Download The Sun - Cornell Astronomy

Lecture #35: The Sun
The Main Point(s)
• The Sun:
– General Properties.
– The Sun's Influence on the Earth.
– Fusion: The Sun's Interior Engine.
– The Sun as a Star.
The Sun is a typical star that generates heat and light
via nuclear fusion reactions deep in its interior.
The Sun is, ultimately, the source of energy
responsible for most life on Earth.
• Reading: Chapter 14
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Basic Properties of the Sun
Overview
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• We live right next to a star!
– The Sun is a star up close.
– Other stars are Suns very far away.
• The Sun's properties are typical, but
the Sun's influence is extraordinary!
– The Sun is a "Main Sequence" star of average to smallish size.
• The Sun is basically a huge ball of "burning" hydrogen.
– Central pressure and temperature are so great
that Hydrogen atoms are converted to Helium
by nuclear fusion reactions (so it’s not really
burning like a fire…)
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Mass = 2x1030 kg (333,400 times Earth).
Radius = 695,000 km (109 times Earth).
Volume = 1.3 million Earths.
Rotation period at equator = 24.7 days.
Density = 1.41 g/cm3.
Solar Co mposition
(by mass)
Temperature of:
Hydrogen 70.0%
– “Surface” (photosphere) = 5800 K.
– Core = 15,000,000 K.
– Upper atmosphere (corona) > 1,000,000 K.
Luminosity = 3.8x1026 Watts.
– 1370 W/m2 at top of Earth's atmosphere.
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Helium
28.0%
Oxygen
Carbon
Neon
Iron
Silicon
Nitrogen
Magnesium
Sulfur
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The Sun as a Star
Observing the Sun
• The Sun appears to be a typical star.
• The largest stars are 50 times more
massive than the Sun and have
surface temperatures 10 times
higher.
• The smallest stars are 10 times less
massive than the Sun and have
surface temperatures 3 times lower.
• Many more details on stars in
Astronomy 101/103...
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• The Sun has been
extensively studied by many telescopes and spacecraft.
• Photosphere studied through filtered visible light.
• Solar eclipses provide short opportunities to study the
Sun's outer atmosphere (but rare!).
• Space missions enable discoveries:
– Can create “artificial eclipses”.
– UV and X-ray wavelengths are blocked
by Earth's atmosphere.
Hertzsprung-Russell diagram
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Solar Vocabulary
The Sun's Interior Engine
From
Inside to Out:
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Collapse of the solar nebula → central concentration of H, He.
As central pressure increased, central temperature increased...
Central pressure and temperature became so great that Hydrogen
atoms could be converted to Helium atoms by nuclear fusion reactions:
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Hydrogen is being "burned" into Helium deep inside the Sun's core!
How fast is the Hydrogen "fuel" being burned up?
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Wow... So when will the fuel run out?
Interior
– 4 H → He + Energy
Photosphere
– The luminosity of the Sun implies 600,000,000 tons of Hydrogen per second.
Chromosphere
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Transition
Region
Corona
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The Sun right now: http://sohowww.nascom.nasa.gov/
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The Sun has been burning Hydrogen for ~ 4.6 billion years.
At 600,000,000 tons per sec, the Sun has converted ~ 4% of its mass from H → He.
This is thought to be about 50% of the mass of H in the Sun's core.
So the Sun is roughly a "middle aged" star, with another ~5 b.y. of life expectancy.
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The Photosphere
Mass-Energy
conversion in
the Sun
• The photosphere is the
visible "surface" of the Sun:
– But this is a thick layer of gases, not a solid “surface”.
– We see just the uppermost layer of gas, ~400 km thick.
– Analogy: Visible clouds on the giant planets.
Mass of 4 protons > Mass of helium-4 nucleus
Δm = 4 • (1.6726×10 -27 kg) – 6.643×10-27 kg
= 6.690×10-27 kg – 6.643×10 -27 kg = 4.7×10-29 kg
• We can see features on the photosphere:
= 0.7% or 0.007 of the original mass.
Converted to Energy according to E=mc2
– Sunspots: Dynamic regions of lower temperature.
One kilogram of matter corresponds to
E = 0.007 kg × (3.108 m/s)2
• Provide a way to measure the Sun's rotation and solar cycles.
= 6.3×1014 J
– Granulation: Dynamic regions of roiling convection.
~ 1% of total electric consumption per day, USA
Sun’s energy output is 3.8×1026 J per second.
3.8 ⋅10 26
= 6.1011 kg = 600 million tons per second.
6.3 ⋅1014
• "Texas-sized" cells of rising and falling photosphere gases.
Requires
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Sunspots
Sunspots and Granules
The Chromosphere
Photospheric granulation
observed over 60 minutes at
Big Bear Solar Observatory.
Field of view 60”x45”.
Waves in the
penumbra of a
sunspot.
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• Next "atmospheric" layer
above the photosphere.
• It is a layer of hot,
transparent gases that was
discovered during eclipses.
• Strongest chromosphere
emission is from Hydrogen.
• Helium discovered in 1868
in chromosphere spectra.
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Solar Rotation
in extreme UV
(195 Å)
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The Corona
Solar Rotation
in X-rays
"Spicules" or
rising filaments
of hydrogen
(H-α light)
• A thin transition region occurs
just above the chromosphere;
T goes from 104 K to > 106 K!
• The corona is the outermost, lowest density,
and hottest layer of the Sun's atmosphere.
• Discovered and studied best during eclipses.
• The reason for the enormous temperatures in the
corona is not well understood, but may relate to
heating by the Sun's magnetic field lines.
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The Active Sun
Coronal Mass Ejections
• The Solar Wind
– The Sun's atmosphere produces a stream of
protons and electrons that flows outward from
the Sun at speeds of 400 km/sec to occasionally
more than 1000 km/sec!
• Prominences
– "clouds" of hot rising gases and flame-like
loops of gas within the Sun's atmosphere.
• Solar Flares
SOHO Spacecraft Data
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– Violent eruptions of material, releasing
enormous amounts of UV and X-ray radiation.
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Solar Flares
The Sun's Influence on the Earth
• The Sun provides the energy that ultimately drives
weather, climate, and life on our planet.
• The interaction of the solar wind and the Earth's
magnetic field produce auroras and also can wreak
havoc on power grids and orbiting satellites.
• Variations in the Sun's luminosity appear to be
correlated with changes in Earth's climate.
• Energetic events like flares can create "solar
storms" and endanger astronauts and space probes.
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H-alpha full disk observations
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Solar Variability and Climate
• The number of sunspots is correlated with solar luminosity.
• Small changes in the Sun's luminosity (< 1%) can have
profound effects on the Earth's temperature and climate.
• How variable is the Sun? What does the future hold?
• 11 year sunspot cycle and 22 year magnetic cycle known. Others?
• Can we separate solar effects from anthropogenic effects?
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11 year cycle
Number of Sunspots
Auroras are
caused by the
solar wind
spiraling in
along Earth's
magnetic field
"Little Ice Age"
"Maunder minimum"
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Year
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Next Lecture...
Summary
• The Sun is the nearest star to Earth.
• The Sun is mostly hydrogen and helium.
• At the enormous P,T of the Sun's core, hydrogen is
fused into helium (4 H → He + Energy).
• The Sun's visible "surface" is the photosphere.
• The Sun's outer atmospheric layers include the
chromosphere, transition region, and corona.
• The Sun is a typical "main sequence" star with 11
and 22 year cycles of activity.
• The Sun's energy fuels the climate and life on Earth.
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• Extrasolar Planets!
– Theory.
– Observations:
• Detection methods.
• Results to date...
– Implications for “Habitable Zones”
Zones”.
• Reading: Chapter 13
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