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Stellar Structure II

Stellar Structure II. AST 112 Lecture 5. Structure of a Star. More than just a big ball of plasma! Pressure and temperature increase toward the core So the plasma behaves differently This gives a star its structure. Stellar Wind. Stream of charged particles expelled by a star

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Stellar Structure II

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  1. Stellar Structure II AST 112 Lecture 5

  2. Structure of a Star • More than just a big ball of plasma! • Pressure and temperature increase toward the core • So the plasma behaves differently • This gives a star its structure

  3. Stellar Wind • Stream of charged particles expelled by a star • Extends to outer edges of our Solar System • The tail of a comet always points away from the Sun

  4. Corona • Low density, high temperature gas • Thought of as top of a star’s “atmosphere” • Some of it escapes (what’s it called?)

  5. Corona

  6. Chromosphere • In the Sun’s chromosphere: • Temperature drops to 17,000 oF • Density increases ~5 million times from top to bottom • Mostly transparent to visual light

  7. Convection Zone • Energy from the core is travelling upward, “piles up” • Creates bubbles that expand and rise • Hot gas rises and cools, falls back through convection zone

  8. Convection Zone

  9. Radiation Zone • Convection does not occur here • Plasma is hot and dense enough to “pass the radiation along” • Energy moves outward primarily as electromagnetic radiation • 18 million oF

  10. Core • Extends to about 0.25 of Sun’s radius • Pressure and temperature high enough for nuclear reactions • 27 million oF (Sun) • Density is 100x of water (Sun) • Pressure is 200 billion x that on Earth’s surface (Sun)

  11. Core • The Sun’s core converts 600 million tons of hydrogen to 596 million tons of helium every second! • So 4 million tons of matter is converted to energy every second!

  12. Core Stability • If the fusion rate increases: • Core expands and decreases fusion rate • If fusion rate decreases: • Core collapses and increases fusion rate

  13. What layer do we see when welook at a star?

  14. Photosphere • Photons are able to travel one mean free path before interacting with matter • Mean free path gets longer as density decreases • When the distance between a photon and the “top” of the star’s atmosphere is about ONE mean free path, the photons can escape! • This layer (from which photons escape) is therefore what we see. It is called the photosphere.

  15. Photosphere of the Sun • Visible surface of the Sun • Top of the Convection Zone • We see the bubbles of plasma from the convection zone • This gives a granular appearance

  16. How Long Do Stars Live? • Stars fuse H into He • The entire star (mostly H) is therefore the “gas tank” • Larger stars “squish” their centers harder • This speeds up reactions; think of a larger “engine” • Larger stars have a more efficient reaction path available • Larger stars live much shorter lives than smaller stars • Sure: the “gas tank” is larger, but the engine gets much larger

  17. Other Stars • Based upon the size of a star, we can estimate: • Location and size of radiation zone • Location and size of convection zone • Different for low-, intermediate-, and high-mass stars

  18. Energy Transport • Nuclear reactions drive energy outward from the core of a star • Two ways: • Photons flow outward, being passed from atom to atom (Radiation Zone) • Hot gas bubbles rise upward (Convection Zone)

  19. Structure of Stars

  20. Structure of Stars • Why the difference? • Consider a blob of plasma inside a star. • If it is hotter than its surroundings, it will rise • It is hot and wants to radiate its heat away. • How easily can it do that?

  21. Structure of Stars • Opacity of the surrounding material determines how easily light travels through it • Low Opacity: More transparent • High Opacity: More Opaque

  22. Structure of Stars • If the surrounding material is less opaque: • Blob radiates its heat away and does not rise • If the surrounding material is more opaque: • Blob can’t radiate heat away easily • Stays hotter than surroundings, rises

  23. Structure of Stars • Also consider temperature gradient: • As the blob rises, if surroundings cool over a short distance, it will keep rising • If surroundings don’t cool much over a short distance, it will cool to the surroundings’ temp, stop rising

  24. Structure of Stars • Putting it all together: • Low mass star: • Cooler, higher opacity • Blob can’t radiate its heat, stays hotter as it rises • Convection zone from core to surface

  25. Structure of Stars • Putting it all together: • Intermediate mass star: • Proton-proton chain reactions a T4 (slow temp change) • Bubbles don’t rise near core; energy flows as photons • Radiation Zone • Closer to surface: cooler, hydrogen is neutral and opaque to UV; photons well up • Convection Zone

  26. Structure of Stars • Putting it all together: • High mass star: • Inner • CNO reactions a T17 (steep temp gradient) • Bubbles can rise from core; convection zone • Outer • Transparent to UV • Temp gradient lessens, does not allow bubbles • Blob can radiate heat and reach temp of surroundings • Radiation Zone

  27. Structure of Stars

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