Stars begin their lives when hydrogen fusion ignites in their dense, hot cores. Once that process starts, it's game on. The gravitational pull of all the mass of the star tries to squeeze it down into a tiny point, but the energy released by fusion pushes outward, creating a delicate balance that can persist for millions or even trillions of years. Small stars live an incredibly long time.
Because of their small stature, they don't need a lot of energy to balance the inward gravitational pull, so they only sip at their hydrogen reserves. In a bonus boost, the atmospheres of these stars constantly circulate, pulling fresh hydrogen down from the outer layers into the core, where it can fuel the continuing fire. All told, a typical red dwarf star will happily burn hydrogen in its core for trillions of years. Not too shabby.
As these small stars age, they steadily become brighter until they just sort of vaguely sputter out, becoming an inert, boring lump of helium and hydrogen just hanging around the universe minding nobody's business but their own. When the massive stars in our universe die, it's much more violent. Because of the increased bulk of these stars, fusion reactions need to happen much faster in order to sustain the balance with gravity.
Despite being so much heavier than their red dwarf cousins, these stars have much shorter life spans: Within only a few million years which given astronomical time scales might as well be next week they die. But when massive stars die, they go out in all their glory. Their huge size, means there's enough gravitational pressure to not only fuse hydrogen, but also helium.
And carbon. And oxygen. If it is made of stiff rubbery material, it will make a fairly large heap, unless, of course, the weight of the material or material piled on top of it is so large that it squashes the material into a smaller heap. A star, like an inflatable bubble, is held up by a balance of internal pressure against gravity. In the normal course of its life, this pressure is provided by the energy produced in nuclear reactions deep in the center of the star.
When those nuclear reactions stop producing energy, the pressure drops and the star falls in on itself. How large a heap will a star make when it collapses? The answer depends on the size of the star.
A star about the size of the Sun will collapse into a heap about the diameter of Earth, or about one-hundredth the original diameter of the star. Such stars are called white dwarf stars because of their small size and because the heat generated by the collapse has made them white hot. A sphere of white dwarf material with a diameter of the size of this would weigh about two pounds, or about a hundred thousand times more than a lead sphere of the same size.
A star about five times as massive as the Sun will undergo a much more violent collapse. The outer layers of the star will be ejected into space in a supernova explosion, leaving behind a collapsed star called a neutron star. Ordinary matter, the kind that we and everything around us is made of, is mostly empty. It is made up of atoms, which are made of electrons, [ ] protons, and neutrons.
The protons and neutrons contain more than The electrons themselves take up little space, but the pattern of their motions, or orbits, defines a size that is the size of the atom. When the protostar starts fusing hydrogen, it enters the "main sequence" phase of its life. Stars on the main sequence are those that are fusing hydrogen into helium in their cores. The radiation and heat from this reaction keep the force of gravity from collapsing the star during this phase of the star's life.
This is also the longest phase of a star's life. Our sun will spend about 10 billion years on the main sequence. However, a more massive star uses its fuel faster, and may only be on the main sequence for millions of years. Eventually the core of the star runs out of hydrogen. When that happens, the star can no longer hold up against gravity. Its inner layers start to collapse, which squishes the core, increasing the pressure and temperature in the core of the star.
While the core collapses, the outer layers of material in the star to expand outward. At this point the star is called a red giant. When a medium-sized star up to about 7 times the mass of the Sun reaches the red giant phase of its life, the core will have enough heat and pressure to cause helium to fuse into carbon, giving the core a brief reprieve from its collapse. Once the helium in the core is gone, the star will shed most of its mass, forming a cloud of material called a planetary nebula.
The core of the star will cool and shrink, leaving behind a small, hot ball called a white dwarf. A white dwarf doesn't collapse against gravity because of the pressure of electrons repelling each other in its core.
A red giant star with more than 7 times the mass of the Sun is fated for a more spectacular ending. These high-mass stars go through some of the same steps as the medium-mass stars. First, the outer layers swell out into a giant star, but even bigger, forming a red supergiant. Next, the core starts to shrink, becoming very hot and dense. Then, fusion of helium into carbon begins in the core.
When the supply of helium runs out, the core will contract again, but since the core has more mass, it will become hot and dense enough to fuse carbon into neon. In fact, when the supply of carbon is used up, other fusion reactions occur, until the core is filled with iron atoms. Up to this point, the fusion reactions put out energy, allowing the star to fight gravity.
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