1 / 27

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

chakra
Download Presentation

Stellar Structure II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related