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

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  (Distance) 2 Area of sphere

Luminosity, Brightness & Distance

Luminosity 1.Measure Brightness: Use detector (Eye, Film, Light meter, CCD) 2.Measure Distance: Use Parallax

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

Measuring Distance by Parallax

Number of Stars vs Luminosity LOWHIGHLuminosity

Surface Temperature 1.Color 2.Spectrum

Color & Temperature Hotter -> Bluer Math Insight 6.2

Hotter -> Bluer

Temperature & Spectrum: Hydrogen Thermometer Visible (Balmer) lines of H formed when electron in 2 nd 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 2 nd level.

Temperature & Spectrum: Hydrogen Thermometer Cool Stars: most electrons in Lowest Energy (ground) state Hot Stars: most electrons ionized (knocked free from atom) Visible (Balmer) lines of H due to Electron transitions to 2 nd energy level Medium Temperature Stars (~10 4 K) -> hot enough for collisions to knock electron up to 2 nd level, not hot enough to knock electron free of atom.

Temperature & Spectrum: Hydrogen Thermometer Only gas at Intermediate Temperature (~10 4 K) has large fraction of H atoms with excited electrons in the 2 nd energy level Colder gas: electrons in lowest enery (ground) state Hotter gas: energetic collisions know electrons out of atom (ionized)

Spectral Classes Hot Cool Sun Vega hydrogen calcium + helium calcium

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 coolest stars hydrogen lines Can you think of a good mnemonic for remembering this order?

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

Doppler Shift

Stellar Masses

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