Lecture 22 - The Sun (3/4/96)

Seeds: Chapter 7

1. What we want to learn about the Sun
   • What is the Sun and what are stars?
   • How were they formed?
   • How do they live and die?
   • What makes them shine?
   • What is inside a star?
   • What is a star made of?
2. The Sun's Physical Properties
   • Four main physical properties: temperature (T), luminosity (L), mass (M), and radius (R)
   • Determine surface temperature by the spectrum -> T(Sun) = 5800 K
   • Determine luminosity by adding up brightness over all wavelengths -> L(Sun) = 3.83 x 10^26 Watts
   • Determine mass by gravitational force on orbiting bodies (planets) -> M(Sun) = 2 x 10^30 kg
   • Determine radius by angular diameter and distance -> R(Sun) = 7 x 10^5 km
   • Number of nucleons (protons & electrons) in Sun: M(Sun)/M(Proton) = 2 x 10^30 kg / 1.67 x 10^-27 kg = 1.2 x 10^50 nucleons!
   • Energy rate (luminosity) per nucleon is 3.2 x 10^-21 W/nucleon = 2 x 10^-12 eV/sec/nucleon
   • or, in more graphic terms, 63 eV/million years/nucleon
   • Since you can get chemically (like burning) only about 13.6 eV per hydrogen atom, the Sun would only last a million year at that rate. Must be something else fueling it.
   • Average density of Sun = Mass/ 4/3 Pi R^2 = 1400 kg/m^3 (water is 1000 kg/m^3)
   • Compare to Earth (5500 kg/m^3) and Jupiter (1400 kg/m^3) -> Sun is gaseous like Jupiter, not rocky like Earth (duh)
3. The Photosphere
   • The photosphere is the region of the Sun from where the light (thermal continuum) comes.
   • The photosphere is about 500 km thick at the surface of the Sun (radius 700000 km) -> thin skin, or surface of ocean
   • The photosphere thickness is determined by from what depth photons can escape to us before scattering off electrons and atoms in the Sun.
   • The deeper in the Sun you go below the photosphere, the hotter and denser you get -> thermal blackbody radiation
   • The outermost parts of the photosphere are thinner and cooler than the parts below -> absorption lines
   • The kinds of absorption lines tell us what kinds of atoms the Sun is made of: most Hydrogen (H), rest mostly Helium (He), traces of life-giving elements Carbon (C), Nitrogen (N) and Oxygen (O), and the heavy metal Iron (Fe).
   • The surface of the photosphere has cooler and thus darker regions (sunspots), and a mottled appearance (granulation).
   • Solar granulation is caused by convection cells about 1500 km across at the surface, and reaching down into the Sun
   • Convection is caused when something is hotter on the bottom than on the top, like heating water on a stove, or the Sun heating the Earth below the cooler atmosphere -> thunderstorms
   • Convection will be seen to be important in carrying heat outward in the Sun and some stars
4. The Chromosphere and Corona
   • The chromosphere is about 10000 km deep just outside the photosphere. It is hotter (10000 K), and thinner, and thus emits emission lines of distinctive colors (hence its name).
   • You can see structures in the chromosphere which are aligned with magnetic fields on the Sun's surface
   • The corona is a very hot (10^6 K) region extending more than 10^7 km (12 solar radii!) outward. It is very hot and thin, and emits lines, but dopper shifts broaden them out into a continuum spectrum.
   • The chromosphere and corona are visible in eclipses when the much brighter photosphere is blocked out.
   • Because the solar corona is so hot, the atoms have very high thermal velocities (200 km/s or more), and thus the fastest can escape from the Sun's gravity at the outermost parts -> solar wind
   • The solar wind is a stream of atoms blown out of the sun by the heat and radiation, and travels along field lines far out into the solar system (way beyond Pluto!). The solar wind hits the Earth's magnetic field, and drops down onto our atmosphere at the poles, causing the Aurora Borealis.
5. Solar Activity
   • The outer layers of the Sun are raging with storms, flares, spots, prominences, loops, and other fascinating and destructive events.
   • Sunspots are regions on the photosphere that are slightly cooler, and thus appear darker next to the hotter neighboring regions (though they are actually bright themselves).
   • Flares, prominences and loops are large explosions of material flowing outward then back down on loops of magnetic field lines. They release tremendous amounts of energy by terrestrial standards (thousands of megatons!) but are infinitesimal compared to the solar luminosity.
   • Because of the solar wind, flares on the Sun can send dangerous radiation and particles to the Earth, which can hamper communication, cause spectacular aurorae, and endanger astronauts in orbit.
   • Though these effects are very spectacular and fascinating, they amount to no more than "weather" or "cosmetics" on the surface of the Sun. We want to see what's going on below the photosphere!

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