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Announcements

Announcements. Star Assignment 4, due Monday March 22 Do Angel quiz, Quiz 2: Monday March 22 Light, Planets, Sun, Observations of stars Chapters 6.4-5, 8-11, 15, 16.1-4 Mid-Term 2: Wednesday March 24 Light, Planets, Sun, Observations of stars Chapters 6.4-5, 8-11, 15, 16.1-4.

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Announcements

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  1. Announcements • Star Assignment 4, due Monday March 22 • Do Angel quiz, • Quiz 2: Monday March 22Light, Planets, Sun, Observations of starsChapters 6.4-5, 8-11, 15, 16.1-4 • Mid-Term 2: Wednesday March 24Light, Planets, Sun, Observations of starsChapters 6.4-5, 8-11, 15, 16.1-4

  2. Confusions from last time • Waves in the Sun • Neutrinos

  3. Objectives • Describe what is meant by Luminosity and Brightness • Describe how to measure a star’s luminosity, brightness, distance, mass and surface temperature • Use stellar spectra to determine the surface temperature and composition of stars

  4. Luminosity & Brightness • Luminosity = Rate of Energy Loss = Energy radiated per second • Apparent Brightness (Flux) = Energy per second per unit areaFarther away -> Fainter appearsApparent Brightness (Flux) = Luminosity/4 p (Distance)2 Area of sphere

  5. Luminosity, Brightness & Distance

  6. Luminosity, Brightness & Distance

  7. Luminosity • Measure Brightness: Use detector (Eye, Film, Light meter, CCD) • Measure Distance: Use Parallax

  8. Measuring Brightness Measures TOTAL Energy impinging on telescope per second Must divide by AREA of Telescope to get BRIGHTNESS = energy per second per unit area Light from star Telescope focuses Light onto Detector which records it onto Computer disk Why build big Telescopes? To collect MORE light

  9. Measuring DistancebyParallax

  10. Number of Stars vs Luminosity 100 0.01 0.0001 1 HIGH Luminosity LOW

  11. Surface Temperature • Color • Spectrum

  12. Color & Temperature Hotter -> Bluer Math Insight 6.2

  13. Hotter -> Bluer

  14. Temperature & Spectrum:Hydrogen Thermometer Visible (Balmer) lines of H formed when electron in 2nd level absorbs a photon and jumps up to a higher level, or when an electron in a higher level emits a photon and jumps down to the 2nd level.

  15. Temperature & Spectrum:Hydrogen Thermometer Hot Stars: most electrons ionized (knocked free from atom) Visible (Balmer) lines of H due to Electron transitions to 2nd energy level 3 2 Medium Temperature Stars (~104K) -> hot enough for collisions to knock electron up to 2nd level, not hot enough to knock electron free of atom. Cool Stars: most electrons in Lowest Energy (ground) state 1

  16. Temperature & Spectrum:Hydrogen Thermometer Only gas at Intermediate Temperature (~104 K) has large fraction of H atoms with excited electrons in the 2nd energy level Colder gas: electrons in lowest enery (ground) state Hotter gas: energetic collisions know electrons out of atom (ionized)

  17. Spectral Classes Hot helium hydrogen Vega Sun calcium+ calcium Cool

  18. Spectral Classification • Movie: Cecelia Payne ( h m) • Activity: Try it yourself

  19. Spectral Classes • Originally classified by strength of hydrogen lines: A, B, C, D • Now converted to a Temperature SequenceO, B, A, F, G, K, M, Lhotest starsstrongest coolest stars hydrogen linesCan you think of a good mnemonic for remembering this order?

  20. Stellar Masses Use Newton’s Version of Kepler’s 3rd Law • Must have TWO binary stars orbiting around each other. • Measure Velocity (by Doppler Shift) • Measure Period (by light curve)

  21. Doppler Shift

  22. Stellar Masses

  23. What should we do with our information? • Luminosities • Surface Temperatures • Masses Look for Relationships

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