# Chapter 14: Our Star

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1. The Sun
• Distance from Earth: 1AU = 149,600,000 km
• Variation from nearest to furthest distance: ~3%
• Light time travel to Earth: ~ 8 minutes
• Radius: 696,000 km or 6.9 x 108 m = 109 X Earth’s radius
• Mass: 1.99 x 1030 kg = 3.3 x 105 Earth’s mass
• Average Density: 1410 kg/m3
• (Earth ~5,500 kg/m³)
• Composition by mass: H – 74%; He - 25%, All others 1%
• Luminosity: 3.8 x 1026 W

average star in terms of mass, luminosity and temperature. it's only special because it's so close to us. no known siblings. the next nearest star is about 270 000 AU away
2. Is the Sun on fire?
no
3. Is the Sun contracting?
no
• yes
• E = mc²
5. Theoretical Model of the Sun
explains pressure, temperature and density as a function of the distance from the Sun's center.

• Key observational factors: the sun is not changing quickly, so it must be in equilibrium.
• -> hydrostatic pressure at any point is exactly enough to support the weight of the overlying gas, so the pressure must increase greatly with depth.
• -> thermal energy is radiated away from surface, it must be re-supplied from below.

sun is in hydrostatic equilibrium and thermal equilibrium ∴ it's not changing in size nor changing in temperature
6. Mechanical, Gravitational or Hydrostatic Equlibrium
energy provided by fusion maintains the pressure
7. Gravitational Contraction
• provided energy that heated core as Sun was forming
• contraction stopped when fusion began
8. Sun's Structure (inside to out)
• 1) core
• 3) convection zone
• 4) photosphere
• 5) chromosphere
• 6) corona
• 7) solar wind
9. Solar Wind
a flow of charged particles from the surface of the Sun
10. Corona
• outermost layer of solar atmosphere
• ~ 1 million K
• appears bright in x-ray photos in places where magnetic fields trap hot gas
11. Chromosphere
• middle layer of solar atmosphere
• ~ 104 - 10K
12. Photosphere
• visible surface of the Sun
• ~ 6000 K
13. Convection Zone
energy transported upward by rising hot gas
energy transported upward by photons
15. Core
• energy generated by nuclear fusion
• ~ 15 million K
16. Why was the Sun's energy source a major mystery?
chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years
17. Why does the Sun shine?
because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion
18. Fusion
• coming together
• small nuclei stick together to make a bigger one
• high temperature enables fusion to happen in the core
• (sun and stars)
19. Fission
• breaking apart
• big nucleus splits into smaller pieces
• (nuclear power plants)
20. Thermonuclear Fusion
• proton-proton chain: how hydrogen fuses into helium in the Sun
• proton-proton cycle: Sun releases energy by fusing 4 hydrogen nuclei into 1 helium nucleus in a 3 step process
• 4¹H ⇒ 14He + energy

• IN: 4 protons
• OUT: 4He nucleus, 2 gamma rays, 2 positrons, 2 neutrinos

Total mass is 0.7% lower
21. Solar Thermostat
• decline in core temperature causes fusion rate to drop so core contracts and heats up
• rise in core temperature causes fusion rate to rise, so core expands and cools down
22. How does the energy from fusion get out of the sun?
• energy gradually leaks out of the radiation zone in the form of randomly bouncing photons
• convection (rising hot gas) takes energy to the surface
• bright blobs on the photosphere is where hot gas is reaching the surface
23. How is energy transported from the center to the surface?
• 1) conduction: not efficient for hot gases
• 2) radiation: diffusion of photons - the primary mechanism for the first ~ 71% of the Sun's radius
• 3) convection: circulation of hot gases - the primary mechanism for the outer ~ 29% of the Sun's radius
24. How do we know what is happening inside the sun?
• by making mathematical models, observing solar vibrations and observing solar neutrinos
• patterns of vibration on the surface tells us about what the sun is like inside
• data on solar vibrations agree with mathematical models of solar interior
• neutrinos created during fusion, fly directly through the Sun
• observing these solar neutrinos can tell us what's happening in the core
25. Neutrino
• a type of fundamental particle that has extremely low mass and responds only to the weak force
• they are leptons and come in 3 types...
• 1) electron neutrinos
• 2) mu neutrinos
• 3) tau neutrinos
26. Solar Neutrino Problem
• early searches for solar neutrinos failed to find the predicted number
• more recent observations find the right number of neutrinos, but some have changed forms
27. The Nobel Prize in Physics, 2015
• Arthur B. McDonald and Takaaki Kajita
• they were cited for the discovery of neutrino oscillations and their contributions to experiments showing that neutrinos change identities
• they determined that neutrinos have mass
28. Cause of Solar Activity
• sunspots: strong magnetic fields, cooler than other parts of the sun. 4000 K
• solar flares: send bursts of X-rays and charged particles into space
• solar prominences: erupt high above the Sun's surface

all are related to magnetic fields. solar activity is like "weather". stretching and twisting of magnetic field lines near the Sun's surface causes solar activity.
29. Sunspots
• are cooler than other parts of the sun's surface
• 4000 K
• regions with strong magnetic fields
• in pictures they look black, but in real life they are actually very bright - just nowhere near the brightness of other parts of the sun
30. Zeeman Effect
• we can measure magnetic fields in sunspots by observing the splitting of spectral lines
• charged particles spiral along magnetic fields
• loops of bright gas often connect sunspot pairs
31. How does solar activity affect human's?
• Coronal mass ejections send bursts of energetic charged particles out through the solar system
• charged particles streaming from the sun can disrupt electrical power grids and can disable communications satellites
32. How does solar activity vary with time?
number of sunspots rise and fall in 11-year cycles
33. Annie Maunder