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Chapter 8 The Sun and Other Stars © 2010 Pearson Education, Inc. Radius: 6.9 108 m (109 times Earth) Mass: 2 1030 kg (300,000 Earths) Luminosity: 3.8 1026 watts © 2010 Pearson Education, Inc. What is the Sun’s structure? Insert TCP 6e Figure 14.3 © 2010 Pearson Education, Inc. Core: Energy generated by nuclear fusion ~ 15 million K © 2010 Pearson Education, Inc. How does nuclear fusion occur in the Sun? © 2010 Pearson Education, Inc. Fission Fusion Big nucleus splits into smaller pieces. Small nuclei stick together to make a bigger one. (Example: nuclear power plants) © 2010 Pearson Education, Inc. (Example: the Sun, stars) High temperatures enable nuclear fusion to happen in the core. © 2010 Pearson Education, Inc. The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus. © 2010 Pearson Education, Inc. IN 4 protons OUT 4He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower. © 2010 Pearson Education, Inc. Radiation Zone: Energy transported upward by photons © 2010 Pearson Education, Inc. How does the energy from fusion get out of the Sun? © 2010 Pearson Education, Inc. Energy gradually leaks out of radiation zone in form of randomly bouncing photons. © 2010 Pearson Education, Inc. Convection Zone: Energy transported upward by rising hot gas © 2010 Pearson Education, Inc. Convection (rising hot gas) takes energy to surface. © 2010 Pearson Education, Inc. Bright blobs on photosphere show where hot gas is reaching the surface. © 2010 Pearson Education, Inc. Photosphere: Visible surface of Sun ~ 6000 K © 2010 Pearson Education, Inc. Chromosphere: Middle layer of solar atmosphere ~ 104–105 K © 2010 Pearson Education, Inc. Corona: Outermost layer of solar atmosphere ~1 million K © 2010 Pearson Education, Inc. Solar wind: A flow of charged particles from the surface of the Sun © 2010 Pearson Education, Inc. Gravitational equilibrium: Energy supplied by fusion maintains the pressure that balances the inward crush of gravity. © 2010 Pearson Education, Inc. Gravitational contraction: Provided the energy that heated the core as Sun was forming Contraction stopped when fusion began. © 2010 Pearson Education, Inc. How we know what is happening inside the Sun? © 2010 Pearson Education, Inc. We learn about the inside of the Sun by … • making mathematical models • observing solar vibrations • observing solar neutrinos © 2010 Pearson Education, Inc. Patterns of vibration on the surface tell us about what the Sun is like inside. © 2010 Pearson Education, Inc. What causes solar activity? © 2010 Pearson Education, Inc. Solar activity is like “weather”. • Sunspots • Solar flares • Solar prominences All these phenomena are related to magnetic fields. © 2010 Pearson Education, Inc. Sunspots Are cooler than other parts of the Sun’s surface (4000 K) Are regions with strong magnetic fields © 2010 Pearson Education, Inc. Loops of bright gas often connect sunspot pairs. © 2010 Pearson Education, Inc. Magnetic activity causes solar flares that send bursts of X rays and charged particles into space. © 2010 Pearson Education, Inc. Magnetic activity also causes solar prominences that erupt high above the Sun’s surface. © 2010 Pearson Education, Inc. The corona appears bright in X-ray photos in places where magnetic fields trap hot gas. © 2010 Pearson Education, Inc. Charged particles streaming from the Sun can disrupt electrical power grids and can disable communications satellites. © 2010 Pearson Education, Inc. Insert TCP 6e Figure 14.21a unannotated The number of sunspots rises and falls in an 11-year cycle. © 2010 Pearson Education, Inc. Properties of Other Stars • Luminosity • Surface Temperature • Mass © 2010 Pearson Education, Inc. How do we measure stellar luminosities? © 2010 Pearson Education, Inc. Luminosity: Amount of power a star radiates (energy per second = watts) Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter) © 2010 Pearson Education, Inc. The amount of luminosity passing through each sphere is the same. Area of sphere: 4 (radius)2 Divide luminosity by area to get brightness. © 2010 Pearson Education, Inc. The relationship between apparent brightness and luminosity depends on distance: Brightness = Luminosity 4 (distance)2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4 (distance)2 (brightness) © 2010 Pearson Education, Inc. Most luminous stars: 106 LSun Least luminous stars: 10–4LSun (LSun is luminosity of Sun) © 2010 Pearson Education, Inc. Apparent Magnitude • • • • Greek astronomer, Hipparchus Brightest stars were magnitude 1 Faintest stars were magnitude 6 Quantitatively redefined by modern scientists: – Difference of five “magnitude” = brightness ratio of 100 – Star Vega = magnitude of zero – Brightness in units of watts/m^2 – Logarithmic scale – mag 1 is 2.512 times mag 2 © 2010 Pearson Education, Inc. Apparent Magnitude (concluded) • • • • • • • • • • Sun Full Moon Venus (max) Mars (max) Mars (min) M31 (Andromeda Galaxy) Best naked eye 7x50 binoculars My telescope Hubble telescope © 2010 Pearson Education, Inc. -26.4 -12.92 -4.89 -2.91 1.84 3.44 7-8 9.5 ?? 10-12 31.5 How do we measure stellar temperatures? © 2010 Pearson Education, Inc. Every object emits thermal radiation with a spectrum that depends on its temperature. © 2010 Pearson Education, Inc. 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 © 2010 Pearson Education, Inc. Lines in a star’s spectrum correspond to a spectral type that reveals its temperature. (Hottest) © 2010 Pearson Education, Inc. O B A F G K M (Coolest) How do we measure stellar masses? Insert TCP 6e Figure 15.7 unannotated © 2010 Pearson Education, Inc. We measure mass using gravity. Direct mass measurements are possible only for stars in binary star systems. 42 p2 = a3 G (M1 + M2) M1 and M2 are the masses of the two stars p = period a = average separation © 2010 Pearson Education, Inc. Most luminous stars: 106 LSun Least luminous stars: 10–4LSun (LSun is luminosity of Sun) © 2010 Pearson Education, Inc. Most luminous stars: 106 LSun Least luminous stars: 10–4LSun (LSun is luminosity of Sun) © 2010 Pearson Education, Inc. Three Major Star Groups • The Main Sequence – Most follow the surface temp – luminosity trend of red=cool and blues=hot – Generate energy by fusing hydrogen in cores – Lower tempertures and luminosities result in longer lives – The term “main sequence” will become selfevident later © 2010 Pearson Education, Inc. Main-Sequence Star Summary High-Mass Star: • • • • High luminosity Short-lived Larger radius Blue Low-Mass Star: • • • • © 2010 Pearson Education, Inc. Low luminosity Long-lived Small radius Red Three Major Star Groups • Giants and Supergiants – The bright red stars in previous slide – Cooler temperature but greater luminosity than our Sun – To be brighter while being cooler means they must have must greater surface area – giants and supergiants – Have run out of hydrogen fuel in core and nearing end of life © 2010 Pearson Education, Inc. Three Major Star Groups • White Dwarfs – – – – Too dim to be seen in the previous slide Very hot (white) but low luminosity Must have a smaller surface area than our Sun Embers of giants that have run out of fuel and blown off outer layers © 2010 Pearson Education, Inc. Sizes of Giants and Supergiants © 2010 Pearson Education, Inc. Pioneers of Stellar Classification • Annie Jump Cannon and the “calculators” at Harvard laid the foundation of modern stellar classification. © 2010 Pearson Education, Inc. What is a Hertzsprung-Russell diagram? © 2010 Pearson Education, Inc. Luminosity An H-R diagram plots the luminosity and temperature of stars. © 2010 Pearson Education, Inc. Temperature Off the Main Sequence • Stellar properties depend on both mass and age: Those that have finished fusing H to He in their cores are no longer on the main sequence. • All stars become larger and redder after exhausting their core hydrogen: giants and supergiants. • Most stars end up small and white after fusion has ceased: white dwarfs. © 2010 Pearson Education, Inc. Mass and Lifetime Sun’s life expectancy: 10 billion years Until core hydrogen (10% of total) is used up Life expectancy of 10MSun star: 10 times as much fuel, uses it 104 times as fast 10 million years ~ 10 billion years 10/104 Life expectancy of 0.1MSun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years 0.1/0.01 © 2010 Pearson Education, Inc. What have we learned? • Why was the Sun’s energy source a major mystery? – Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years. • Why does the Sun shine? – The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion. © 2010 Pearson Education, Inc. What have we learned? • What is the Sun’s structure? – From inside out, the layers are: • • • • • • Core Radiation zone Convection zone Photosphere Chromosphere Corona © 2010 Pearson Education, Inc. What have we learned? • How does nuclear fusion occur in the Sun? – The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton–proton chain. – Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature. © 2010 Pearson Education, Inc. What have we learned? • How does the energy from fusion get out of the Sun? – Randomly bouncing photons carry energy through the radiation zone. – Rising of hot plasma carries energy through the convection zone to photosphere. • How do we know what is happening inside the Sun? – Mathematical models agree with observations of solar vibrations and solar neutrinos. © 2010 Pearson Education, Inc. What have we learned? • What causes solar activity? – Stretching and twisting of magnetic field lines near the Sun’s surface cause solar activity. • How does solar activity affect humans? – Bursts of charged particles from the Sun can disrupt radio communication and electrical power generation and damage satellites. • How does solar activity vary with time? – Activity rises and falls with an 11-year period. © 2010 Pearson Education, Inc.