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Starlight and What it Tells Us

Starlight and What it Tells Us. Brightness of Stars. Variations in distance and intrinsic brightness Scale based on one by Hipparcos 500 B.C. Magnitude: Large Numbers = Fainter One magnitude = 2.5 x Five magnitudes = 100 x. Magnitudes. Planet around nearby star: 30 Pluto: 13

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Starlight and What it Tells Us

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  1. Starlight and What it Tells Us

  2. Brightness of Stars • Variations in distance and intrinsic brightness • Scale based on one by Hipparcos 500 B.C. • Magnitude: Large Numbers = Fainter • One magnitude = 2.5 x • Five magnitudes = 100 x

  3. Magnitudes • Planet around nearby star: 30 • Pluto: 13 • Faintest Naked-Eye Star: 6 • Big Dipper Stars: 2 • Sirius (Brightest Star) -1.6 • Venus -4 • Full Moon -12 • Sun -27

  4. Absolute Magnitude • Altair and Deneb are about equally bright as seen from Earth • Altair is 16 l.y. away, Deneb 1600 • Hence Deneb must be about 10,000 times brighter

  5. Absolute Magnitude • How bright a star would be at a distance of 32.6 l.y. (10 parsecs) • Sun: 4.5 (inconspicuous naked-eye star) • Altair: 2.2 • Deneb: -7.1 (bright as crescent moon) • Note: Deneb - Altair about 10 magnitudes = 100 x 100 = 10,000 times

  6. Black-Body Radiation • Objects Emit Radiation Because They Are Hot • Why “Black”? Because None of the Radiation is Reflected from Some Other Source • The Sun Emits Black-Body Radiation, the Moon Does Not

  7. Why Black-Body Radiation is so Important • Color is directly related to temperature • Temperature is the only determinant of color • Energy per unit area is the same if temperature is the same • If two stars have the same color and distance, difference in brightness is due to difference in size • Dwarf and giant stars are literally dwarfs or giants

  8. Spectroscopy • Different atoms absorb or emit specific wavelengths of light • When light spread into a spectrum, the absorbed wavelengths show up as dark (missing) bands • These spectral lines are indicators of: • Chemical composition • Physical conditions

  9. The Solar Spectrum

  10. Spectral Lines are Affected By: • Electrical and Magnetic Fields • Number of Electrons Atoms Have Lost (Indicates Temperature and Pressure) • Motion (Doppler Effect) • Blue-shifted if Motion Toward Observer • Red-shifted if Motion Away From Observer

  11. The Doppler Effect

  12. What the Doppler Effect Tells Us • Radial Motion • Rotation of Stars • Approaching side of star blue-shifted, receding side red-shifted • Unseen Companions (Stars or Planets) • Star oscillates around center of mass • Surface and Interior Motions • Changes in Size • Interior Oscillations

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