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Stars: Other Suns

Stars: Other Suns. The Solar-Stellar Connection. Physical Properties. Luminosity Mass Size (diameter/radius) Surface temperature Chemical composition. Distances. Heliocentric stellar parallax Inverse relation : Smaller parallax, greater the distance

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Stars: Other Suns

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  1. Stars: Other Suns The Solar-Stellar Connection

  2. Physical Properties • Luminosity • Mass • Size (diameter/radius) • Surface temperature • Chemical composition

  3. Distances • Heliocentric stellar parallax • Inverse relation: Smaller parallax, greater the distance • Hipparcos satellite measured over 100,000 stars precisely (±1 mas), over 1 million with less precision

  4. Luminosities • Measure flux at earth • Imagine a sphere with radius equal to distance to star; catches all flux from star • Apply inverse square law for light • Watch out for interstellar dust! (dims the starlight)

  5. Masses • Measure directly only with binarysystems of stars (lots!) • Revolve around center of mass • Apply Kepler’s 3rd law to get sum of masses from orbital period, separation (need distance!)

  6. Sizes • Angular diameters mostly too small (mas!) to measure directly • New optical techniques work on some stars (more to come!) • Angular diameter + distance => physical diameter

  7. Chemical Compositions • Examine spectra (most show absorption lines) • Match darks lines to those for known elements • Gives composition of photosphere only (mostly H, He)

  8. Temperatures • From color (hottest, bluish white; coolest, reddish) • Or from wavelength at peak in the continuous spectra • Assume radiate like blackbodies(many do!)

  9. Mass-luminosity relation • A star’s mass and luminosity are related: a little more mass means a lot more luminosity! • Luminosity directly proportional to Mass4 = M x M x M x M! • Ex: 2 solar mass star is L ≈ 2 x 2 x 2x 2 =16 solar luminosities!

  10. Stellar lifetimes • Fuel reserve depends on mass; fuel use depends on luminosity • Lifetime depends on reserve/use, or M/M4 or 1/M3 • More mass => shorter lifetime (by a lot!)

  11. Stellar ages • Lifetime is total span of active life from fusion reactions • Age is time elapsed since fusion began • Sun’s lifetime about 10 billion years; age about 5 billion years (middle age!)

  12. Hertzsprung-Russell Diagram • Plot of stellar luminosities (low to high) versus surface temperature (hot to cool) • A sorting tool: stars fall into different regions • Main sequence, giants, supergiants, white dwarfs

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