16. Our Star, the Sun

16. Our Star, the Sun• Sun data• Hydrogen fusion produces the Sun’s energy• Sun’s energy moves from core to surface• Sunquakes give information about the interior• The problem of the missing solar neutrinos• Photosphere: The 1st atmospheric layer• Chromosphere: The 2nd atmospheric layer• Corona: The 3rd atmospheric layer• Sunspots are relatively cool magnetic storms• Sunspots exhibit a 22-year cycle• Other magnetic effects on the Sun

Sun Data (Table 16-1)

The Sun Is An “Average” Star• The usual descriptions

– The Sun’s diameteris about midway

– The Sun’s massis about midway

– The Sun’s surface temperatureis about midway

– The Sun’s chemical compositionis about midway

• More accurate descriptions– Sun is in the middle of possible star masses

• Least massive star can be ~ 0.08 x the Sun’s mass• Most massive star can be ~ 110 x the Sun’s mass

– Sun is in the middle of possible star luminosities• Least luminous star can be ~ 10-4 x the Sun’s luminosity• Most luminous star can be ~ 10 6 x the Sun’s luminosity

About 95% of all stars are less massive than the Sun

Some Important Concepts• Reflection vs. Emission

– Planets, asteroids & comets shine by reflecting light– Sun shines by emitting

light

• Luminosity– Total energy emitted per second

• For the Sun, this is 3.9 . 1026 watts• For the Sun, this is 3.9 . 1026 joules . sec-1

• The Sun’s spectrum– This is very nearly perfect blackbody radiation

Hydrogen Fusion Makes the Sun’s Energy• Early speculation about Sun’s energy source

– Chemical combustion• Each combusting atom releases ~ 10-19 joules• This would require ~ 3.9 . 1045 atoms . sec-1

– The Sun contains ~ 1057 atoms• Sun could produce chemical energy for ~ 3.0 . 1011 sec• Sun would burn itself out in only 10 thousand years

• Modern understanding of Sun’s energy source– Einstein’s Special Theory or Relativity

• e = m . c2

– c [speed of light] is a very large number, so c2 is huge• 4 hydrogen atoms fuse into 1 helium atom

– Mass lost per helium atom = 4.8 . 10-26 gOnly ~ 0.7%

= 4.3 . 10-12 joules• Sun converts ~ 6.0 . 1011 kg . sec-2 of H2 into He• Sun will exhaust core hydrogen after ~ 10 billion years

Critical Terminology• Misleading astronomical terminology

– Hydrogen burning• Misleading because it implies chemical combustion

• Precise scientific terminology– Hydrogen fusion

• Not misleading because it clearly indicates nuclear fusion

Thus:

Always use “hydrogen fusion”Never use “hydrogen burning”

Hydrogen Fusion: Deuterium Synthesis

Hydrogen Fusion: 3He Synthesis

Hydrogen Fusion: 4He Synthesis

Energy Moves from Core to Surface• Extremely high density ~ 1.6 . 105 kg . m-3

– ~ 14 times as dense as lead• Extremely high pressure ~ 3.4 . 1011 atmos.

– ~ 340 billion times as dense as Earth’s atmosphere• Extremely high temperature > 1.0 . 107 K• Hydrostatic equilibrium

– Long-term pressure stability inside the Sun– Downward force = Upward force

• Thermal equilibrium– Long-term temperature stability inside the Sun– Heat generation rate = Heat escape rate

• Heat travels from hot areas to cool areas

Hydrostatic Equilibrium In Water

Hydrostatic Equilibrium In the Sun

Generic Heat Transport Mechanisms• Conduction

– Energy transfer by contact between adjacent atoms• Atoms vibrate around an essentially fixed location• Relatively inefficient in the Sun because it is gaseous

• Convection– Energy transfer by circulation of atoms

• Atoms can move great distances• Relatively efficient where pressure is relatively low

• Radiative diffusion– Energy transfer by photon absorption & re-emission

• The Sun’s gases are dense enough to permit this• Relatively efficient where pressure is relatively high

Sun’s Heat Transport Mechanisms• Computer models of the Sun’s interior

– Different models use same physics equations– Different models use different assumptions

• All models produce nearly identical internal structures• Accepted model for the Sun’s heat transport

– Deepest regions• Radiative diffusion is dominant heat transport mechanism• The core

~ 25% the Sun’s radius– Intermediate regions

• Radiative diffusion is dominant heat transport mechanism• The radiative zone ~ 71% the Sun’s radius

– Shallowest regions• Convection is dominant heat transport

mechanism• The convective zone 100% the Sun’s radius

– ~ 170,000 years for a photon to escape the Sun

Sun’s Interior Physical Properties

The Sun’s Internal Structure

Seismic Waves Probe Sun’s Interior• The Sun vibrates at many frequencies

– Discovered by Robert Leighton of Cal Tech in 1960• Extremely high-precision Doppler shift analyses

– Many possibilities exist• Move ~ 10 meters every 5 minutes

~ 3.3 mm . sec-1

~ 0.003 hertz [cycles . sec-1]~ 13 octaves lower than humans can hear

• Longer periods from 20 to 160 minutes• The science of helioseismology

– Sunquakes comparable to earthquakes• Substantial evidence regarding the Sun’s interior

– Set limits on the amount of He in Sun’s core & convective zone– Estimate layer thickness between radiative & convective zones– Convective zone is thicker than computer models predict

The Vibrating Sun

The Missing Neutrino Problem• Hydrogen fusion releases abundant neutrinos

– Abundant energy~ 1.0 . 1038 neutrinos . sec-1 produced by solar H fusion~ 1.0 . 1014 neutrinos . sec-1 penetrate every m2 of Earth

– No electrical charge– Little or no mass

< 1.0 . 10-4 times the mass of an electronTravel very slightly slower than light in vacuum

– Extremely little interaction with matter• Neutrino detectors

– 3 detector types search for different phenomena• Brookhaven National Lab ~ 35% of expected

value• GALLEX & SAGE ~ 55% of expected

value• Kamiokande ~ 45% of expected

value

Interpretation of Neutrino Flux• Our models of the Sun’s interior are wrong

– The Sun’s core is cooler than predicted by models• 10% temperature reduction would account for neutrinos• 10% temperature reduction would reduce Sun’s diameter

• Our understanding of neutrinos is wrong– Neutrinos may behave in unpredicted ways

• There are actually three kinds of neutrinos– Only one kind of neutrino is produced in the Sun– Detectors are designed to detect only this kind of neutrino– If neutrinos change types, an answer might be at hand

• Super Kamiokande– Neutrino oscillation may take place– Super Kamiokande was severely damaged in 12 November 2001– Super Kamiokande was back in full operation in June 2006

Photosphere: 1st Atmospheric Level• The Sun has no solid surface

– Gas pressure decreases smoothly moving outward– A 400 km thick layer of the Sun is “visible”

• This is only 0.057% the Sun’s radius• This seems like a very well defined surface• Limb darkening results

– Density is ~ 10-4 times Earth’s atmospheric pressure• The photosphere approximates a blackbody

– Produces a continuous spectrum

Hot high-density• Temperature of ~ 5,800 K

– Produces an absorption spectrum

Cool low-density• Fraunhofer lines produced by coolest upper photosphere• Temperature of ~ 4,400 K

~ 24% drop– Not “cool” by Earth standards– Very “cool” by Sun standards

Fraunhofer Lines in the Solar Spectrum

Solar Photosphere: “Light” Sphere

The Quiet Sun: Granulation• Small-scale convection in the photosphere

– The convection is broken up into small cells– Granules average ~ 1,000 km in diameter

• The size of Texas + Oklahoma• Temp. drops ~ 300 K from center to edge of a granule• The center is ascending, the edge is descending

– Granules last only a few minutes– The Sun has ~ 4 million granules at any one time

Granules & Photosphere Convection

The Quiet Sun: Supergranules• Medium-scale convection in the photosphere

– Another scale of convection is superimposed– Supergranules are similar to granules– Supergranules average ~ 35,000 km in diameter– Supergranules last about one day

Solar Supergranule Convection

Chromosphere: 2nd Atmospheric Level• Chromosphere means “sphere of color”

– Invisible under ordinary viewing conditions– Density is ~ 10-8 x Earth’s atmospheric pressure

• Chromosphere is dominated by emission lines– Characteristic of hot low-density gases– The Ha line at 656.3 nm is very strong

• An electron falls from the n = 3 to the n = 2 energy level• Color is a very vibrant red• Color appears pink during a solar eclipse

– Gas density is very low, so there are very few atoms emitting• Ha filter makes chromosphere visible without an eclipse

– Chromosphere temperature increases with altitude• Lowest chromosphere level is ~ 4,400 K• Highest chromosphere level is ~ 25,000 K

Chromosphere: Color Emission

The Chromosphere’s Spicules• The chromosphere has many tall spikes

– Rapidly rising gas jets• ~ 20 km . sec-1

~ 45,000 mph

– Typical spicules last ~ 15 minutes– ~ 300,000 spicules exist at any one time

• They cover only ~ 1% of the Sun’s surface

– Typical spicules form at supergranule edges

Corona: 3rd Atmospheric Level• Corona means “crown”

– Invisible under ordinary viewing conditions• ~ 1.0 . 10-6 times as bright as the photosphere• This is the brightness of the full moon

– Visible during a total solar eclipse• The corona is dominated by emission lines

– Extremely unusual emission lines• Green line at 530.3 nm is from highly ionized iron

– 13 of 26 electrons have been stripped away– Temperature > 2.0 . 106 K

– Heated by magnetic energy from the photosphere

The Corona & the Solar Wind• The Sun’s atmosphere

– Retained by the Sun’s extremely strong gravity

• The Sun’s escaping matter– Temperatures are extremely high

• Very high speeds of atoms & molecules– ~ 1 million km . hr-1

• Statistically, some will exceed escape velocity– This is the solar wind

The Solar Corona: The “Crown”

Solar Upper Atmosphere Temperatures

An X-Ray View of the Sun

Sunspots Basics• The active Sun

– Several features that vary considerably over time• Sunspots

– These are one type of active Sun feature– Irregular dark regions imbedded in the photosphere– Typically several thousand kilometers in diameter– Typically last a few hours to a few months– Typical sunspots have two parts

• Umbra Cool central part Appears red– ~ 4,300 K ~ 30% as much energy as the

photosphere• Penumbra Warm outer part Appears

orange– ~ 5,000 K ~ 55% as much energy as the

photosphere

Sunspots Imaged Close-Up

A mature sunspot Overlapping sunspots

Sunspot Movement

The Sunspot Cycle: A First Look• Heinrich Schwabe 1843

– Had observed many years of sunspot activity– Sunspots vary cyclically in number

• Times of minimal sunspots– Recently 1976 1986 1996

2010• Times of abundant sunspots

– Recently 1978 1989 20002013?

– Sunspots vary cyclically in latitude• Times of minimal sunspots

– Sunspots first appear at ~ 30° latitude on the Sun• Times of abundant sunspots

– Sunspots migrate to < 5° latitude on the Sun

Sunspots: Cool Magnetic Storms• George Ellery Hale 1908

– Discovered that sunspots are magnetic storms– Basic scientific principles

• Zeeman effect Magnetic fields can split spectral lines

Magnitude depends on field strength• Plasma At least some atoms are ionized

Moving plasma generate magnetic fieldsPlasma can be deflected by the Sun

– Basic observations• Large sunspots have strong magnetic fields• Opposite sunspot polarities in opposite hemispheres

– Basic tool• Magnetograms show sunspot polarities

– Leading sunspots in the N hemisphere have onepolarity

– Leading sunspots in the S hemisphere have oppositepolarity

Sunspots: Strong Magnetic Fields

Mapping the Sun’s Magnetic Field

Sunspots Exhibit a 22-Year Cycle• The visual sunspot cycle

– Repeats on an approximately 11 year cycle• The magnetic sunspot cycle

– Repeats on an approximately 22 year cycle• Sun’s North Pole is magnetic north for 11 years• Sun’s North Pole is magnetic south for 11 years

• A proposed cause– The magnetic-dynamo model

• Sun’s differential axial rotation stretches magnetic field• They eventually stretch so far that they “snap”

• Some anomalies– Extremely few sunspots from 1645 to 1715

• Europe’s “Little Ice Age” & western U.S. severe drought– Excessive sunspots during 11th & 12th centuries

• Earth was warmer than it is today

The Sunspot Cycle

Babcock’s Magnetic Dynamo

Rotation Rates in the Solar Interior

Other Magnetic Effects on the Sun• Magnetic heating of the corona

– Twisting & short-circuiting magnetic loops• Other magnetic phenomena

– Plages• Chromosphere precursors of many sunspots

– Filaments• Relatively cool dark streaks in the chromosphere

– Prominences• Filaments seen against a dark background seem bright

– Flares• Eruptive events associated with major sunspots

– Coronal mass ejections (CME’s)• ~ 1.0 . 1012 kg of gas ejected into space at high speeds• Super-sized versions of solar flares• Cause aurorae in Earth’s atmosphere

Magnetic Heating of the Sun’s Corona

A Solar Prominence

A Coronal Mass Ejection (CME)

Before the CME Early stages 16 minutes after (b)

Coronal Mass Ejection Mechanism

• The Sun as an “average” star– In the middle of mass & luminosity– More massive than 95% of all stars

• The Sun’s energy source– Chemical reactions: 10

thousand years– Nuclear reactions: 10 billion

years• 4 H fuse to form 1 He atom + energy• Einstein’s E = m c2 equation• ~ 6.0 . 1011 kg . sec-2 of hydrogen

– Critical terminology• Always use “hydrogen fusion”

• Models of the Sun’s interior– Hydrostatic & thermal equilibrium

• No major changes in the Sun– Three interior regions

• CoreRadiative transfer

• Radiative zone Radiative transfer• Convective zone Convection

– The problem of the missing neutrinos• Solar interior models may be wrong• Neutrino models may be wrong

• Observations of the Sun’s atmosphere– Three levels

• PhotosphereContinuous spectrum

• ChromosphereEmission spectrum

• CoronaEmission spectrum

– Sunspots• Magnetic storms in the photosphere• Follow a 22 year magnetic cycle

– Magnetograms as observational tools• Polarity in opposite hemispheres• The magnetic-dynamo model

• Other magnetic phenomena– Filaments & prominences– Coronal mass ejections (CME’s)

• Cause aurorae in Earth’s atmosphere

Important Concepts