EVOLUTION OF STARS LIKE THE SUN
The star starts out mostly hydrogen, with about 9% helium.
As time goes by, it fuses hydrogen into helium, and the core becomes nearly pure helium.
The core becomes an inert "ash" of helium, while fusion continues to turn hydrogen to helium in the layers above the core.
| Stage 8 |
Lasts: 100 million years Central T: 50x106 K Surface T: 4000 K Size: several x Sun | Hydrogen shell burning stage. Lack of heat in core causes imbalance between pressure and gravity, and core shrinks. Fusion in layer outside core causes outer layers to heat up and the resulting pressure causes the outer layer to expand. Surface temperature drops, but overall luminosity increases because of increased volume of burning |
| Stage 9 |
Lasts: 100 thousand years Central T: 100x106 K Surface T: 4000 K Size: 70 x Sun; ~ orbit of Mercury Central density: ~1000 x Sun | Red Giant stage. Core, containing 25% of star's mass, continues to shrink. Stellar winds may eject mass (perhaps up to 20-30%) from the star's surface. Winds from (cool) red giants are a source of new gas and dust in interstellar medium |
| HELIUM FLASH Runaway reaction rapidly raises temperature of core and converts it to rapid helium burning. | ||
| Stage 10 |
Lasts: 50 million years Central T: 200x106 K Surface T: 5000 K Size: 10 x sun | Horizontal branch. Helium burning in core; hydrogen burning in shell around core. |
| Stage 11 |
Lasts: 10 thousand years Central T: 250x106 K Size: > orbit of Mars | Asymptotic giant branch (red supergiant) |
| Stage 12 |
Lasts: 100 thousand years Central T: 300x106 K Surface T: 100,000 K Size: 1/10 x Sun | Carbon core. Some helium burning at edge of core. |
| Planetary nebula. Outer layers cool enough for atoms to recombine with electrons and become opaque; radiation pushes envelope outwards. Looks like a planet in a small telescope. | ||
| Stage 13 |
Core T: 100x106 K Surface T: 50,000 K Size: ~ Earth Density: million times Earth's | White dwarf. Very dim. Stellar core continues to cool and contract. E.g. Sirius B. |
| Stage 14 |
Temperature: low. Size: ~ Earth. | Black dwarf. A "cinder." Impossible to see in a telescope. |
OBSERVATIONAL EVIDENCE
Examples of most of stages of stellar evolution can be observed, but not the evolution itself.
H-R diagrams of clusters show "snapshots" of stars all of the same age. Diagrams of clusters of different ages show how the distribution of stars on the H-R diagram changes with time.
EVOLUTION OF STARS MORE MASSIVE THAN THE SUN
a whole other story!
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