Chapter 10 The Deaths of Stars - Chapter 10: The Deaths of...

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Chapter 10: The Deaths of Stars-What looks like a new star in the sky, is either a nova, the eruption of a very old dying star, or a supernova, the violent explosive death of an aging star10.1 Giant StarsExpansion into a Giant-As soon as the energy generation starts to die down, gravity begins making the core contract-Soon, hydrogen fusion begins in a spherical layer or shell around the exhausted core of the staroThe hydrogen-fusion shell creeps outward, leaving helium ash behind and increasing the mass of the helium core-The flood of energy produced by the hydrogen-fusion shell pushes toward the surface, heating the outer layers of the star and forcing them to expand dramaticallyoAs the outer layers of gas expand, energy is absorbed in lifting and expanding the gasThe loss of that energy lowers the temperature of the gasDegenerate Matter-Although the hydrogen-fusion shell can force the envelope of the star to expand, it can’t stop the contraction of the helium coreoNormally, the pressure in a gas depends on its temperatureThe hotter a gas is, the faster its particles move, and the more pressure it exerts-Quantum mechanics says that the moving electrons confined in the star’s core can have only certain amounts of energy, just as the electron in an atom can occupy only certain energy levels-The second quantum mechanical law says that two identical electrons can’t occupy the same energy leveloTwo electrons can occupy a single energy level if they spin in opposite directions-If a gas becomes very dense nearly all of the lower energy levels are occupiedoIn such a gas, a moving electron can’t slow down; slowing down would decrease its energy, and there are no open energy levels for it to drop down toWhen a gas is so dense that the electrons are not free to change their energy, astronomers call it degenerate matterThe degenerate gas resists compression-The pressure of degenerate gas does not depend on temperatureoChanging the temperature of the gas has almost no effect on the pressure-Many stars collapse into white dwarfs – these tiny stars are made of degenerate matterHelium fusion-Three helium nuclei can collide to make a carbon nucleus in what is called the triple-alpha process-Stars more massive than about 3 solar masses contract rapidly, their helium-rich cores heat up, and helium fusion begins gradually
-Less massive stars evolve more slowly, and their cores contract so much that the gas becomes degenerateoThe pressure-temperature thermostat does no regulate energy production-When the temperature becomes hot enough, helium fusion begins to make energy and the temperature rises, but pressure does not increase because the gas is degenerateoThe higher temperature increases the helium fusion even further, and the result is a runaway explosion called the helium flashin which, for a few minutes, the core of a star can generate more energy per second than does an entire galaxy-

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Term
Fall
Professor
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Tags
Physiology, Mass Transfer, Supernova, White dwarf

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