final study guide

final study guide - LIFETIMEOFSTARS 15:55 Stars begin as...

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LIFETIME OF STARS 15:55 Stars begin as protostars , which is a large mass formed from a gas cloud As cloud contracts, it increases in temp (gravitational energy to thermal kinetic energy) Shrinks in radius Spins as a disk, and grows as it gains more matter When it is done with accretion process, it is called Pre-main sequence star (PMS) Stars are powered primarily by nuclear reactions which convert H to He in their cores. They are in hydrostatic equilibrium. They are in energy equilibrium: energy generated in the core equals energy emitted at surface. The nuclear reactions occur only in the core, which is about 1⁄2 the mass of the star. Stars spend most of their lifetime on the main sequence “burning” hydrogen But when the hydrogen runs out… As hydrogen “burns”, helium “ash” builds up in core, which interferes with hydrogen burning Star’s core contracts to become hotter and denser to continue the “burning” Outer layers of star expand and cools down while core contracts and heats up So the star appears to become cooler in the HR diagram
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Fundamental principle: The star is in a battle against gravity trying to crush it. The star’s weapon in the war is nuclear “burning” to create energy. It will try everything it can, and changes in the core are mirrored in its physical appearance to us. Evolution of a 1 solar mass star (like our sun) 1. H “burning” in the core on the main sequence for 10 billion years. As it reached the end of this the core is contracting and the outer layers expanding. 2. Core becomes so filled with helium that fusion stops Hydrogen shell burning – core contracts and becomes hot and dense enough layers beyond the core star undergoing nuclear reactions Star’s outer layers expand and become redder Hydrogen shell burning continues and the shell moves outward creating a hotter and denser helium filled core.
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3. Helium core becomes hot enough to allow the triple alpha reaction to occur Helium in core burns into carbon, which is a new short-lived source of energy for the star Star expands further to become a red giant Red giant is a K or M star with luminosity class III 100 times or more the size of sun, due to H-shell burning and helium core burning What drives size increase? Think of pot boiling – turn up heat, boils more furiously and froth expands 4. For a star like our sun, this is the end of the line for nuclear reactions core becomes hotter and denser but not hot enough for further reactions Surface is boiling off and being pushed out into space As outer layers are pushed out, hotter layers underneath are revealed and star becomes bluer Can push ½ of mass or more into space As mass is lost, weight bearing down on core decreases, so temp and density decreases The core of the star is no longer hot enough to generate energy from fusion but it is still plenty hot 5. As layers are removed, hot core is revealed, and contracts to size of earth, called a white dwarf Very hot surface illuminates the material thrown into space to create planetary nebula
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This note was uploaded on 01/13/2012 for the course ASTR 100 taught by Professor Hayes-gehrke during the Spring '06 term at Maryland.

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final study guide - LIFETIMEOFSTARS 15:55 Stars begin as...

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