PHY121 Ch 10 Notes

PHY121 Ch 10 Notes - Astronomy Chapter 10: The Death of...

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Astronomy Chapter 10: The Death of Stars When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die. A nova appears to be a new star that becomes visible and then fades after a few weeks. High-mass stars will die fastest, in a gigantic explosion called a supernova . Supernovae are more luminous and last longer. Both are associated with the deaths of stars. Main sequence stars like the sun can expand and become giant stars when they use up the hydrogen fuel in their cores. As that happens, the core contracts and heats up. Hydrogen fusion begins in a spherical layer around the core – a hydrogen-fusion shell. Energy from the hydrogen-fusion shell swells the star into a cool giant 10 to 100 times larger in diameter than the sun. Massive stars swell to become supergiants up to 1000 times larger in diameter than the sun. The contraction of the star’s core eventually ignites helium, first in the core and later in a shell. If the star is massive enough, it can eventually fuse carbon and other elements. You can see evidence of stellar evolution in the H-R diagrams of star clusters. Stars in both open clusters and in globular clusters evolve in similar ways. The stars begin their evolution at the same time but evolve at different rates, depending on their masses. The most-massive stars leave the main sequence first and are followed later by progressively less-massive stars. This makes the evolution of stars visible in the H-R diagram. You can estimate the age of a star cluster from the turnoff point in its H-R diagram. In old clusters, stars fusing helium follow a loop in the H-R diagram that is visible in the diagram of star clusters as the horizontal branch . Red Dwarf (less than 0.4 solar masses (M o )) Red Dwarf Step by Step account:
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1. Fusion of H proceeds in the star core via the proton-proton chain, 4 x 1H ?4He + energy. 2. Stars contract and heat up, but they never get hot enough to fuse He. 3. The whole volume of these stars is convective, so H and He are mixed, shell fusion (He burning) never begins, and consequently Red Dwarfs do not expand into Giant Stars. 4. As Red Dwarfs die, they move downward off the Hertzsprung-Russell Main Sequence, becoming less luminous. Eventually they cool into dense solid He objects, containing matter that may or may not be degenerate, depending on the mass, radius, and temperature. In degenerate matter, the density is so high that quantum mechanical effects prevent electrons from changing their energies. Such matter is very difficult to compress, and its pressure does not depend on its temperature. 5. Red Dwarfs should live longer than the age of the universe due to their low masses, τ= , so step #4 has not occurred for any Red Dwarf, and is based only on predictions 6. They will remain on the main-sequence for many times the present age of the universe.
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Sun-like Stars (0.4 and 3 M o o – spec class A)
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PHY121 Ch 10 Notes - Astronomy Chapter 10: The Death of...

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