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The Observed Properties of Stars Stars look like points of light in the sky. How similar are they to the Sun? What do we need to measure about these stars in order to make “models” that can be compared to our model of the Sun? The Observed Properties of Stars Clicker question: Which of the following things did we NOT need to know about the Sun in order to compute an accurate model of its interior? A. Chemical composition B. Luminosity C. Mass D. Diameter E. We needed to know all of the above 1 Finding the luminosity [Fig. 11.1/11.2] L F = 4πr2 [11.1] L = 4πr2 F • Luminosity = Energy/unit time • But we measure flux incident on Earth = Energy/unit time /unit area Î Also must know distance r • For nearby stars, use parallax: [Fig. 11.2/11.3] Stellar masses [11.1] • Binary stars • Use Newton’s form of Kepler’s 3rd law: 4π 2 a3 P = G m1 + m2 2 constant applet Orbits are about center of mass. 2 Mass - Luminosity Relation Luminosity (Lsun) Î • Key observational result for theoretical interpretation of different types of stars. Mass (solar masses)Î [Fig. 5.10] Emitted energy per unit surface area Î Finding the star’s diameter 5000o [fig 4.8] 4000o 3000o Wavelength Î Í Energy • Total energy emitted per unit surface area Stefan-Boltzmann Law: • Total energy from whole star: E = σ T4 Surface area of sphere of diameter D L = E x (surface area) = (σ T4) x (πD2) Luminosity 3 Two Ways: • Thermal radiation curve • Spectroscopy Emitted energy per unit surface area Î Taking a star’s (surface) temperature [Fig. 5.10] 5000o 4000o 3000o Wavelength Î Í Energy Sodium Hydrogen Calcium Mercury Neon Wavelength Î Different stars have very different-looking spectra. G Ionized helium K Flux Æ O B Low ionization & neutral atoms A Excited hydrogen F G M Molecules Low ionization & neutral atoms Wavelength Æ What causes these differences? 4 1. Ionized atom has entirely different energy level structure ÎEntirely different set of absorption lines. REVIEW: 2. Excitation, ionization can also be caused by collisions between atoms. 3. Higher temperatures Î More energetic collisions Î • electrons in higher levels on average. • atoms in higher ionization states on average. E = hf = [Fig. 5.8/5.9] hc λ Stellar spectral types O B A F G A Temperature Sequence Higher temperatures Î More energetic collisions Î • electrons in higher levels on average. • atoms in higher ionization states on average. K M Excited Relative Strength Î Excited [see Table 11.1] Spectral Type O Temperature 30,000 B A 10,000 F G K 5800 M 3500 5 O B A F G K Measuring the surface temperatures of stars Michigan State Extra-Credit Lottery! for best OBAFGKM mnemonic. • 3 clicker points for entering. • 3 clicker points for best answer that can be repeated in class. • enter by email to [email protected], before Midterm 3. M Excited Relative Strength Î Excited [see Table 11.1] Spectral Type O Temperature 30,000 O B B A 10,000 F G K 5800 M 3500 Measuring the surface temperatures of stars A F G K M Excited Relative Strength Î Excited [see Table 11.1] Spectral Type O Temperature 30,000 B A 10,000 F G K 5800 M 3500 6