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Where does a stars helium content increase the fastest
in it's core where the temperatures are highest and the burning is fastest
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hydrogen-shell burning stage
- core is full of helium and not hot enough to fuse, contracts due to lack of energy balance
- shell around core heats up, higher temp than found in core, star burns helium in shell much faster and continues to increase as helium shell contracts
- star gets brighter, more energy produced once fusion in the core stops
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how long does it take a normal main-sequence star to become an elderly red giant
100 million years
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what goes on with start temperature between stages 8-9
- stage 8, surface temperature has fallen to the point at which much of the interior is opaque to the radiation from within. Convections carries the cores energy to the surface
- temperature remains nearly constant between these stages
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stage 9 - radius and luminosity
- luminosity man hundred times solar value
- radius is 100 times larger
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red giants core density (stage 9)
- 10^8 kg/m^3
- contrast with 10^-3 in the giants most outer layer 5000 average density for earth and 150,000 in the present core of the sun
- about 25% of the mass of the entire star is packed into its planet sized core
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what are Helium-4 nuclei traditionally know as
alpha particles
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stage 10 laws
governed by the laws of quantum mechanics
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pauli exclusion prinicple
prohibits the electrons in the core from being squeezed together - electron degeneracy ensues.
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Helium flash energy
- results in reduction of energy output
- surface temperature gets high, luminosity gets lower
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stage 9-10 transition time (helium flash)
100,000 years, quick
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asymptotic-giant branch
- the second ascent of the star into the giant branch.
- Follows life after the horizontal branch
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black dwarfs are about what size
size of earth
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sirius b
a white dwarf in orions dog
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what stage is the main sequence
stage 7
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brown dwarfs
the end product of low-mass protostars unable even to fuse hydrogen in their cores
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why are stellar clusters good tests sites for the theory of stellar evolution
they formed at the same time, from the same interstellar cloud, and with virtually the same composition
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turnoff
astronomers refer to the high-luminosity end of the observed main sequence as the main sequence turnoff.
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turnoff mass
the mass of star that is just evolving off the main sequence at any moment is known as the turnoff mass
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when did all of the globular clusters in our galaxy form
10-12 billion years ago
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roche lobe of two stars
- the roche lobes of two stars meet at a point on the line joining them - the inner lagrangian point - gravitational pulls of the two stars exactly balance each other
- if a star expands past it's roche lobe a period of rapid mass transfer ensues
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