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The Sun

The Sun. Review Question. What does it mean to say the Sun is in hydrostatic equilibrium?. Review Question. What does it mean to say the Sun is in thermal equilibrium?. Discussion.

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The Sun

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  1. The Sun

  2. Review Question What does it mean to say the Sun is in hydrostatic equilibrium?

  3. Review Question What does it mean to say the Sun is in thermal equilibrium?

  4. Discussion If the Sun is in thermal equilibrium and it is generating heat in its core via thermonuclear fusion, what must happen to that energy?

  5. Heat Transport in the Sun • Conduction – particles transfer energy via collisions • Convection – energy transferred by movement of material from hotter to cooler regions • Radiative Diffusion – energy transferred via photons

  6. Discussion Which would you rather do, put your hand in an oven at 450 degrees F or put you hand on a 450 degree F stove top?

  7. Radiative Diffusion Radiative zone – inner 71 percent of the Sun’s Interior were all atoms are ionized. Takes a photon 170,000 years to reach the convective zone. Each time a photon is absorbed it loses energy.

  8. Convection Convective Zone – outer 29 percent of Sun’s interior. Bottom of convective zone is cool enough for heavy atoms to regain electrons and absorb light.

  9. Discussion What happens to the bottom layer of the convection zone as it absorbs light from the radiative zone.

  10. Solar Granulation

  11. Solar Granulation

  12. Discussion Can all the hydrogen in the Sun be converted to helium? Why or why not?

  13. Discussion Will observations of the properties of the photons emitted by the Sun reveal much information about the interior of the Sun? Why or why not?

  14. Physical properties

  15. Physical properties Temperature Distance Mass Luminosity Radius Composition Velocity Age

  16. Surface temperatures We can determine a star’s surface temperature from its color using Wien’s law, the temperature of a blackbody is proportional to the peak frequency of the blackbody radiation.

  17. Stellar spectra Spectrograph’s spread out the star light Getting a star’s spectrum is time consuming

  18. How do we measure a star’s color? UBVIR photometry We measure the stars apparent brightness through a number of colored filters. U – ultraviolet I – infrared B – blue R – red V – visual

  19. Band pass filters

  20. Color index The ratio of the brightness through various filters can be compared to blackbody curves of different temperatures.

  21. Discussion We’ve discussed two potential problems with measuring a stars temperature using its color. What are they?

  22. Distances to the Stars The most accurate method of determining stellar distances is geometric parallax.

  23. The parsec With a baseline of 1 AU, a star that has a parallax of 1 arcsec has a distance of 1 parsec. Thus distance in parsecs is given by d = 1/p where p is the parallax in arcsec.

  24. Discussion From Earth we can only measure a star’s parallax to about 100 pc. The distance to the center of the galaxy is 8 kpc or 80 times this distance. Why are parallax measurements so limited?

  25. Hipparcos Measured the parallax of the brighter stars (118,000 of them) from Earth orbit out to about 1000 pc or 1 kpc.

  26. Discussion How do you think astronomers determine the masses of stars? Hint: Most stars are binaries.

  27. Kepler’s 3rd law The square of the sidereal period is proportional to the cube of the semimajor axis of the orbit: p2 a3 Newton’s laws require:

  28. Discussion After observing this binary system for an entire orbit, what else do we need to know to determine the star’s mass?

  29. Discussion What are some of the things stellar spectra can tell us?

  30. Stellar spectroscopy

  31. Discussion The strength of the hydrogen absorption lines does not correlate well with the amount of hydrogen present. Almost all stars have about 75% hydrogen, 24% helium and 1% metals. Why do stars have absorption lines in their spectra?

  32. Discussion The hydrogen Balmer lines are produced by electrons absorbing a photon and jumping from the 2nd energy level to a higher energy level. How can this not take place with hydrogen still present the stellar atmosphere?

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