Nucleus Positive charge Most of mass of atom

1. Inside the Nucleus • Electron • Charge -1 • Proton • Charge +1 • Neutron • Charge 0 • Nucleus • Positive charge • Most of mass of atom Q. 65: What Holds the Nucleus Together?

2. The Strong Nuclear Force • At long distances, electric forces cause protons to repel • At short distances, nucleons attract each other very strongly • A huge amount of energy is released

3. The Strong Nuclear Force Hydrogen • Most hydrogen has just one proton • Most helium has two protons and two neutrons • Other nuclei are more complicated Helium Iron

4. Nuclear “Burning” of Hydrogen • Not really burning • Needs high temperature • 16 million K (for Sun) • Uses up fuel • Makes energy

5. Fusion: The Net Reaction 4 Hydrogen nuclei + 2 electrons 1 Helium + 2 neutrinos + energy + + + What’s a Neutrino? • Charge 0 • Speed of light • Can penetrate most anything

6. Neutrino Detectors 4 H + 2e - He + 2 neutrino + energy Sudbury Neutrino Observatory Homestake neutrino detector • Neutrinos come directly to the Earth • No delay • They confirm the energy is coming from fusion

7. Neutrino Detectors The Sun imaged in neutrinos Super Kamiokande

8. Measuring Stars Easy Hard Binary Stars What We Want to Know • Brightness • Temperature • Composition • Distance • Luminosity • Size (Radius) • Mass lpeakT = 2900 Km • Spectrum tells you composition (M+m)P2 = a3 • Spectrum also tells you much more

9. Luminosity and Brightness • The LuminosityL is how much power something is putting out • The BrightnessB is how brightsomething appears • They are related: Sphere: A = 4d2 d L = 4d2B • The brightness is always easy to determine • If we can get one of the distance or the luminosity, we can get the other. Q. 66: Brightness, Luminosity, and Distance

10. Finding the Distance Easy Hard L = 4d2B • Brightness • Temperature • Composition • Distance • Luminosity • Size (Radius) • Mass • If we can get the distance, we can get the luminosity too • We will use a new unit for measuring distance, the light year • The distance light goes in a year ly = 9.46  1015 m = 63,240 AU • Real astronomers use parsecs • But we won’t

11. Methods for Finding Distance • Radar • Solar System only • Excellent accuracy • Parallax • Nearby stars (< 10,000 ly) • Moderate accuracy • Spectroscopic parallax • Main sequence stars only • Poor accuracy

12. Radar Distance Earth Venus d 2d = ct, solve for d • We know what an AU is • Effectively no error

13. Parallax • The distance to an object can be judged if you view it from two angles • The difference in the angle you see it from is called parallax • The more distant, the smaller the parallax  

14. Parallax p p • The farther apart you put your “two eyes”, the better you can judge distance • The smaller p is, the farther away the star is. d • p in arc-seconds (The distance 3.26 ly is also known as a parallax second) parsec nearest stars several ly away  Centauri C = Proxima Centauri : 4.2 ly Sirius: 9 ly

15. Spectral Type • The following are all equivalent information: • The surface temperature of a star • The color of the star • The spectral type of the star • From hottest to coldest, OBAFGKM • Subdivided 0-9, with 0 the hottest • Sun is a G2 star • The spectral type is easy to determine Why I hate astronomers “Oh Be A Fine Girl, Kiss Me.” Q. 67: Spectral Type

16. Spectral Type