Copyright © 2009 Pearson Education, Inc. Chapter 12 – Star StuffCopyright © 2009 Pearson Education, Inc. Carl Sagan “We are made of star stuff”Copyright © 2009 Pearson Education, Inc. How do we measure stellar temperatures?Copyright © 2009 Pearson Education, Inc. Every object emits thermal radiation with a spectrum that depends on its temperature.Copyright © 2009 Pearson Education, Inc. An object of fixed size grows more luminous as its temperature rises. Relationship Between Temperature and LuminosityCopyright © 2009 Pearson Education, Inc. Properties of Thermal Radiation 1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average energy.Copyright © 2009 Pearson Education, Inc. Hottest stars: 50,000 K Coolest stars: 3000 K (Sun’s surface is 5800 K)Copyright © 2009 Pearson Education, Inc. Solid Molecules Neutral Gas Ionized Gas (Plasma) Level of ionization also reveals a star’s temperature. 10 K 102 K 103 K 104 K 105 K 106 KCopyright © 2009 Pearson Education, Inc. Absorption lines in a star’s spectrum tell us its ionization level.Copyright © 2009 Pearson Education, Inc. Lines in a star’s spectrum correspond to a spectral type that reveals its temperature: (Hottest) O B A F G K M (Coolest)Copyright © 2009 Pearson Education, Inc. (Hottest) O B A F G K M (Coolest) Remembering Spectral Types • Oh, Be A Fine Girl/Guy, Kiss Me • Only Boys Accepting Feminism Get Kissed MeaningfullyCopyright © 2009 Pearson Education, Inc. Thought Question Which of the stars below is hottest? A. M star B. F star C. A star D. K starCopyright © 2009 Pearson Education, Inc. Thought Question Which of the stars below is hottest? A. M star B. F star C. A star D. K starCopyright © 2009 Pearson Education, Inc. Pioneers of Stellar Classification • Annie Jump Cannon and the “calculators” at Harvard laid the foundation of modern stellar classification.Copyright © 2009 Pearson Education, Inc. How do we measure stellar masses?Copyright © 2009 Pearson Education, Inc. Orbit of a binary star system depends on the strength of gravity.Copyright © 2009 Pearson Education, Inc. Types of Binary Star Systems • Visual binary • Eclipsing binary • Spectroscopic binary About half of all stars are in binary systems.Copyright © 2009 Pearson Education, Inc. Visual Binary We can directly observe the orbital motions of these stars.Copyright © 2009 Pearson Education, Inc. Eclipsing Binary We can measure periodic eclipses.Copyright © 2009 Pearson Education, Inc. Spectroscopic Binary We determine the orbit by measuring Doppler shifts.Copyright © 2009 Pearson Education, Inc. Isaac Newton We measure mass using gravity. Direct mass measurements are possible only for stars in binary star systems. p = period a = average separation p2 = a3 4π2 G (M1 + M2)Copyright © 2009 Pearson Education, Inc. Need two out of three observables to measure mass: 1. Orbital period (p) 2. Orbital separation (a or r = radius) 3. Orbital velocity (v) For circular orbits, v = 2pr / p r MvCopyright © 2009 Pearson Education, Inc. Most massive stars: 100MSun Least massive stars: 0.08MSun (MSun is the mass of the Sun.)Copyright © 2009 Pearson Education, Inc. What have we learned? • How do we measure stellar luminosities? — If we measure a star’s apparent brightness and distance, we can compute its luminosity with the inverse square law for light. — Parallax tells us distances to the nearest stars. • How do we measure stellar temperatures? — A star’s color and spectral type both reflect its temperature.Copyright © 2009 Pearson Education, Inc. What have we learned? • How do we measure stellar masses? — Newton’s version of Kepler’s third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a).Copyright © 2009 Pearson Education, Inc. 11.2 Patterns Among Stars Our goals for learning: • What is a Hertzsprung–Russell diagram? • What is the significance of the main sequence? • What are giants, supergiants, and white dwarfs?Copyright © 2009 Pearson Education, Inc. What is a Hertzsprung–Russell diagram?Copyright © 2009 Pearson Education, Inc. Temperature Luminosity An H-R diagram plots the luminosities and temperatures of stars.Copyright © 2009 Pearson Education, Inc. Generating an H-R DiagramCopyright © 2009 Pearson Education, Inc. Most stars fall somewhere on the main sequence of the H-R diagram.Copyright © 2009 Pearson Education, Inc. Stars with lower T and higher L than main-sequence stars must have larger radii: giants and supergiants large radiusCopyright © 2009 Pearson Education, Inc. small radius Stars with higher T and lower L than main-sequence stars must have smaller radii: white dwarfsCopyright © 2009 Pearson Education, Inc. A star’s full classification includes spectral type (line identities) and luminosity class (line shapes, related to the size of the star): I — supergiant II — bright giant III — giant IV — subgiant V — main sequence Examples: Sun — G2 V Sirius — A1 V Proxima Centauri — M5.5 V Betelgeuse — M2 ICopyright © 2009 Pearson Education, Inc. Temperature Luminosity H-R diagram depicts: Temperature Color Spectral type Luminosity RadiusCopyright © 2009 Pearson Education, Inc. Which star is the hottest? A B C DCopyright © 2009 Pearson Education, Inc. Which star is the hottest? A A B C DCopyright © 2009 Pearson Education, Inc. Which star is the most luminous? A B C DCopyright © 2009 Pearson Education, Inc. Which star is the most luminous? C A B C DCopyright © 2009 Pearson Education, Inc. Which star is a main-sequence star? A B C DCopyright © 2009 Pearson Education, Inc. Which star is a main-sequence star? D A B C DCopyright © 2009 Pearson Education, Inc. Which star has the largest radius? A B C DCopyright © 2009 Pearson Education, Inc. Which star has the largest radius? C A B C DCopyright © 2009 Pearson Education, Inc. What is the significance of the main sequence?Copyright © 2009 Pearson Education, Inc. Main-sequence stars are fusing hydrogen into helium in their cores, like the Sun. Luminous main-sequence stars are hot (blue). Less luminous ones are cooler (yellow or red).Copyright © 2009
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