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Announcements

Announcements. Welcome back to Standard Time! Wednesday night star parties begin this week, 8:45 pm, weather permitting. Attend one for 4 points extra credit! (Staff signature required.) Milky Way bars! (For a bigger one, solve the Monty Python song puzzler.)

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Announcements

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  1. Announcements • Welcome back to Standard Time! • Wednesday night star parties begin this week, 8:45 pm, weather permitting. Attend one for 4 points extra credit! (Staff signature required.) • Milky Way bars! (For a bigger one, solve the Monty Python song puzzler.) • Telescopic trick-or-treat tomorrow at 1444 Binford Street (weather permitting) • Turn in Homework 9 • Pick up Homework 10 • Sorry about the delay in grading projects, tests…

  2. Nuclear Reactions in Stars 30 October 2006

  3. Today: • Measuring masses of stars • What makes the stars shine?

  4. Binary Star Systems • Period of orbit is determined by m1+m2 • Relative amounts of motion are determined by mass ratio m1/m2

  5. Binary Star Systems • Period of orbit is determined by m1+m2 (and orbital size) • Relative amounts of motion are determined by mass ratio m1/m2

  6. Summary of Stellar Properties

  7. Masses of Stars

  8. Masses of Stars • The most massive stars area about 100 times as massive as our sun (msun = 2 x 1030 kg). • The least massive stars have about 1/10 the mass of our sun. • The full range of masses occurs among main-sequence stars, for which mass correlates to temperature and luminosity. • “Red giant” stars tend to be more massive than our sun, but otherwise are no more massive than main-sequence stars. • “White dwarf” stars tend to have about the same mass as our sun.

  9. What makes the stars shine? Stars give off huge amounts of radiant energy over very long time periods, yet undergo little noticeable change. Where does all this energy come from?

  10. Types of energy Motion (“kinetic”) Gravitational Elastic Thermal Chemical Nuclear Electrical Radiant (light) Energy can be converted from one type to another, but cannot be created or destroyed. The total amount of energy in the universe never changes.

  11. Possible energy sources… • Chemical. 1030 kilograms of gasoline, each providing 10,000 Calories of energy (40 MJ), would yield 4 x 1037 joules of energy. If the energy is released at a rate of 4 x 1026 J/s, this fuel would last 1011 seconds, or about 3000 years. Not long enough! ?

  12. Possible energy sources… • Gravitational. Our sun formed from a collapsing gas cloud, and may still be contracting. As material falls inward, gravitational energy is converted to kinetic and then thermal energy. From the sun’s mass and present size, Kelvin calculated an age of about 20 million years (at its current luminosity).

  13. Possible energy sources… • Nuclear! Nuclear reactions typically give off a million times as much energy, per atom, as chemical reactions, so the sun could last billions of years. • Problem: The sun is made mostly of stable isotopes, 1H and 4He. Its energy source is not ordinary radioactive decay!

  14. Nuclear Fusion Reactions 4 1H (protons) • Small nuclei (such as hydrogen) combine to form larger nuclei (helium, etc.), releasing energy. • But: First the nuclei must “touch” each other, despite electrostatic repulsion. This requires high-speed collisions, that is, very high temperatures (except at the U. of U. Chemistry Department). 4He

  15. Nuclear Fusion DetailsFor the sun and most main-sequence stars (Not shown: Positrons (beta+) annihilate with electrons into gamma rays.) Net result: 4 protons plus 2 electrons convert to a 4He nucleus, 2 neutrinos, and 6 gamma-ray photons

  16. How much energy is released? E = mc2 Conversion factor, numerically equal to 3x108 m/s, squared Energy content of any object (including all forms except kinetic energy of overall motion), in joules Mass of object, in kilograms The total energy content of a 1-kg object is 9 x 1016 joules! Chemical reactions typically involve mass changes of only one part in a billion -- too small to measure. Nuclear reactions typically involve mass changes of a few parts in a thousand -- easy to measure.

  17. How much energy is released? For each kilogram of hydrogen converted to helium, the mass decreases by 7 grams, so the energy released is 6 x 1014 joules. In one second, the sun releases 4 x 1026 joules of energy, so its mass must decrease by about 4 billion kilograms! Hans Bethe, 1906 - 2005

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