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The Sun – Our StarChapter 12

Most important, this chapter gives us our first detailed look at a star. The chapters that follow will discuss the many kinds of stars that fill the heavens, but this chapter shows us that each of them is both complex and beautiful; each is a sun.

Intro

I. The Solar AtmosphereA. Heat Flow in the SunB. The PhotosphereC. The ChromosphereD. The Solar CoronaE. Helioseismology

II. Solar ActivityA. Sunspots and Active RegionsB. The Sunspot CycleC. The Sun's Magnetic CycleD. Magnetic Cycles on Other StarsE. Chromospheric and Coronal ActivityF. The Solar Constant

Outline

III. Nuclear Fusion in the SunA. Nuclear Binding EnergyB. Hydrogen FusionC. The Solar Neutrino Problem

Outline (continued)

General Properties

• Average star

• Absolute visual magnitude = 4.83 (magnitude if it were at a distance of 32.6 light years)

• Central temperature = 15 million 0K

• 333,000 times Earth’s mass

• 109 times Earth’s diameter

• Consists entirely of gas (av. density = 1.4 g/cm3)

• Only appears so bright because it is so close.

• Spectral type G2

• Surface temperature = 5800 0K

Very Important Warning:

Never look directly at the sun through

a telescope or binoculars!!!

This can cause permanent eye damage – even blindness.

Use a projection technique or a special sun viewing filter.

The Solar Atmosphere

Hea

t F

low

Solar interior

Temp. incr. inward

Only visible during solar eclipses

Apparent surface of the sun

• Apparent surface layer of the sun

The Photosphere

The solar corona

• Depth ≈ 500 km• Temperature ≈ 5800 oK• Highly opaque (H- ions)• Absorbs and re-emits radiation produced in the solar interior

Energy Transport in the PhotosphereEnergy generated in the sun’s center must be transported outward.

In the photosphere, this happens through

Convection:

Bubbles of hot gas rising up

Cool gas sinking down

≈ 1000 kmBubbles last for ≈ 10 – 20 min.

Granulation

… is the visible consequence of convection

The Chromosphere

Chromospheric structures visible in Ha emission (filtergram)

• Region of sun’s atmosphere just above the photosphere.

• Visible, UV, and X-ray lines from highly ionized gases

• Temperature increases gradually from ≈ 4500 oK to ≈ 10,000 oK, then jumps to ≈ 1 million oK

Transition region

Filaments

The Chromosphere (2)

Spicules: Filaments of cooler gas from the photosphere, rising up into the chromosphere.

Visible in Ha emission.

Each one lasting about 5 – 15 min.

The Layers of the Solar Atmosphere

Visible

Photosphere

Ultraviolet

Chromosphere

Coronal activity, seen in visible

light

Corona

Sun Spot Regions

The Magnetic Carpet of the Corona• Corona contains very low-density, very hot (1 million oK) gas

• Coronal gas is heated through motions of magnetic fields anchored in the photosphere below (“magnetic carpet”)

Computer model of the magnetic carpet

The Solar Wind

Constant flow of particles from the sun.Velocity ≈ 300 – 800 km/s

Þ Sun is constantly losing mass:

107 tons/year

(≈ 10-14 of its mass per year)

Sun Spots

Cooler regions of the photosphere (T ≈ 4240 K).

Only appear dark against the bright sun. Would still be

brighter than the full moon when placed on the night sky!

Sun Spots (2)

Active Regions

Visible

Ultraviolet

Face of the Sun

Solar Activity, seen in soft X-rays

Magnetic Fields in Sun SpotsMagnetic fields on the photosphere can be measured through the Zeeman effect

Sun Spots are related to magnetic activity on the photosphere

Sun Spots (3)Magnetic field in sun spots is about 1000 times stronger than average.

In sun spots, magnetic field lines emerge out of the photosphere.

Magnetic North Poles

Magnetic South Poles

Magnetic Field Lines

Magnetic North Pole

Magnetic South Pole

Magnetic Field Lines

The Solar Cycle

11-year cycle

Reversal of magnetic polarity

After 11 years, North/South order of leading/trailing sun spots is reversed

=> Total solar cycle = 22 years

The Solar Cycle (2)Maunder Butterfly Diagram

Sun spot cycle starts out with spots at higher latitudes on the sun

Evolve to lower latitudes (towards the equator) throughout the cycle.

The Sun’s Magnetic Dynamo

This differential rotation might be responsible for magnetic activity of the sun.

The sun rotates faster at the equator than near the poles.

Magnetic Loops

Magnetic field lines

The Sun’s Magnetic Cycle

After 11 years, the magnetic field pattern becomes so

complex that the field structure is re-arranged.

New magnetic field structure is similar to the

original one, but reversed!

New 11-year cycle starts with reversed magnetic-field

orientation

The Maunder Minimum

Historical data indicate a very quiet phase of the sun, ~ 1650 – 1700: The Maunder Minimum

The sun spot number also fluctuates on much longer time scales:

Magnetic Cycles on Other Stars

H and K line emission of

ionized Calcium indicate magnetic

activity also on other stars.

Prominences

Looped Prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops

Relatively cool gas (60,000 – 80,000 oK)

May be seen as dark filaments against the bright background of

the photosphere

Eruptive Prominences

(Ultraviolet images)

Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis).

Space Weather

Solar Aurora

Sound waves

produced by a

solar flare

~ 5

min

utes

Coronal mass ejections

Coronal Holes

X-ray images of the sun reveal coronal holes.

These arise at the foot points of open field lines and are the origin of the solar wind.

Energy ProductionEnergy generation in the sun

(and all other stars):

Nuclear Fusion

= fusing together 2 or more lighter nuclei to produce heavier ones.

Nuclear fusion can produce energy up to the production of iron;

For elements heavier than iron, energy is gained by nuclear fission.

Binding energy due to strong force = on short range, strongest of the 4 known forces: electromagnetic, weak, strong, gravitational

Energy Generation in the Sun: The Proton-Proton Chain

Basic reaction:

4 1H 4He + energy

4 protons have 0.048*10-27 kg (= 0.7 %) more mass than 4He.

Þ Energy gain = Dm*c2

= 0.43*10-11 J

per reaction.

Need large proton speed ( high temperature) to overcome

Coulomb barrier (electromagnetic repulsion between protons).

Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity.

T ≥ 107 0K = 10 million 0K

The Solar Neutrino ProblemThe solar interior can not be observed directly because it is highly opaque to radiation.

But neutrinos can penetrate huge amounts of material without being absorbed.

Davis solar neutrino experiment

Early solar neutrino experiments detected a much lower flux of neutrinos than expected ( the “solar neutrino problem”).Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem.

sunspotgranulationconvectionsupergranulelimblimb darkeningtransition regionfiltergramfilamentspiculecoronagraphmagnetic carpetsolar windhelioseismologyactive regionZeeman effectMaunder butterfly diagram

differential rotation

dynamo effectBabcock modelprominenceflarereconnectionauroracoronal holecoronal mass ejection (CME)

solar constantMaunder minimumweak forcestrong forcenuclear fissionnuclear fusionCoulomb barrierproton–proton chaindeuteriumneutrino

New Terms

1. What energy sources on Earth cannot be thought of as stored sunlight?

2. What would the spectrum of an auroral display look like? Why?

3. What observations would you make if you were ordered to set up a system that could warn astronauts in orbit of dangerous solar flares? Such a warning system exists.

Discussion Questions

Quiz Questions

1. What effect does the formation of negative hydrogen ions in the Sun's photosphere have on solar observations?

a. We can view the Sun's interior through special filters set to the wavelength of the absorption lines created by such ions.b. Concentrations of such ions form sunspots that allow us to track solar rotation.c. It divides the Sun's atmosphere into three distinct, easily observable layers.d. The extra electron absorbs different wavelength photons, making the photosphere opaque.e. These ions produce the "diamond ring" effect that is seen during total solar eclipses.

Quiz Questions

2. What evidence do we have that the granulation seen on the Sun's surface is caused by convection?

a. The bright centers of granules are cooler than their dark boundaries.b. The bright centers of granules are hotter than their dark boundaries.c. Doppler measurements indicate that the centers are rising and edges are sinking.d. Both a and c above. e. Both b and c above.

Quiz Questions

3. Which layer of the Sun's atmosphere contains the cooler low density gas responsible for absorption lines in the Sun's spectrum?

a. The photosphere.b. The chromosphere.c. The corona.d. The solar wind.e. All of the above.

Quiz Questions

4. Which of the following is true about granules and supergranules?

a. They are both about the same size.b. Granules and supergranules each fade in about 10 to 20 minutes.c. They are both due to convection cells in layers below.d. Both a and c above.e. Both b and c above.

Quiz Questions

5. What is revealed by observing the Sun at a very narrow range of wavelengths within the 656-nanometer hydrogen alpha line?

a. The structure of the photosphere.b. The structure of the chromosphere.c. The structure of the corona.d. We can see the electrons make the transition from energy level 3 to level 2.e. Nothing is seen; all light is absorbed at this wavelength.

Quiz Questions

6. What are the general trends in temperature and density from the photosphere to the chromosphere to the corona?

a. The temperature increases and density decreases.b. The temperature increases and density increases.c. The temperature decreases and density decreases.d. The temperature decreases and density increases.e. The temperature and density remain constant.

Quiz Questions

7. What physical property of the Sun is responsible for "limb darkening"?

a. The chromosphere is hotter than the photosphere.b. The chromosphere is cooler than the photosphere.c. The lower photosphere is cooler than the upper photosphere.d. The lower photosphere is hotter than the upper photosphere.e. Both a and d above.

Quiz Questions

8. The spectrum of the corona has bright spectral lines of highly ionized elements. What does this reveal?

a. The corona is a very hot, high density gas.b. The corona is a very hot, low density gas.c. The corona is very irregular in shape.d. The corona extends out to 20 solar radii.e. Both b and d above.

Quiz Questions

9. What heats the chromosphere and corona to high temperatures?

a. Long-wavelength electromagnetic radiation emitted by layers below.b. Visible light emitted by layers below.c. Short-wavelength electromagnetic radiation emitted by layers below.d. Sungrazing comets, giving up their energy of motion as they vaporize in these two layers.e. Fluctuating magnetic fields from below that transport energy outward.

Quiz Questions

10. How are astronomers able to explore the layers of the Sun below the photosphere?

a. Short-wavelength radar pulses penetrate the photosphere and rebound from deeper layers within the Sun.b. Long-wavelength radar pulses penetrate the photosphere and rebound from deeper layers within the Sun.c. Highly reflective space probes have plunged below the photosphere and sampled the Sun's interior.d. By measuring and modeling the modes of vibration of the Sun's surface.e. By observing solar X-rays and gamma rays with space telescopes. These shorter wavelengths are emitted from hotter regions below the photosphere.

Quiz Questions

11. What is responsible for the Sun's surface and atmospheric activity?

a. The Sun's magnetic field.b. Many comets impacting the Sun.c. Gravitational contraction of the Sun.d. The Sun sweeping up interstellar space debris.e. Gravitational interactions between the Sun and the planets.

Quiz Questions

12. What is the source of the Sun's changing magnetic field?

a. The differential rotation of the Sun.b. Convection beneath the photosphere.c. The Sun's large iron core.d. Both a and b above.e. Both a and c above.

Quiz Questions

13. What evidence do we have that sunspots are magnetic?

a. The spectral lines of sunspots are split by the Zeeman Effect.b. Observations show that the north pole and south pole sunspots attract one another and move closer together over time.c. Observations at far ultraviolet show material arched above the Sun's surface from one sunspot to another.d. Both a and b above.e. Both a and c above.

Quiz Questions

14. Which active feature in the Sun's atmosphere, seen from a different point of view, corresponds precisely to the dark filaments that are observed with an hydrogen alpha filter?

a. A sunspot.b. A solar flare.c. A prominence.d. A spicule.e. A coronal hole.

Quiz Questions

15. What does a Maunder Butterfly Diagram show?

a. During the 11-year sunspot cycle, the spots begin at high latitude and then form progressively closer to the equator.b. Between the years 1645 and 1715 the low activity on the Sun correlates with the Little Ice Age.c. The Sun's magnetic field is simple at the beginning of a sunspot cycle and grows progressively more complex due to differential rotation.d. Planetary nebulae do not all have spherical symmetry.e. When a butterfly flaps its wings in Brazil it affects the climate worldwide.

Quiz Questions

16. How constant is the solar constant; that is, by how much has the solar constant of 1360 joules per square meter per second been observed to vary over a few years?

a. About 20%.b. About 10%.c. About 5%.d. About 1%.e. About 0.1%.

Quiz Questions

17. How does the Sun maintain its energy output?

a. Gravitational contraction. b. Fusion of hydrogen nuclei.c. The impact of small meteoroids.d. Coal burning in pure oxygen.e. Fission of Uranium 235.

Quiz Questions

18. Why does nuclear fusion require high temperatures?

a. Protons have positive charge, and like charges repel one another.b. Two protons must get close enough together to overcome the Coulomb barrier.c. Two protons must get close enough for the strong force to bind them together.d. Both a and b above.e. All of the above.

Quiz Questions

19. What happens to the neutrinos that are produced in the proton-proton chain?

a. They collide immediately with other particles, thus adding to the gas pressure that supports the Sun against gravitational contraction.b. They combine with antineutrinos and form a pair of gamma rays.c. They head out of the Sun at nearly the speed of light.d. They are blocked by the Coulomb barrier and remain inside the Sun.e. They spiral out along magnetic field lines to become cosmic rays.

Quiz Questions

20. What solved the solar neutrino problem?

a. The discovery that neutrinos oscillate between three different types.b. The standard model of energy production within the Sun was modified.c. It was discovered that electron neutrinos do not penetrate rock as easily as expected.d. Some of the radioactive argon gas was found leaking out of the neutrino detector undetected.e. The finding that chlorine does not interact with electron neutrinos as predicted.

Answers

1. d2. e3. a4. c5. b6. a7. d8. b9. e10. d

11. a12. d13. e14. c15. a16. e17. b18. e19. c20. a