Class notes
Astr 112 Exam 2 Prep Notes
- Course
- ASTR 114
- Institution
- State University Of New York - Binghamton
Detailed and comprehensive exam Preparatory Notes on Exam 2 for Astr 114. *Essential Study Material!!
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LECTURE 10
Physical properties of the sun
● Interior structure of the Sun:
● Outer layers are not to scale
● The core is where nuclear fusion takes place
● Luminosity—total energy radiated by the Sun— can be calculated from the
fraction of that energy that reaches Earth.
● Solar constant—the amount of Sun's energy reaching Earth—is 1400 W/m2.
● Total luminosity is about 4 × 1026 W—the equivalent of 10 billion 1-megaton
nuclear bombs per second.
● Radius—6.9x108m (109 x Earth)Mass—1.99x1030kg (300,000 Earths)
How is the Sun Stable If All this Radiation is Given Off?
● Weight Of Upper Layers compresses lower layers.
The Solar Interior – Gravitational Equilibrium
● Mathematical models, consistent with observation and physical principles,
provide information about the Sun’s interior
● In equilibrium, inward gravitational force must be balanced by outward
pressure
The Solar Interior – Energy Balance
● Energy Balance:
○ Therateat which energy radiates from the surface of the Sun must be the
same as the rate at which it is released by fusion in the core.
● Solar wind:
○ A flow of charged particles from the surface of the Sun
● Corona: Outermost layer of solar atmosphere
○ ~1 million K
● Chromosphere: Middle layer of solar atmosphere
○ ~ 104–105 K
● Photosphere: Visible surface of Sun
○ ~ 6000 K
● Convection Zone: Energy transported upward by rising hot gas
● Radiation Zone: Energy transported upward by photons
● Core: Energy generated by nuclear fusion
○ ~ 15 million K
Review
● Why does the Sun shine?
○ Chemical and gravitational energy sources could not explain how the Sun
could sustain its luminosity for more than about 25 million years.
, ○ The Sun shines because gravitational equilibrium keeps its core hot and
dense enough to release energy through nuclear fusion.
● What is the Sun's structure?
○ From inside out, the layers are:
• Core
• Radiation zone
• Convection zone • Photosphere
• Chromosphere
• Corona
Nuclear fusion in the sun
● Fission
○ Big nucleus splits into smaller pieces.
○ (Example: nuclear power plants)
● Fusion
○ Small nuclei stick together to make a bigger one.
○ (Example: the Sun, stars)
● High temperatures enable nuclear fusion to happen in the core.
● • The Sun releases energy by fusing four hydrogen nuclei into one helium
nucleus.
● The proton–proton chain is how hydrogen fuses into helium in the Sun.
○ IN- 4 protons
○ OUT- 4He nucleus 2 gamma rays 2 positrons 2 neutrinos
○ Total mass is 0.7% lower.
Solar energy transport
● Energy transport: The radiation zone is relatively transparent;;the
coolerconvection zone is opaque
Random photon walk
● Energy gradually leaks out of the radiation zone in the form of randomly
bouncing photons.
Granulation of the Sun – Evidence of Convection
● Convection (rising hot gas) takes energy to the surface.
● Bright blobs on photosphere show where hot gas is reaching the surface.
Review
● How does nuclear fusion occur in the Sun?
○ The core's extreme temperature and density are just right for nuclear
fusion of hydrogen to helium through the proton–proton chain.
○ Gravitational equilibrium acts as a thermostat to regulate the core
temperature because fusion rate is very sensitive to temperature.
● How does the energy from fusion get out of the Sun?
○ Randomly bouncing photons carry energy through the radiation zone.
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