Transcript February 13
The Sun Discussion Why does the Sun shine? Discussion How does the Sun stay hot for billions of years? Discussion What is the proton-proton chain? Proton-Proton chain Discussion Fusion keeps the Sun hot, but fusion requires the Sun to be hot. How did the Sun ever get hot enough to start fusion? Modeling the Sun 1. Hydrostatic equilibrium 2. Thermal equilibrium Pressure increases toward the center of the Sun To maintain equilibrium, the pressure below each layer of the Sun must be greater than the pressure above that layer. Discussion What does this tell you about how the density changes with depth in the Sun? Discussion What does this tell you about how the temperature changes with depth in the Sun? Discussion According to the previous graphs, where is fusion taking place in the Sun? Explain. Discussion What would happen if the Sun started to contract? What would happen if the Sun started to expand? Discussion What would happen if all fusion ended in the Sun? 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 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? 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. 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. Discussion What happens to the bottom layer of the convection zone as it absorbs light from the radiative zone. Solar Granulation Solar Granulation Discussion Can all the hydrogen in the Sun be converted to helium? Why or why not? 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? Physical properties Discussion What are some things that you might like to know about the stars? Physical properties Temperature Distance Mass Composition Velocity Luminosity Radius Age Discussion How can we determine the temperature of a star? 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. Stellar spectra Spectrograph’s spread out the star light Getting a star’s spectrum is time consuming How do we measure a star’s color? UBVIR photometry We measure the stars apparent brightness through a number of colored filters. U – ultraviolet B – blue V – visual I – infrared R – red Band pass filters Color index The ratio of the brightness through various filters can be compared to blackbody curves of different temperatures. Discussion We’ve discussed two potential problems with measuring a star’s temperature using its color. What are they? Distances to the Stars The most accurate method of determining stellar distances is geometric parallax. 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. 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? 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? What could you do to get parallax measurements for more distant stars? Hipparcos Measured the parallax of the brighter stars (118,000 of them) from Earth orbit out to about 1000 pc or 1 kpc. Discussion How do you think astronomers determine the masses of stars? Hint: Most stars are binaries. 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: 4 3 p a G(M1 M 2 ) 2 2 Discussion After observing this binary system for an entire orbit, what else do we need to know to determine the star’s mass? Discussion What are some of the things stellar spectra can tell us? Stellar spectroscopy 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? 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? Discussion Classification of stars using spectral line strengths tells you what about a star? OBAFGKM Oh Be A Fine Girl (Guy) Kiss Me. Oh Brother, Astronomers Frequently Give Killer Midterms. O stars are the hottest M stars are the coolest The Sun is a G2 star.