1. Physics 55 Friday, October 28, 2005 1.The Sun as a star. 2.Structure of the Sun. 3.Nuclear reactions that power the Sun. 4.How do we know what we know?

2. Talk by Dr. Jocelyn Bell Burnell Tonight UNC 7pm Morehead Planetarium Jocelyn Bell in 1968

3. Mars, the Pleiades, Taurus, the Hyades WARNING: Do not confuse the star Aldebaran in Taurus with Alderaan, the home planet of Princess Leia in the Star Wars movies!

4. Mars Won’t Be This Close for 13 Years

5. Our Sun as a Star: Chapter 15 Why does it shine? What is its structure? How are the elements produced? How does the Sun affect Earth? How do we know what we know?

6. Particular Properties of the Sun Note: Total human energy consumption about 10 11 watts

7. Structure of the Sun 15,000,000 K 6000K 10,000 K 1,000,000 K Who remembers role of solar wind in formation of solar system?

8. Other Stars Have Similar Structures

9. What Determines the Size of the Sun? Gravitational Equilibrium Balance of gravitational force toward center and pressure outward (from thermal motion): “gravitational equilibrium” “hydrostatic equilibrium”

10. Attempts to Estimate Lifetime of Sun in 1800s Led to a Mystery: Too Short! “Calculating the age of the Earth and Sun” by Arthur Stinner, Physics Education 37(4) 296-305 (2002). Helmholtz: Assume Sun with mass 10 30 kg is made of combusting coal: Kelvin’s meteoric theory: energy of Sun supplied by meteors steadily falling into Sun, releasing gravitational potential energy. Anyone see a way to test this idea? Kelvin and Helmholtz: Assume energy released by conversion of gravitational potential energy into kinetic energy of mass falling inward toward center: ~ 60,000,000 years with Sun shrinking only about 80 m in radius per year. All known mechanisms fail to give age consistent with geology and biology of billions of years, what is going on?

11. Radioactivity

12. Nuclear Reactions