The Beginnings: The life of a star begins in a giant molecular cloud. These huge clouds of dust and gas are leftover from the formation of galaxies, and are an ideal place for new stars to form. As the more dense parts of a molecular cloud clump together, gaseous cores are formed. This core starts to collapse into itself, and once it has its own unique gravity it is classified as a protostar.
Stars that are not big enough to sustain hydrogen fusion in their cores are called brown dwarfs.
Stars that are not big enough to sustain hydrogen fusion in their cores are called brown dwarfs.
A Young Star: Once the star begins to burn hydrogen in its core and convert it to helium, the star is a zero age main sequence star. Once the star burns enough hydrogen and becomes more luminous, it is classified as a main sequence star. 90% of a star’s life is spent in the main sequence, and as the star moves along the main sequence, it gets smaller and cooler. Early main sequence stars are more massive and hot, while later main sequence stars are smaller and cooler.
The most common type of star in our galaxy is the red dwarf. Because of their relatively small mass, they can sustain hydrogen fusion for trillions of years. Eventually, if the red dwarf never turns into a red giant, it will run out of hydrogen and become a white dwarf (see planetary nebulae). Red Giants: If the star is massive enough, it will become a red giant. Once the star exhausts the hydrogen in its core, the star starts to collapse in on itself, just like in a smaller star, and it will start to fuse helium. At some point, the heat and pressure are great enough that more hydrogen is brought into the core and fusion resumes, increasing the star’s luminosity and overall surface area. For stars less massive than about nine times the mass of the sun, the star will become an asymptotic giant branch star. At this point the star has an inert carbon/oxygen core, surrounded by burning layers of helium and hydrogen. Depending of its mass and chemical composition, once the core finally collapses the star will become either a planetary nebula, a supernova, or a cepheid star. |
Early main sequence stars are more massive and hot, while later main sequence stars are smaller and cooler. A star's evolutionary track, or how a star will evolve, can be determined solely by its mass and its chemical composition. This is called the Vogt-Russell Theorem. |
Nebulae: When the star can no longer support fusion, thermo-gravitational equilibrium (Liberation of energy from the interior of the star balanced by the energy released at the surface of the star) is disrupted, and the star beings to collapse inward. The pressure in the core increases until is too great, the star ultimately explodes, and its outer layers are forced far out into space. This explosion lasts thousands of years, and once it is complete the core of the star is left behind. The core heats up the gases and dust of the outer layers (which has now become a giant molecular cloud), causing them to brilliantly glow.
The Carbon-Oxygen core left behind is called a White Dwarf. These cores have very high temperatures and are very small. White dwarfs are formed as a star contracts and its electrons are forced into lower energy levels, which is called electron degeneracy. As all of the lowest energy levels are filled, this creates pressure which prevents the mass from contracting anymore, and the white dwarf star is formed.
The Carbon-Oxygen core left behind is called a White Dwarf. These cores have very high temperatures and are very small. White dwarfs are formed as a star contracts and its electrons are forced into lower energy levels, which is called electron degeneracy. As all of the lowest energy levels are filled, this creates pressure which prevents the mass from contracting anymore, and the white dwarf star is formed.
Two subtypes of neutron stars include magnetars, which have extremely strong magnetic fields and pulsars, which are neutron stars that are rotating at very high speeds while emitting radiation so from earth they appear to be pulsing. |
Supernovae: The largest stars have the most dramatic ends to their lives. Similar to a planetary nebula, once the star can no longer support fusion, the core collapses in on itself and eventually explodes. These events are much bigger and more violent than planetary nebulae, and can last up to 200 years. Supernovae are classified as I, II, III, IV, or V based on the frequency of the light they give off and their absorption lines. For supernovae, instead of leaving behind a white dwarf core, they leave behind neutron stars. They get their name because in this core protons and electrons fuse together to form neutrons. Neutron stars are the densest and smallest stars currently known to exist. Neutron stars are formed when an extremely massive star has enough energy available to take electrons and protons and form neutrons. As the energy levels are filled, the pressure eventually stops the star from contracting any further, and a neutron star is formed.
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