1	Chapter 12 Star Stuff
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3	Evolution of Low-Mass Stars 1. The Sun began its life like all stars as an intersteller cloud. 2. This cloud collapses due to gravity into a dense core. 3. In about a million years a small, hot, dense core called a protostar forms.
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9	Schematic Illustration of protostellar disk-jet structure
10	"4." 4. When the temperature reaches 10 million Kelvin in the core, fusion begins and transforms the protostar into a zero age main-sequence star. 5. Low mass stars like the Sun remain on the main-sequence for about 10 billion years. Massive stars stay on the main-sequence for about 1 billion years.
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13	"6." 6. Hydrogen fusion begins in a shell around the core and the star expands into a Red Giant. 7. After most of the hydrogen is fused into helium, helium fusion begins in an event called the Helium Flash. 8. Stars can then become unstable and turn into pulsating stars like RR Lyrae Variables or Cephied Variables.
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16	Core Structure of helium burning star.
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19	"9." 9. As a star burns helium into carbon the radiation pressure pushes the star's outer atmosphere away from the core creating a Planetary Nebula. Electron degeneracy pressure halts any further collapse. Fusion process in the core stops. 10. This leaves an exposed core called a White Dwarf. These have about the same diameter as the Earth.
20	The life track of a 1M₈ star from main-sequence to white dwarf.
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24	Evolution of High-Mass Stars 1 to 5. Same as before… intersteller cloud ¢ dense core ¢ ¢ protostar ¢ zero-age main-sequence star ¢ main-sequence star 6. When a high-mass star exhausts the hydrogen fuel in its core the star leaves the main sequence and begins to burn helium.
25	"7." 7. The star becomes a Red Supergiant after millions of years of helium fusion. 8. When helium is depleted, fusion of heavier elements begins. This process is called nucleosynthesis. H ¢ He ¢ C ¢ O ¢ Si ¢ Fe
26	Stellar Nucleosynthesis
27	The Multiple Layers Of Nuclear Burning In The Core Of A High Mass Star During Its Final Days
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29	"9." 9. Fusion stops with iron (Fe) and a star with an iron core is out of fuel. Reason: Iron atoms cannot fuse and release energy. 10. The core collapses due to reduced pressure converting the iron core into mostly neutrons. 11. The core pressure then surges and lifts the outer layers from the star in a titanic explosion - a supernova!
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41	End of Chapter