Chapter 12 Star Stuff Copyright 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 Luminosity Copyright 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 106 K 105 K 104 K Ionized Gas Plasma 103 K Neutral Gas 102 K Molecules 10 K Solid Copyright 2009 Pearson Education Inc Level of ionization also reveals a star s temperature 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 Copyright 2009 Pearson Education Inc O B A F G K M Coolest Remembering Spectral Types Hottest O B A F G K M Coolest Oh Be A Fine Girl Guy Kiss Me Only Boys Accepting Feminism Get Kissed Meaningfully Copyright 2009 Pearson Education Inc Thought Question Which of the stars below is hottest A B C D Copyright 2009 Pearson Education Inc M star F star A star K star Thought Question Which of the stars below is hottest A B C D Copyright 2009 Pearson Education Inc M star F star A star K star 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 We measure mass using gravity Direct mass measurements are possible only for stars in binary star systems 2 4 p2 a3 G M1 M2 p period Isaac Newton Copyright 2009 Pearson Education Inc a average separation 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 Copyright 2009 Pearson Education Inc v r M 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 Luminosity An H R diagram plots the luminosities and temperatures of stars Copyright 2009 Pearson Education Inc Temperature Generating an H R Diagram Copyright 2009 Pearson Education Inc Most stars fall somewhere on the main sequence of the H R diagram Copyright 2009 Pearson Education Inc large radius Stars with lower T and higher L than mainsequence stars must have larger radii giants and supergiants Copyright 2009 Pearson Education Inc Stars with higher T and lower L than main sequence stars must have smaller radii white dwarfs small radius Copyright 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 II III IV V supergiant bright giant giant subgiant main sequence Examples Sun G2 V Sirius A1 V Proxima Centauri M5 5 V Betelgeuse M2 I Copyright 2009 Pearson Education Inc H R diagram depicts Temperature Luminosity Color Spectral type Luminosity Radius Copyright 2009 Pearson Education Inc Temperature C B D A Copyright 2009 Pearson Education Inc Which star is the hottest C B D A Copyright 2009 Pearson Education Inc Which star is the hottest A C B D A Copyright 2009 Pearson Education Inc Which star is the most luminous C B Which star is the most luminous C D A Copyright 2009 Pearson Education Inc C B D A Copyright 2009 Pearson Education Inc Which star is a mainsequence star C B Which star is a mainsequence star D D A Copyright 2009 Pearson Education Inc C B D A Copyright 2009 Pearson Education Inc Which star has the largest radius C B Which star has the largest radius C D A Copyright 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 mainsequence stars are hot blue Less luminous ones are cooler yellow or red Copyright 2009 Pearson Education Inc High mass stars Low mass stars Copyright 2009 Pearson Education Inc Mass measurements of main sequence stars show that the hot blue stars are much more massive than the cool red ones High mass stars Low mass stars Copyright 2009 Pearson Education Inc The mass of a normal hydrogen burning star determines its luminosity and spectral type The core temperature of a higher mass star needs to be higher in order to balance gravity Hydrostatic Equilibrium Copyright 2009 Pearson Education Inc A higher core temperature boosts the fusion rate leading to greater luminosity
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