© 2010 Pearson Education, Inc. Chapter 8 The Sun and Other Stars

© 2010 Pearson Education, Inc. Radius: 6.9  10 8 m (109 times Earth) Mass: 2  kg (300,000 Earths) Luminosity: 3.8  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 Big nucleus splits into smaller pieces. (Example: nuclear power plants) Fusion Small nuclei stick together to make a bigger one. (Example: the Sun, stars)

© 2010 Pearson Education, Inc. 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 4 He 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 ~ 10 4 –10 5 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. The number of sunspots rises and falls in an 11-year cycle. Insert TCP 6e Figure 14.21a unannotated

© 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: Luminosity Brightness = 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: 10 6 L Sun Least luminous stars: 10 –4 L Sun (L Sun 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 times mag 2

© 2010 Pearson Education, Inc. Apparent Magnitude (concluded) Sun-26.4 Full Moon Venus (max)-4.89 Mars (max)-2.91 Mars (min)1.84 M31 (Andromeda Galaxy)3.44 Best naked eye7-8 7x50 binoculars9.5 My telescope?? Hubble telescope31.5

© 2010 Pearson Education, Inc. 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. (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 Meaningfully

© 2010 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)

© 2010 Pearson Education, Inc. 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. M 1 and M 2 are the masses of the two stars p = period a = average separation p 2 = a 3 4  2 G (M 1 + M 2 )

© 2010 Pearson Education, Inc. Most luminous stars: 10 6 L Sun Least luminous stars: 10 –4 L Sun (L Sun is luminosity of Sun)

© 2010 Pearson Education, Inc. Most luminous stars: 10 6 L Sun Least luminous stars: 10 –4 L Sun (L Sun 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 self- evident later

© 2010 Pearson Education, Inc. Main-Sequence Star Summary High-Mass Star: High luminosity Short-lived Larger radius Blue Low-Mass Star: Low luminosity Long-lived Small radius Red

© 2010 Pearson Education, Inc. 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. Temperature Luminosity An H-R diagram plots the luminosity and temperature of stars.

© 2010 Pearson Education, Inc. 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 Life expectancy of 10M Sun star: 10 times as much fuel, uses it 10 4 times as fast 10 million years ~ 10 billion years  10/10 4 Life expectancy of 0.1M Sun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years  0.1/0.01 Until core hydrogen (10% of total) is used up

© 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.