure is no longer sufficient to balance the gravitational force.This process, though, causes the core temperature to rise allowing helium to fuse into carbon. At this point the star has expanded and become a red giant.
The next phase in the star's evolution is completely dependent on the mass of the star. If it is a low mass star, like our Sun, it will eventually blow off its outer layers, creating a planetary nebula with a white dwarf in the middle.
High mass stars, however, will explode in a supernova. The core of the original star is left behind as either a neutron star or a black hole.
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