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26 Oct 2000 ASTR103, GMU, Dr. Correll 1
Ch 9--The SunCh 9--The Sun
26 Oct 2000 ASTR103, GMU, Dr. Correll 2
What do you think?What do you think?
• What fraction of the solar system’s mass is in the Sun?
• Does the Sun have a solid and liquid interior, like the Earth?
• What is the surface of the Sun like?• Does the Sun rotate?• What makes the Sun shine?
26 Oct 2000 ASTR103, GMU, Dr. Correll 3
Sun’s Vital StatisticsSun’s Vital Statistics
26 Oct 2000 ASTR103, GMU, Dr. Correll 4
Solar StructureSolar Structure
• Atmosphere– Photosphere ~ 500 km
thick--main light emitting surface
– Chromosphere ~ 2000 km or more--middle layer giving absorption bands
– Corona ~ 106 km or more--outer layer, very thin, very hot
26 Oct 2000 ASTR103, GMU, Dr. Correll 5
Photospheric SpectrumPhotospheric Spectrum
• Photospheric absorption spectrum - contains several tens of thousands of absorption lines– Strongest absorption lines due to Ca II– Hydrogen Balmer series fairly prominent– Most absorption lines due to Fe I and Fe II– About 70 of 92 naturally occurring elements observed in
photospheric spectrum– About 20 molecules observed in photospheric spectrum
• Chemical composition can be determined from spectrum and knowledge of temperature and density of photosphere
26 Oct 2000 ASTR103, GMU, Dr. Correll 6
Photospheric GranulationPhotospheric Granulation
• Photospheric granulation - “rice-grained” pattern of convection cells– Due to magnetic
field bundles
26 Oct 2000 ASTR103, GMU, Dr. Correll 7
ChromosphereChromosphere
• Chromospheric emission spectrum– Many emission lines matching wavelengths of photospheric
absorption lines– Bright yellow line produced by helium– Chromospheric temperature is up to 30,000 K at highest
level and gas density is lower than photosphere– From this, one concludes that temperature must rise rapidly
up through chromosphere
26 Oct 2000 ASTR103, GMU, Dr. Correll 8
Chromospheric SpiculesChromospheric Spicules
• Spicules - chromosphere stippled with jet-like spikes of gas– Best seen in red H
spectroheliogram– Dark features, like blades of
grass, outlining interiors of supergranule cells
• Reveals supergranules– spanned by hundreds of
granules
26 Oct 2000 ASTR103, GMU, Dr. Correll 9
CoronaCorona
• Corona - halo of pale white glowing gas extending several solar radii (several million kilometers) out from photosphere
• Creates a solar wind– millions of tons of H+,
e-, and He++ blown away each second
26 Oct 2000 ASTR103, GMU, Dr. Correll 10
Corona (cont)Corona (cont)
• Coronal spectrum– All emission lines originate in highly excited ions of
familiar elements• Temperatures in the millions of degrees Kelvin• Example - iron from which as many as 15 electrons
have been stripped in corona's extremely hot, tenuous gaseous
– Densities must be quite low compared to photosphere to produce emission spectrum from gas that is transparent to photospheric radiation
26 Oct 2000 ASTR103, GMU, Dr. Correll 11
Quiet and Active SunQuiet and Active Sun
• Quiet Sun - average solar conditions devoid of various phenomena which make up solar activity
• Active Sun - cycle of transient activity which includes variety of phenomena– Sunspot groups (chief identifier of 11 cycle of activity)– Flares– Filaments and prominences– Coronal holes, quiet regions, active regions
26 Oct 2000 ASTR103, GMU, Dr. Correll 12
The Sun’s Magnetic FieldThe Sun’s Magnetic Field
• Global structure throughout solar system
26 Oct 2000 ASTR103, GMU, Dr. Correll 13
The Sun’s Magnetic FieldThe Sun’s Magnetic Field
• Local variations throughout surface of sun
26 Oct 2000 ASTR103, GMU, Dr. Correll 14
The Sun’s Magnetic FieldThe Sun’s Magnetic Field
26 Oct 2000 ASTR103, GMU, Dr. Correll 15
Photospheric SunspotsPhotospheric Sunspots
• Sunspots - cooler structures than normal photosphere– Defined by
magnetic field
26 Oct 2000 ASTR103, GMU, Dr. Correll 16
Solar Activity CycleSolar Activity Cycle
• Sunspot cycle - more than century ago sunspots discovered to come and go in roughly 11 year cycle– Magnitude and period of cycle not exact– Polarity (north or south seeking) of magnetic field reverses
each cycle• 22-year magnetic cycle far more repeatable than 11-year
frequency cycle
26 Oct 2000 ASTR103, GMU, Dr. Correll 17
Sunspot Cycle - Frequency Sunspot Cycle - Frequency
26 Oct 2000 ASTR103, GMU, Dr. Correll 18
Chromospheric FlareChromospheric Flare
26 Oct 2000 ASTR103, GMU, Dr. Correll 19
Soft X-RaySoft X-RayJuly 7, 1998July 7, 1998
Coronal HoleCoronal Hole
Coronal Active Coronal Active RegionRegion
Coronal Quiet Coronal Quiet RegionRegion
EquatorEquator
PolePole
PolePole
26 Oct 2000 ASTR103, GMU, Dr. Correll 20
Coronal ProminencesCoronal Prominences
• Prominences - Chromospheric material extending upward into corona– Seen against
photospheric or chromospheric disk known as filaments
ProminenceProminence
26 Oct 2000 ASTR103, GMU, Dr. Correll 21
Coronal Prominences (cont)Coronal Prominences (cont)
• Properties– Much cooler than surrounding corona– Lifetimes up to 90 days
26 Oct 2000 ASTR103, GMU, Dr. Correll 22
ProminencesProminences
26 Oct 2000 ASTR103, GMU, Dr. Correll 23
The Sun’s InteriorThe Sun’s Interior
26 Oct 2000 ASTR103, GMU, Dr. Correll 24
• Thermonuclear fusion - fusion of small mass nuclei to form more massive nuclei with resulting direct conversion of mass into energy by E = mc2
• Nuclear forces (“strong” and “weak”) are much more energetic than chemical forces (“electromagnetic”)
Thermonuclear FusionThermonuclear Fusion
26 Oct 2000 ASTR103, GMU, Dr. Correll 25
Hydrogen “Burning”Hydrogen “Burning”
Proton-proton chain
26 Oct 2000 ASTR103, GMU, Dr. Correll 26
Stellar StructureStellar Structure
• Energy Transport– conduction--molecular
collisions– radiation--light waves– convection--fluid flow
26 Oct 2000 ASTR103, GMU, Dr. Correll 27
Stellar StructureStellar Structure
• Our Sun– energy generated by
fusion in the core– energy radiates outward
via radiation through the hot interior
– cooler outer layer is “opaque” to the radiation and absorbs the energy
– convection transfers energy outwards to photosphere
26 Oct 2000 ASTR103, GMU, Dr. Correll 28
Stellar StructureStellar Structure
• Hydrostatic Equilibrium--balance of gravitational attraction and pressure at all layers of the star
26 Oct 2000 ASTR103, GMU, Dr. Correll 29
Stellar StructureStellar Structure
• Equations of stellar structure are solved on computers to give description of stellar interior and energy radiated out into space
26 Oct 2000 ASTR103, GMU, Dr. Correll 30
Oscillatory MotionsOscillatory Motions
• 1960, vertical oscillatory motions detected in and above solar granulation via Doppler shifted spectra– Typical excursion is on order of 50 to 100 km– Period of oscillations almost exactly 5 minutes with
velocities of about 0.5 km/s– 1984, Sun's closest stellar neighbor, Alpha
Centauri, was also shown to be undergoing 5-minute oscillations
26 Oct 2000 ASTR103, GMU, Dr. Correll 31
Oscillatory Motions (cont)Oscillatory Motions (cont)
Helioseismology
26 Oct 2000 ASTR103, GMU, Dr. Correll 32
Source of Stellar EnergySource of Stellar Energy
• Theory & Observations of the sun– Luminosity is known– Nuclear physics well
understood– Predicted central
temperature of sun agrees with helioseismology
– But, neutrino flux about 1/3 too low
C2Cl4 (cleaning fluid) tank in South Dakota
26 Oct 2000 ASTR103, GMU, Dr. Correll 33
Source of Stellar EnergySource of Stellar Energy
• Superkamiokande--detected different species of massive neutrinos– Neutrinos may change type travelling between Sun and Earth
26 Oct 2000 ASTR103, GMU, Dr. Correll 34
What do you think?What do you think?
• What fraction of the solar system’s mass is in the Sun?– 99.85%
• Does the Sun have a solid and liquid interior, like the Earth?– No, the Sun is composed entirely of hot gases
• What is the surface of the Sun like?– The photosphere is composed of hot, churning gases
• Does the Sun rotate?– The Sun’s surface rotates differentially, varying between once
every 25 days near its poles and once every 35 days at its equator
• What makes the Sun shine?– Thermonuclear fusion at the Sun’s core is the source of the Sun’s
energy
26 Oct 2000 ASTR103, GMU, Dr. Correll 35
Questions for ThoughtQuestions for Thought
• Describe the interior layers (structure) of the Sun, how energy is generated, and how the energy makes its way to the outer layers of the Sun to be radiated into space