Transcript the Sun - University of Redlands

THE SUN
The star we see by day
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Goals
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Summarize the overall properties of the Sun.
What are the different parts of the Sun?
Where does the light we see come from?
The scientific method: solar neutrinos.
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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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Vital Statistics
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Radius = 100 x Earth (696,000 km)
Mass = 300,000 x Earth (1.99 x 1030 kg)
Surface temp = 5,800 K
Core temp = 15,000,000 K
Luminosity = 4 x 1026 Watts
Solar “Day” =
– 24.9 Earth days (equator)
– 29.8 Earth days (poles)
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Structure
• ‘Surface’
– Photosphere
• ‘Atmosphere’
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Chromosphere
Transistion zone
Corona
Solar wind
• ‘Interior’
– Convection zone
– Radiation zone
– Core
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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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Do you see the light?
• Everything in the solar system reflects light.
• Everything also absorbs light and heats up
producing blackbody radiation.
• Q: Where does this light come from?
• A: The Sun.
• But where does the Sun’s light come from?
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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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Nuclear Fusion
• 4H  He
• The mass of 4 H nuclei (4 protons):
4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg
• The mass of He nuclei: = 6.643 x 10-27 kg
• Where does the extra 4.7 x 10-29 kg go?
• ENERGY!  E = mc2
• E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2
• E = hc/l  l = 4.6 x 10-14 m (gamma rays)
• So: 4H  He + light!
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The Radiation Zone
• This region is transparent to light.
• Why?
– At the temperatures near the core all atoms are ionized.
– Electrons float freely from nuclei
– If light wave hits atom, no electron to absorb it.
• So: Light and atoms don’t interact.
• Energy is passed from core, through this region,
and towards surface by radiation.
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The Convection Zone
• This region is totally opaque to light.
• Why?
– Closer to surface, the temperature is cooler.
– Atoms are no longer ionized.
– Electrons around nuclei can absorb light from below.
• No light from core ever reaches the surface!
• But where does the energy in the light go?
• Energy instead makes it to the surface by
convection.
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Convection
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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 blackbody radiation
we see.
• Why?
– At the photosphere, the density is so low that the gas is
again transparent to light.
– The hot convection cell tops radiate energy as a function of
their temperature (5800 K).
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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The Solar
Atmosphere
HOT
You
COOL
• Above the photosphere:
– Thin cool gas
– Mostly transparent to light again
• Unlike radiative zone, here atoms not totally
ionized.
• Therefore, there are electrons in atoms able to
absorb light.
• Absorption lines in solar spectrum are from these
layers in the atmosphere.
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The Chromosphere
• Hydrogen most common.
• Brightest hydrogen line – Ha.
• Chromosphere = color
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Ha Sun
Photo by Big Bear Solar Observatory
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Prominences
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Corona
• Magnetic activity carry energy up to the Transition
Zone.
• 10,000 km above photosphere.
– Temperature climbs to 1,000,000 K
– Remember photosphere is only 5800 K
• The hot, low density, gas at this altitude emits the
radiation we see as the Corona.
– But corona very faint compared to photosphere.
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Solar Wind
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At and above the corona:
Gas is very hot
Very energetic
Like steam above our boiling pot of water, the gas
‘evaporates’.
• Wind passes out through Coronal Holes
• Solar Wind 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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Aurorae
• The solar wind
passes out
through the
Solar System.
• Consists of electrons, protons and other charged
particles stripped from the Sun’s surface.
• When charged particles and magnetic fields interact:
light!
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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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Magnetic fields and Sunspots
• At kinks, disruption in convection cells.
• Sunspots form.
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11-year sunspot cycle.
Center – Umbra: 4500 K
Edge – Penumbra: 5500 K
Photosphere: 5800 K
Sunspots
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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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Solar Neutrino Problem
• We observe:
– Sun’s luminosity (total light radiated).
• We hypothesize:
– 4H  He + light + neutrinos
• We can test:
– Observe number neutrinos reaching Earth
• Does or test agree with hypothesis?
• No
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What to Do?
• For 30 years:
– Theorists certain of nuclear reaction.
– Observers positive of observations.
– Detected only 1/3 the hypothesized neutrinos.
• What to do?
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Neutrino Flavors
• 3 types of neutrinos
– Electron neutrino
– Tau neutrino
– Muon neutrino
• Nuclear reactions produce
only electron neutrino.
• Previous detectors only
detected electron
neutrinos.
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Neutrino Fluctuations
• New detector (2002) gives number of all three
flavors.
• Total number agrees with number predicted in
core of Sun.
• Conclusion:
– Nuclear hypothesis is correct.
– Neutrinos change flavor.
– Neutrinos have mass (used to be thought massless).
• Problem solved  new science discovered.
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