Just What is a Star, Anyway? * The Periodic Table * Neutron Stars
A star is a massive fusion reactor, fusing hydrogen into helium.
Carbon, nitrogen and oxygen (CNO) act as catalysts in the nuclear forces which fuse hydrogen into helium. A first step in this fusion process is the movement from hydrogen (one proton, one electron) to heavy hydrogen (the isotope deuterium) whereby a hydrogen atom takes on a neutron.
As two hydrogen atoms are fused into one helium atom, a single photon of light is emitted.
When hydrogen runs out in the core the star begins fusing hydrogen in the layers just outside, causing the star to swell into a red giant. If the core heats up sufficiently as the star ages, it can enter into a stage of helium fusion, whereby helium is fused into carbon.
Fusion up to the element iron generates energy. Fusing elements iron and heavier requires energy.
Why does astronomy care about the Periodic Table? It is essential to understanding the lives of stars.
There are all sorts of variations, but it generally it looks like this:
Notice the most common element in the universe is first in the table: hydrogen. No surprise, it's the fuel of stars. The second most common element in the universe is helium, and it is number 2 in the table. Notice a pattern? The lightest elements are what stars fuse into the heavier elements. Other common elements include carbon, nitrogen, oxygen and so forth.
All the basic elements in the periodic table are created by stars. The heavier elements are rarer because they require larger forces in order to fuse simpler elements into them. That is why gold, for instance, is so rare on the earth.
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If a star is greater than about 10 solar masses and less than about 20 to 40 solar masses, it becomes a neutron star after exploding as a supernova. |
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Neutron stars are composed entirely of neutrons, squeezed so tightly together that they cannot be compressed any further. (White dwarfs are composed entirely of electrons, but neutron stars are much smaller and their density much larger than a white dwarfs). |
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Neutron stars spin incredibly fast. According to the law of the conservation of angular momentum, if a star is reduced in size it must increase its speed of rotation proportionately. Therefore if a star the size of the sun shrunk to a size of 10 kilometers it would increase its speed of rotation from once a month to once a second. |
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Neutron stars have an extremely high magnetic field because they are so small. |
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Neutron stars have a very high gravitational field. Because all the mass is concentrated so close to the center, the inverse square law of gravitation means that the force of gravity on the surface of a neutron star is staggering. The gravity on a neutron star would be over 1000 times greater than that of the sun. With that much gravity, if you dropped a marshmallow it would land with the kinetic energy of an atomic bomb. |
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Neutron stars are very faint. |
The inverse square law: Any point source which spreads out equally in all directions without a limit to its range will obey the inverse square law. Coming entirely from geometry, the intensity of the influence at any given radius r is the source strength divided by the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena such as gravity, electric fields, light, sound, radiation and so on.
