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The Sun: Home Star

The Sun: Home Star. Solar-Stellar Connection. The Sun. Key to solar-stellar connection Close-up model for other stars Local “lab” for testing ideas about the physics of stars Energy source for most life on earth. Sun: Properties. Luminosity Mass Radius

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The Sun: Home Star

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  1. The Sun: Home Star Solar-Stellar Connection

  2. The Sun • Key to solar-stellar connection • Close-up model for other stars • Local “lab” for testing ideas about the physics of stars • Energy source for most life on earth

  3. Sun: Properties • Luminosity • Mass • Radius Must know distance to find these!

  4. Sun: Properties • Surface temperature • Chemical composition Analyze spectrum to find these; do not need to know distance!

  5. Distance • 1 AU! About 150 million km • How? Bounce radar signals off Venus! Know distance in AU from Kepler’s laws. Know speed of light, measure travel time => distance in km, known fraction of AU; get AU in km

  6. Luminosity • At earth, measure flux: Energy passing through an area per second => 1370 W/s • Imagine a sphere around the sun with radius 1 AU • Collects 4 x 1026 W, sun’s luminosity or power

  7. Mass • Apply: Newton’s version of Kepler’s 3rd or Newton’s 2nd law plus law pf gravitation • Need earth’s orbital period (365 days) and distance, 150 million km (1 AU) • Result: 2 x 1030 kg, 330,000 times that of earth

  8. Diameter (Radius) • Angular diameter from earth about 1/2 degree => actual diameter/distance = 1/110 • Know distance (1 AU in km) • Actual diameter = (1/110) x 150 x 106 km = about 1.4 million km, or 100 times earth!

  9. Chemical Composition • Absorption-line spectrum: Match lines to those of known elements • Information only about photosphere, where lines form • Mostly H (72%) and He (26%)

  10. Surface Temperature • Assume: Radiates like a blackbody (Planck curve of continuous emission) • Then: Match overall curve or use peak of emission • Get: Temp about 5800 K

  11. Solar Energy • Source: Fusion reactions(H => He) in core, where temperature 10 to 15 million K • How: E = mc2; thermonuclear reactions convert mass to energy (heat & light) • Energy flow: From core to surface takes light about 200,000 y (opaque!)

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