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
diameter
– The Sun’s
mass
– The Sun’s surface temperature
– The Sun’s chemical composition
is about midway
is about midway
is about midway
is about midway
• More accurate descriptions
– The 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
– The 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 joules . sec-1
• For the Sun, this is 3.9 . 1026
watts
• 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 g
= 4.3 . 10-12 joules
Only ~ 0.7%
• 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 yet still gaseous
• 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 electron
Travel very slightly slower than light in vacuum
– Extremely little interaction with matter
• Neutrino detectors
– 3 detector types search for different phenomena
• Brookhaven National Lab
• GALLEX & SAGE
• Kamiokande
~ 35% of expected value
~ 55% of expected value
~ 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”
Coronation
– 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
– ~ 4,300 K
• Penumbra
– ~ 5,000 K
Cool central part
Appears red
~ 30% as much energy as the photosphere
Warm outer part
Appears orange
~ 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
2000
2014?
• Times of abundant sunspots
– Recently
1978
1989
– 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
• Plasma
Magnetic fields can split spectral lines
Magnitude depends on field strength
At least some atoms are ionized
Moving plasma generate magnetic fields
Plasma 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
one
polarity
– Leading sunspots in the S hemisphere have opposite polarity
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 its magnetic field
• Field lines 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
Important Concepts
•
– In the middle of mass & luminosity
– More massive than 95% of all stars
•
– The problem of the missing neutrinos
The Sun as an “average” star
The Sun’s energy source
• Solar interior models may be wrong
• Neutrino models may be wrong
•
– Three levels
– Chemical reactions: 10 thousand years
– Nuclear reactions: 10 billion years
• Photosphere
• Chromosphere
• Corona
• 4 H fuse to form 1 He atom + energy
• Einstein’s E = m c2 equation
• ~ 6.0 . 1011 kg . sec-2 of hydrogen
• Magnetic storms in the photosphere
• Follow a 22 year magnetic cycle
• Always use “hydrogen fusion”
– Magnetograms as observational tools
Models of the Sun’s interior
• Polarity in opposite hemispheres
• The magnetic-dynamo model
– Hydrostatic & thermal equilibrium
• No major changes in the Sun
– Three interior regions
• Core
• Radiative zone
• Convective zone
Radiative transfer
Radiative transfer
Convection
Continuous spectrum
Emission spectrum
Emission spectrum
– Sunspots
– Critical terminology
•
Observations of the Sun’s atmosphere
•
Other magnetic phenomena
– Filaments & prominences
– Coronal mass ejections (CME’s)
• Cause aurorae in Earth’s atmosphere