Term
Core Collapses (process)--1st -When they start to die? |
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Definition
Hydrogen fusion only occurs in core during main sequence. As stars burn H to He, He builds up in the core. Stars begin to “die” when they run out of hydrogen in their core. In the absence of H fusion, the core collapses |
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Term
Core Collapse (process)--2nd -Hydrogen shell burning? |
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Definition
As the core collapses, its temperature increases (gravitational energy) This start a hydrogen shell “burning" Release of energy from burning shell causes star to enlarge. Core continues to collapse. |
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Term
Core Collapse (process) --3rd -Changes in Star -How does it become stable? |
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Definition
Star expands, cools Moves from main sequence, to red-giant branch on H-R diagram Solar winds are strong Pressure and temp increase He fusion begins (He fuses to form carbon) in a “helium flash” Energy output decreases Star contracts. Star becomes temporarily stable (horizontal branch) |
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Term
Core Collapse (process)--4th Helium Fusion |
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Definition
During He fusion, carbon ash builds up The star eventually runs out of He fuel Core begins another collapse What happens next in a dying star depends on the mass of the star… |
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Term
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Definition
Collapse ends when carbon atoms become “electron degenerate” That is, the force of atomic electrons keeps atoms from further collapse He shell around core stars to burn into C Stellar position on the H-R diagram moves along asymptotic giant branch expands and cools luminosity increases |
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Term
| Formation of Planetary Nebula |
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Definition
Shell burning is notstable –occurs in bursts Bursts of fusion push off outer layers of star All of the outer layers of the star are expelled Forms planetary nebula ~60% of mass is lost in planetary nebula |
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Term
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Definition
Expanding shell of hot gas around a dead star Emission Nebula ionized by hot, dense core |
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Term
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Definition
Small, dense, electron degenerate carbon core 1 tsp. of a white dwarf would weigh 5 tons! ~ size of the Earth Slowly cools Very small, less area to lose heat Cools to become a black dwarf White dwarf cannot be larger than 1.4 M = Chandrasekhar LimitSirius |
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Term
| High Mass Stars (M > 5 M) |
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Definition
High mass stars have: - More mass
- Greater gravity
- Higher core temperatures and pressures
- Fusion reactions do not stop with Helium burning in the core as for smaller stars
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Term
| When Star becomes a giant (from a small mass star) |
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Definition
Star becomes giant (for small-mass star)… - Helium burning ends in core
- Core contracts
- Temp and pressure in core increase
- He shell burning begins
- Core continues collapse
- Carbon fuses into higher-mass elements
Same process repeats- Fusion of different elements continues through neon, oxygen, silicon and finally iron
- Note: all fusion occurs only in the very core
Star expands to become a Supergiant Star moves back and forth on the HR diagram with each type of fusion. Each stage of burning lasts for a shorter period of time…
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Term
| Death of a High mass Star |
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Definition
Iron builds up in the core Iron cannot be fused and produce more energy What keeps iron core from collapsing? - Initially: electron degeneracy
After core has a mass greater than 1.4 M - (Chandresekharlimit) the electron degeneracy is not sufficient to stop collapse
- Electrons are forced to combine with the protons to create neutrons
- Core collapses until pressure from physical force of neutrons bouncing against each other stops it
- Core rebounds and runs into outer material which is still falling inward.
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Term
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Definition
Collision produces huge shock wave pushing all material outward in an immense explosion Explosion can be as bright as an entire galaxy (billions of stars) for a few days Some energy creates elements heavier than iron…these elements are dispersed to the rest of the galaxy Supernova leaves a large shell of expanding material around a central core (remnant) |
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Term
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Definition
All stars in a cluster are formed at the same time Age of a cluster can be determined by looking at what point the stars depart the main sequence (the so-called “turn-off point”) Age of Cluster = Lifetime of star at turn-off point |
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