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Transcript
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 Confusions from last time • Waves in the Sun • Neutrinos 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 Luminosity & Brightness • Luminosity = Rate of Energy Loss = Energy radiated per second • Apparent Brightness (Flux) = Energy per second per unit area Farther away -> Fainter appears Apparent Brightness (Flux) = Luminosity/4 p (Distance)2 Area of sphere Luminosity, Brightness & Distance Luminosity, Brightness & Distance QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Luminosity 1. Measure Brightness: Use detector (Eye, Film, Light meter, CCD) 2. Measure Distance: Use Parallax Measuring Brightness Measures TOTAL Energy impinging on telescope per second Light from star Telescope focuses Light onto Detector which records it onto Why build big Telescopes? To collect MORE light Computer disk Must divide by AREA of Telescope to get BRIGHTNESS = energy per second per unit area Measuring Distance by Parallax Number of Stars vs Luminosity 100 HIGH 1 0.01 Luminosity 0.0001 LOW Surface Temperature 1. Color 2. Spectrum Color & Temperature Hotter -> Bluer max 2,900,000nm/T(K) Math Insight 6.2 Hotter -> Bluer 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. Temperature & Spectrum: Hydrogen Thermometer Hot Stars: most electrons ionized (knocked free from atom) 3 2 1 Visible (Balmer) lines of H due to Electron transitions to 2nd energy level 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 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) Hot helium hydrogen Vega Spectral Classes Sun calcium+ calcium Cool Spectral Classification • Movie: Cecelia Payne ( h m) • Activity: Try it yourself Spectral Classes • Originally classified by strength of hydrogen lines: A, B, C, D • Now converted to a Temperature Sequence O, B, A, F, G, K, M, L hotest stars strongest hydrogen lines coolest stars Can you think of a good mnemonic for remembering this order? Stellar Masses Use Newton’s Version of Kepler’s 3rd Law 3 3 D (AU) V P M1 M 2 2 P (yrs) 4 pG • • • Must have TWO binary stars orbiting around each other. Measure Velocity (by Doppler Shift) Measure Period (by light curve) Doppler Shift Stellar Masses What should we do with our information? • Luminosities • Surface Temperatures • Masses Look for Relationships