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Starlight and What it Tells Us

Starlight and What it Tells Us. The Stars in the Sky. Vary in Brightness Distance Size Vary in Color Color = Temperature. Star Names. Proper star names mostly Arabic Greek Letters, Numbers Catalog Identifiers Faint stars usually have no name. The Names of Sirius.

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Starlight and What it Tells Us

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  1. Starlight and What it Tells Us

  2. The Stars in the Sky Vary in Brightness • Distance • Size Vary in Color • Color = Temperature

  3. Star Names • Proper star names mostly Arabic • Greek Letters, Numbers • Catalog Identifiers • Faint stars usually have no name

  4. The Names of Sirius • Alpha Canis Majoris (Bayer, 1603) • 9 Canis Majoris (Flamsteed, 1725) • BD -16 1591 (Bonner Durchmusterung 1859-1903) • HR 2491 (Harvard Revised Catalog, 1908) • HD 48915 (Henry Draper, 1918-1924) • ADS 5423 (Aitken Double Star Catalog, 1932) • HIP 32349 (HIPPARCOS, 1997)

  5. The Heavens Are Not Changeless • The Stars Move • Most of our constellations would have been unrecognizable to Neanderthal Man • The Solar System Moves • Very few of our nearby stars would have been visible to the first humans • Stars are Born, Live and Die • Many of our brightest stars did not exist in the days of the dinosaurs

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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, Mars Does Not • Close Example of pure Black-Body radiation: Peephole in a pottery kiln

  11. Black Body Radiation

  12. What’s The Source of the Light?

  13. Color = Temperature

  14. 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

  15. Sirius and the Pup

  16. Sirius and the Pup • Sirius M = -1.5; Pup M = 8.5 • 10 magnitude difference • 100 x 100 = 10,000 times brightness distance • Sirius and the Pup are same color, therefore same temperature (Pup is hotter) • Pup must have 1/10,000 the apparent area of Sirius = 1/100 the diameter

  17. 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

  18. Atoms and Radiation

  19. The Solar Spectrum

  20. Spectra and Spectral Lines • Continuous Spectrum: Incandescent solids or liquids (steel mill) and dense hot gases (Sun’s photosphere) • Emission Spectrum: Thin hot gases (fireworks, sodium or mercury vapor lights, Sun’s chromosphere • Absorption Spectrum: Light shining through thin gases (Sun and star light)

  21. How the Chromosphere Works

  22. 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

  23. The Doppler Effect

  24. 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

  25. Spectral Classification of Stars • W – very hot young stars expelling their outer layers • Main Sequence: O, B, A, F, G, K, M (hottest to coolest) • “Oh be a fine girl/guy, kiss me” • Subdwarfs: L, T, Y (hottest to coolest) • Chemically Peculiar Stars: C, N, R, S • White Dwarfs: D

  26. Spectral Signatures of Stars • O: Ionized Helium • B: Neutral Helium • A: Strongest Hydrogen Lines • F: Ionized Calcium • G: Strongest Calcium Lines + Neutral Metals • K: Neutral Metals Dominate • M: Titanium Oxide

  27. The Hertzsprung-Russell Diagram

  28. The Main Sequence: O • 30,000-60,000 K (Blue-white) • Absolute Magnitude -5 • 1,000,000 times Sun’s Luminosity • 16 times Sun’s Diameter • 64 times Sun’s Mass • Lifetime: Less than a million years • Examples: Orion's Belt

  29. The Main Sequence: B • 10,000-30,000 K (Blue-white) • Absolute Magnitude -3 • 20,000 times Sun’s Luminosity • 7 times Sun’s Diameter • 18 times Sun’s Mass • Lifetime: 10 million years • Examples: Spica

  30. The Main Sequence: A • Temperature: 7500-10,000 K (White) • Absolute Magnitude +0.5 • 40 times Sun’s Luminosity • 2 times Sun’s Diameter • 3 times Sun’s Mass • Lifetime: 600 million years • Examples: Vega, Sirius

  31. The Main Sequence: F • Temperature: 6000-7500 K (Yellow-White) • Absolute Magnitude +2.5 • 6 times Sun’s Luminosity • 1.5 times Sun’s Diameter • 1.7 times Sun’s Mass • Lifetime: 2.5 billion years • Examples: Procyon

  32. The Main Sequence: G • Temperature: 5000-6000 K (Yellow) • Absolute Magnitude +5 • 1 times Sun’s Luminosity • 1 times Sun’s Diameter • 1 times Sun’s Mass • Lifetime: 10 billion years • Examples: Sun, Alpha Centauri A

  33. The Main Sequence: K • Temperature: 3500-5000 K (Orange) • Absolute Magnitude +6 • 0.4 times Sun’s Luminosity • 0.9 times Sun’s Diameter • 0.8 times Sun’s Mass • Lifetime: 10 billion years • Examples: Alpha Centauri B

  34. The Main Sequence: M • Temperature: 2000-3500 K(Red) • Absolute Magnitude +10 to +15 • 0.04 times Sun’s Luminosity • 0.5 times Sun’s Diameter • 0.4 times Sun’s Mass • Lifetime: 5 trillion years • 75% + of all stars • Examples: Barnard's Star, Proxima Centauri

  35. Sub-Dwarfs • L: 1300-2000 K, Borderline stars with alkali metals and metal hydrides • T: 700-1300 K, Substellar, methane in spectra • Y <700 K, Substellar, ammonia in spectra (predicted)

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