1. Q. 65: What Holds the Nucleus Together?
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. The Sun imaged in neutrinos
Neutrino Detectors
The Sun imaged in neutrinos
Super Kamiokande

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

9. Luminosity and Brightness
The Luminosity L is how much power something is putting out
The Brightness B is how bright something 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 L = 4d2B ly = 9.46  1015 m = 63,240 AU
Brightness
Temperature
Composition
Distance
Luminosity
Size (Radius)
Mass
Easy
If we can get the distance, we can get the luminosity too
Hard
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. nearest stars several ly away
Parallax
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 p
(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. “Oh Be A Fine Girl, Kiss Me.”
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