Structure of the SunStructure of the Sun

The Core is where all the The Core is where all the action is.action is.

The core is the only place in the Sun where the The core is the only place in the Sun where the temperature (10 million K) and density are high temperature (10 million K) and density are high enough to support nuclear fusionenough to support nuclear fusion

(hydrogen bomb needs atomic bomb for ignition!) (hydrogen bomb needs atomic bomb for ignition!)

Every second, about 600 million tons of Every second, about 600 million tons of Hydrogen are fused into 596 million tons of Hydrogen are fused into 596 million tons of helium.helium.

The remaining mass (4 million tons) is converted The remaining mass (4 million tons) is converted to energy in line with Einstein’s formulato energy in line with Einstein’s formula

E = mcE = mc22

Nuclear Fusion in the SunNuclear Fusion in the Sun

Hydrogen is Hydrogen is constantly being constantly being transformed into transformed into helium in the Sun’s helium in the Sun’s core.core.

The proton-proton chain ReactionThe proton-proton chain Reaction(main branch)(main branch)

- neutrinos; + - positrons; - gamma rays.

4411H --> H --> 44He + energyHe + energy ( ( E = E = mcmc2 2 ))mass(4He) = 0.993{mass(H)+mass(H)+mass(H)+mass(H)}

mass loss is 0.007{mass(4H)} = 5 x 10-29 kg

E=mc2=(5 x 10-29)(3 x 108)2 = 4 x 10-12 joules

How many fusions per second?

Solor Luminosity = 4 x 1026 joules/sec

4 x 1026 joules/sec----------------------- = 1038 fusions/sec = 200 million tons/sec!!!4 x 10-12 joules

Actually, about 500 million tons/sec are needed!

Balance in the starsBalance in the stars

ThermalPressure

GravitationalContraction

Pressure and Temperature Pressure and Temperature of a Gasof a Gas

This balance between weight and pressure is called hydrostatic equilibrium.

The Sun's core, for example, has a temperature of about 16 million K.

How does a star hold itself?

Air temperature inside drops belowthe set point of the thermostat

Electrical signal triggersignition of furnace

Furnace burns oil/gasand generates heat

Heat is absorbed by waterrunning near furnacecausing water temperatureto rise

Hot water is circulated throughout house

Radiators transfer energyfrom water to air

Air warms thermostat untilelectrical signal is shut off

Furnace shuts off

My House’s Thermostat

Outward thermal pressure of coreis larger than inward gravitational pressure

Core expands

Expanding core cools

Nuclear fusion ratedrops dramatically

Outward thermal pressureof core drops (and becomessmaller than inward grav. pressure)

Core contracts

Contracting core heats up

Nuclear fusion raterises dramatically

The Solar Thermostat

BalanceBalance If the star contracts at all, the If the star contracts at all, the

temperature goes up, the rate of temperature goes up, the rate of fusion increases, the pressure fusion increases, the pressure increases, and the star re-increases, and the star re-expands.expands.

If the star expands too much, the If the star expands too much, the temperature drops, the rate of temperature drops, the rate of fusion drops, the pressure drops, fusion drops, the pressure drops, and the star contracts.and the star contracts.

How is the energy How is the energy transported from the transported from the center to the surface?center to the surface? Radiative transferRadiative transfer

Photons are continually absorbed and re-Photons are continually absorbed and re-emitted (emitted (reprocessing of radiationreprocessing of radiation), and ), and when they are re-emitted from more when they are re-emitted from more external layers, which are colder, the loose external layers, which are colder, the loose energy (but total energy is conserved, energy (but total energy is conserved, because more photons are emitted)because more photons are emitted)

ConvectionConvection Large scale motions of gas. Hot gas moves Large scale motions of gas. Hot gas moves

upward and carries energy there. Cold gas upward and carries energy there. Cold gas move downward to restore balancemove downward to restore balance

Radiative TransportRadiative Transport

Photons are absorbed and re-emitted in Photons are absorbed and re-emitted in random walk motion.random walk motion.

Each photon mean-free path is very, very Each photon mean-free path is very, very shortshort

It takes each γ photon produced by It takes each γ photon produced by nuclear fusion million years to reach the nuclear fusion million years to reach the surface, and when it does, it has been surface, and when it does, it has been reprocessed into ~1,800 cooler visible-reprocessed into ~1,800 cooler visible-light photons (energy is consesrved!!)light photons (energy is consesrved!!)

Convection Activity

Structure of the SunStructure of the Sun

An aside about nuclear An aside about nuclear fusionfusion

as a viable energy sourceas a viable energy sourceA few key points about the P-P Chain:A few key points about the P-P Chain:

It is a “clean” form of energy production.It is a “clean” form of energy production. Put 4 protons (Hydrogen) in and get one Helium and Put 4 protons (Hydrogen) in and get one Helium and

some energy back out.some energy back out. Given fuel (in the form of Hydrogen) it is a self-Given fuel (in the form of Hydrogen) it is a self-

sustaining reaction.sustaining reaction. It requires *very* high temperatures (10It requires *very* high temperatures (1077 K) K)

The energy emitted by the The energy emitted by the Sun is producedSun is produced

in a small region at the very center in a small region at the very center

of the Sun.of the Sun. uniformly throughout the entire Sun.uniformly throughout the entire Sun. throughout the entire Sun but more throughout the entire Sun but more

in the center than at the surface.in the center than at the surface. from radioactive elements created in from radioactive elements created in

the Big Bang.the Big Bang.

1) energy output

2) radius

3) electric fields

4) magnetic fields

The bulk of the violent surface activity on the Sun is due to “seasonal” variations in the Sun’s _________.

What is burning in stars?What is burning in stars?

Gasoline Gasoline Nuclear fissionNuclear fission Nuclear fusionNuclear fusion Natural gas Natural gas

Sun Activity: or Weather on the Sun

• Two key points to remember here:

1. nearly all “solar weather” is a result of changes in the magnetic fields that penetrate the Photosphere.

2. Magnetic Fields are the result of convection in the Sun (=large scale motions of electrically charged particles generate manetic fields)

Due to change in magnetic field

Magnetic field lines

Measured by Zeeman effect

Why sunspot is colder

Differential Rotation of the SunDifferential Rotation of the Suncauses magnetic cyclescauses magnetic cycles

Rotation period: about a month

The middle rotates faster than the north or south.

Time-lapse (27 days) movie of x-ray emission from the Sun's corona. Source: Yohkoh Public Outreach Project.

Babcock Model

The Solar The Solar CycleCycle

Solar activity peaks roughly each 11 years.

The Solar ConstantThe Solar Constant The S.C tells us the amount of energy the Sun The S.C tells us the amount of energy the Sun

produces:produces: It is 1360 Joules per second per square meter…It is 1360 Joules per second per square meter… ……or about the luminosity (=ENERGY PER or about the luminosity (=ENERGY PER

SECOND) of 60 W bulb every square millimeter.SECOND) of 60 W bulb every square millimeter. A stable change of ~1% in the S.C. would change A stable change of ~1% in the S.C. would change

Earth’s temperature by 1-2 degreesEarth’s temperature by 1-2 degrees Random variation ~0.1% over days or weeksRandom variation ~0.1% over days or weeks Long-term, cyclical variation 0.018% per year Long-term, cyclical variation 0.018% per year

(period longer than the 22 years of the sunspots)(period longer than the 22 years of the sunspots) Small random fluctuations do not affect Earth’s Small random fluctuations do not affect Earth’s

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