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Astronomy 1020-H Stellar Astronomy Spring_2014 Day-29

Astronomy 1020-H Stellar Astronomy Spring_2014 Day-29. Course Announcements. 1 st Quarter Observing – Mon. 4/7 @8:30pm Archwood parking lot OR atrium of SSB Rain, shine, sleet, snow … it’s on Lunar Eclipse … Mon-Tues. 4/14-15/2014

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Astronomy 1020-H Stellar Astronomy Spring_2014 Day-29

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  1. Astronomy 1020-H Stellar Astronomy Spring_2014 Day-29

  2. Course Announcements • 1st Quarter Observing – Mon. 4/7 @8:30pm • Archwood parking lot OR atrium of SSB • Rain, shine, sleet, snow … it’s on • Lunar Eclipse … Mon-Tues. 4/14-15/2014 • IF CLEAR, we’ll be at the observatory from about midnight-ish on. • Dark night, 4/23/2014 (Wed.) weather dependent.

  3. Astronomy in the Fall, 2014 Astr 1010 - Planetary Astronomy + Lab (H,R) Astr 1020 - Stellar Astronomy + Lab (R) Astr 2010 - Problems in Planet Astronomy Astr 2011 - Intro. to Observational Astronomy Astr 3005 - Observational Astronomy + Lab Astr 4010 – Intro. to Stellar Astrophysics Phys 3701 - Advanced Lab (this one will be astronomy based)

  4. Stars are constantly radiating energy. • The energy available from fusion is very large, but finite. • Eventually, the fusion sources change, then run out.

  5. The star’s luminosity, size, or temperature will change. • A star’s life depends on mass and composition. • Stars of different masses evolve differently.

  6. The rates and types of fusion depend on the star’s mass. • Generally, stars with M < 3 M share many characteristics: low-mass stars. • Intermediate-mass stars: 3 M < M < 8 M • High-mass stars: M > 8 M

  7. Higher temperature and pressure means faster nuclear fusion. • We can figure out main-sequence lifetimes:lifetime = (energy available) / (rate used).

  8. More mass = more fuel available. • Rate energy used = luminosity. • More massive stars have much higher luminosity. • They use their fuel up more quickly and leave the MS faster.

  9. MATH TOOLS 16.1 • Estimates can be made of star lifetimes, based on mass. • The mass-luminosity relationship: • The lifetime of a star depends on the amount of fuel (M) and how quickly it is used (L). • Can use this to compare other stars to the Sun:

  10. Main-sequence stars fuse hydrogen to helium in their cores. • Eventually, much of the core H is converted to He. • A core of He ash is built up (does not fuse at this point).

  11. Helium Core Is Degenerate • H fusion only takes place in a shell around the 100 percent He core: hydrogen shell burning. • If H fusion is not happening in the core, the star is no longer main sequence. • Since the He is not fusing, gravity begins to win over the pressure, crushing the He. • The core becomes more dense, and becomes electron-degenerate. • This means pressure is not from moving atoms, but from a quantum mechanical effect: There’s a limit to how tightly electrons can be packed together.

  12. When the fuel runs out of the core, the luminosity increases. Why? • When the core shrinks, its gravitational pull gets stronger. • Weight of the outer layers increases.

  13. This results in increased pressure: Fusion in the shell goes faster. • Faster nuclear reactions release more energy. • This leaves the star’s surface at a higher rate (higher luminosity).

  14. Increase in pressure and luminosity results in increased size and decreased surface temperature: red giant. • H-R diagram: Star moves up and to the right.

  15. He core is small, dense, electron-degenerate. • Outer envelope is greatly expanded, cooler. • Fusion of H in shell creates more He, making He nuclei in core denser and hotter.

  16. Once hot enough, fusion of He begins in the degenerate core. • He fuses to carbon (C) via the triple-alpha process starts suddenly in the helium flash. • Star shrinks and heats up.

  17. After the helium flash, the star is on the horizontal branch of the H-R diagram. • At first, He  C in the core, H  He in a shell around the core. • Star is smaller and hotter.

  18. Helium is then used up in the core. • He fusion in an inner shell and H fusion in an outer shell all surrounding a C core. • Star gets more luminous and cool, and enters the asymptotic giant branch (AGB).

  19. As an AGB star, the star expands even more than as a red giant, and cools. • H-R diagram: moves up and to the right again. • Dense, electron-degenerate carbon core.

  20. After the AGB: Planetary Nebula • The star is very thinly spread. • Cannot hold on to the outer layers easily. • Outer layers are ejected into space, due to instabilities in the interior.

  21. After the AGB: Planetary Nebula • The ejected material creates a planetary nebula. • The core shrinks and first gets very hot, but eventually cools into a compact white dwarf.

  22. If the conditions are right, the star will ionize the gas in the expanding outer layers. • Will last for about 50,000 years before the gas expands too far and disperses.

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