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Stellar Structure I

AST 112. Stellar Structure I. Our Star. The Sun is the STAR at the center of the Solar System. What is the difference between a star and a planet?. Stars vs. Planets. Stars have enough gravity to initiate and sustain nuclear fusion reactions at their cores Planets do not

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Stellar Structure I

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  1. AST 112 Stellar Structure I

  2. Our Star • The Sun is the STAR at the center of the Solar System

  3. What is the difference between a star and a planet? Stars vs. Planets

  4. Stars have enough gravity to initiate and sustain nuclear fusion reactions at their cores • Planets do not • Cores of planets are hot, but are not producing nuclear reactions Stars Vs. Planets

  5. The Sun (and all the other stars) spend their lifetimes fusing H into He Depletion of H signals the beginning of the end of a star’s life The initial amount of H and the rate at which the star fuses H into He determine a star’s lifetime Stars Don’t Last Forever

  6. Composition of the Sun What is the Sun made of and why?

  7. Composition of the Sun Mostly H, some He, and a small amountof heavier elements (nitrogen, oxygen,iron, etc.)

  8. Yellow Main Sequence Star • Diameter: 864,000 miles (109DEarth) • Mass: 330,000 MEarth • Density: (water is 1000 kg/m3) • Surface: 5.0 x 10-6 kg/m3 • Core: 162,200 kg/m3 • The surface is 10,000 oF • Lifespan: 10 billion years The Sun

  9. Blue Main Sequence Star Diameter: 1,480,000 miles (1.77 DSun) Mass: 2.02 MSun Luminosity: 25.4 LSun The surface is 17,500 oF Lifespan: 1 billion years Sirius (FAKE PICTURE)

  10. Red Supergiant • Diameter: 1,000,000,000 miles (1180 DSun) • Mass: 19 MSun • Density: (water is 1000 kg/m3) • Surface: 1/100000 that of air at sea level on Earth • Luminosity: 140,000 LSun • The surface is 5840 oF • 100,000,000 years old and maybe on its way out Betelgeuse

  11. Plasma • PLASMA is the phase of matter in a star • Tenuous (gas-like) at outer surface, much more dense toward core • The high temperatures do not allow electrons to stay bound to nuclei

  12. Ancients believed that the Sun was a type of fire Wood burning, coal burning, and other chemical processes insufficient to generate the Sun’s energy (Not) Why Stars Shine

  13. Late 1800’s, an incorrect idea: • The Sun slowly contracts gravitationally, generating large amounts of thermal energy • Recall conservation of energy • Potential energy  Thermal energy • Calculations gave 25 million years worth of shining • Fossils and rocks on Earth were known to be way more than 25 million years old (Not) Why Stars Shine

  14. Nuclear reactions convert H to He. Some of the mass is lost to pure energy: E = mc2 Why Stars Shine

  15. When H fuses into He: • Two smaller nuclei mash into one larger one • The larger one weighs less than the sum of the two smaller ones • The mass deficit is converted to energy Why Stars Shine

  16. Nuclei have + charge • Should repel, right? • The Coulomb force does cause them to repel, but: • At extremely short distances, nuclear strong force takes over and binds nucleons together • High pressure and high temperature are required Strong vs. Coulomb

  17. Isn’t there a “contradiction” going on in a star? • Doesn’t gravity still try to collapse the star? • Don’t the nuclear reactions try to blow it apart? Hydrostatic Equilibrium

  18. Hydrostatic Equilibrium • They balance! • Gravity pulls matter inward • Pressure from hotter layers and nuclear reactions pushes matter outward

  19. Hydrostatic Equilibrium • A stack of acrobats: • Bottom one is pressed the hardest, and pushes the hardest • Middle one presses less, is pushed less • Top one isn’t pressed from above, doesn’t push

  20. Hydrostatic equilibrium keeps the Sun’s size stable What happens when the Sun has fused all of its hydrogen into helium? Hydrostatic Equilibrium

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