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THE SUN

The star we see by day

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The Sun, Our Star

• The Sun is an average star.• From the Sun, we base our

understanding of all stars in the Universe.

• No solid surface.

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Interior Properties

• Core = 20 x density of iron

• Surface = 10,000 x less dense than air

• Average density = Jupiter

• Core = 15,000,000 K• Surface = 5800 K

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Scientific Method

• How do we know this stuff?• Three examples:

– Fusion in the core (core temperature).– Different zones in interior.– Solar activity and Earth

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1. The Core

• Scientific Method:– Observations– Make hypothesis (a model)

• Models make predictions– Test predictions

• Compare results of predictions with observations– Revise model if necessary.

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Testing the Core

• Observe Sun’s:– Mass (how?)– Composition (how?)– Radius– Luminosity (total energy output)

• Use physics to make a model Sun.• Predict:

– Surface temp/density (how do you test?)– Surface Luminosity (how do you test?)– Core temp/density Fusion Rate neutrino rate

(test?)

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In The Core

• Density = 20 x density of Iron

• Temperature = 15,000,000 K

• Hydrogen atoms fuse together.

• Create Helium atoms.

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Concept Question

How do the nuclear reactions in the Sun’s core produce energy?a. The mass of the product of the reaction is

greater than the mass of the hydrogen atoms that enter the reaction.

b. The mass of the product of the reaction is less than the mass of the hydrogen atoms that enter the reaction.

c. The chemical potential energy of the product of the reaction is greater than the chemical potential energy of the hydrogen atoms that enter the reaction.

d. The chemical potential energy of the product of the reaction is less than the chemical potential energy of the hydrogen atoms that enter the reaction.

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2. Helioseismology• Continuous

monitoring of Sun.– Ground based

observatories– One spacecraft

(SOHO)• Surface of the Sun is

‘ringing’• Sound waves cross

the the solar interior and reflect off of the surface (photosphere).

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Solar Interior

• Core– Only place with

fusion• Radiation Zone

– Transparent• Convections

Zone– Boiling hot

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Convection

• A pot of boiling water:

• Hot material rises.• Cooler material

sinks.• The energy from the

pot’s hot bottom is physically carried by the convection cells in the water to the surface.

• Same for the Sun.

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Solar Cross-Section

• Progressively smaller convection cells carry the energy towards surface.

• See tops of these cells as granules.

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The Photosphere

• This is the origin of the 5,800 K thermal radiation we see. l = k/T = k/(5800 K) l = 480 nm (visible light)

• This is the light we see.• That’s why we see this as the “surface.”

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Concept Question

How can we observe nuclear fusion in the Sun’s core? a. We can’t.b. We can observe neutrinos produced in

the core.c. We can observe positrons produced in

the core.d. We can observe high energy gamma

rays produced in the core.

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3. Solar Activity and Earth

• Is there a connection between Solar Activity and Earth’s Climate?

• Observation:– Little Ice Age– Maunder Minimum

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What is Solar Activity?

• Sunspots• Magnetic Fields• Coronal Mass Ejections• Solar Wind• Magnetic Storms• Aurora• Other effects?

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Sunspots

• 11-year sunspot cycle.• Center – Umbra: 4500 K• Edge – Penumbra: 5500 K• Photosphere: 5800 K

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Magnetic fields and Sunspots

• At kinks, disruption in convection cells.• Sunspots form.

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Magnetic fields and Sunspots

• Where magnetic fields “pop out” of Sun, form sunspots.

• Sunspots come in pairs.

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Concept Question

Why are sunspots cooler than the rest of the photosphere?a. They are where cooler gas sinks as part of the

convection cells bringing heat to the photosphere.

b. They are areas of slightly different composition that absorbs radiative energy from below less efficiently than the rest of the photosphere.

c. They are areas of magnetic fields that inhibit convective transport of heat from below.

d. They are regions of denser gas.e. They are at higher altitudes, where

temperatures are slightly lower, than the surrounding photosphere.

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Prominences

Hot low density gas = emission lines

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Stereo

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Corona and Solar Wind

• Hot, low density, gas emits the radiation we see as the Corona: 1,000,000 K

• Solar Wind: Like steam above our boiling pot of water, the gas ‘evaporates’.

• Carries away a million tons of Sun’s mass each second!

• Only 0.1% of total Sun’s mass in last 4.6 billion years.

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Solar Cycle

• Increase in solar wind activity- Coronal Mass Ejections

• Increase in Auroral displays on Earth• Increase in disruptions on and around

Earth.

Courtesy of SOHO/LASCO/EIT consortium.

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Aurora

• The solar windpasses outthrough theSolar System.

• Consists of electrons, protons and other charged particles stripped from the Sun’s surface.

• Magnetic fields herd charged particles into atmosphere at poles.

• Charged particles excite electrons in atoms. Light!

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2003 CME

Credit: E. Woldt

Oklahoma 10/29/2003

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Homework #9

• Due Friday 24:• Read Bennett Ch9• Do Ch9 Quiz online