Ch. 6: The Sun

Chemical energy?

The Sun’s luminosity is about 4x1026 joules per second.Its mass is about 2x1030 kg. What is its energy source?

Chemical energy?

The Sun’s luminosity is about 4x1026 joules per second.Its mass is about 2x1030 kg. What is its energy source?

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= 10 Cal/g= 108 J/kg

2x1030 kg

4x1026 Joules

second

108 joules

kgx 1 year

3x107 secondsx

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≈17,000 years

Is it CONTRACTING?

Luminosity

Gravitational Potential Energy~ 25 million years

It can be powered by NUCLEAR ENERGY!

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

E = mc2

- Einstein, 1905

The fusion of hydrogen via the proton-proton chain

positron

neutrino

4He nucleu

s

• Neutrinos created during fusion fly directly through the Sun

• Observations of these solar neutrinos can tell us what’s happening in core

Weight of upper layers compresses lower layers

The Sun is in hydrostatic equilibrium

Gravity pushes inward

Fusion generates energy

- heats the gas

- increases the pressure

- balances the inward force of gravity

Core:

Energy generated by nuclear fusion

~ 15 million K

Radiation Zone:

Photons don’t know which way is up - they escape by a “random walk”

Energy gradually leaks out of radiation zone

Convection Zone:

Energy transported upward by rising hot gas

Photosphere:

Visible surface of Sun

~ 6,000 K

Chromosphere:

Middle layer of solar atmosphere

~ 104 - 105 K

Corona:

Outermost layer of solar atmosphere

~1 million K

Solar wind:

A flow of charged particles from the surface of the Sun

Limb darkening

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The layers of the Sun are determined by how energy is transported through and out of that layer

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I = I0e−τ

Radiation transport (radiative transfer)

Sunspots

cooler than other parts of the Sun’s surface (~4000 K)

Contain strong magnetic fields

cool gas is sinking

41.2 Zeeman effect

Atomic energy levels, and their spectral lines, are split when the atoms are in a magnetic field

Experimental confirmation of the quantization of angular momentum

Why do the lines split?

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rμ =I

r A

Magnetic moment of a current carrying loop:

I

Current I = charge/time

•Electrons are moving charges•They respond to external B•Their energy levels change when in a B

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μ =e

2mL

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L = mvr = nh

Potential energy of this interaction:

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U = −r μ ⋅

r B

Loops of bright gas often connect sunspot pairs

X-ray emission

Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space

Magnetic activity also causes solar prominences that erupt high above the Sun’s surface

Corona appears bright in X-ray photos in places where magnetic fields trap hot gas

Insert TCP 5e Figure 14.20

Coronal mass ejections send bursts of energetic charged particles out through the solar system

Charged particles streaming from Sun can disrupt electrical power grids and can disable communications satellites

Number of sunspots rises and falls in 11-year cycle

Solar activity varies with time

Little Ice Age1300-1850

Winter sports became very popular!

Historical implications of the Little Ice Age

• Greenland settlement collapsed in 1400• Glaciers advanced, crushing villages in Switzerland• 1780: New York Harbor froze• Stradivari built his famous violins• Armies could march acropss frozen rivers and straits• Snow fell in summer in Northern Europe: famine ensued