Astronomy 111 – Lecture 18
Our Sun
An Overview
Basic Concepts
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Size and Composition of Sun
Age and Energy Problem
Models of the Sun
The Atmosphere of the Sun
The Future of the Sun
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Basic Sun Data
• Distance to Earth
– Mean Distance : 1 AU = 149,598,000 km (light travel
time to Earth = 8.32 min)
– Maximum : 152,000,000 km
– Minimum : 147,000,000 km
• Mean angular diameter : 32 arcmin
• Radius : 696,000 km = 109 Earth radii
• Mass : 1.9891 x 1030 kg = 3.33 x 105 Earth Masses
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Basic Sun Data
• Composition (by mass)
– 74% Hydrogen, 25% Helium, 1% other elements
• Composition (by number of atoms)
– 92.1% Hydrogen, 7.8% Helium, 0.1% other elements
• Mean Density : 1.4 g cm-3
• Mean Temperatures
– Surface 5800 K, Centre 15.5 Million K (!)
• Total Luminosity : 3.86 x 1026 W
(1 sec  enough energy for 1 million years for humankind)
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How old is the Sun ?
• Really problematic : Sun does not come
with a time-stamp, shows no wrinkles etc…
• Start with Earth & Solar System
– Date geological formations, rocks
• Various geochronometrical methods using
radioactive decay properties  >4.2 billion years
– Find oldest meteorites  4.5-4.7 billion years
• Conclusion : Sun at least that old !
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What fuel does the Sun use ?
• A truly gigantic energy crisis emerges:
– Sun is billions of years old.
– Life on Earth as well.
•  Sun must shine roughly like today for
already a long time.
• How does it create all the energy ?
• What keeps the Sun so hot ?
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The Solar Energy Source
• First guesses (early 18th century) :
– Chemical Processes (‘burning’ fuel)
• Maximum burning time : 10,000 years !
– Using gravitational energy (Kelvin-Helmholtz
mechanism, mid-1800s)
• Slow contraction of Sun releases energy, heats gas
up -> energy radiated into space
• Maximum contraction time : 25 million years !
• Only important at very early stage (birth of Sun)
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A new Energy Source
• Problem :
– Need more energy released per atom
• Solution :
– Albert Einstein :
E = mc2
• m = mass, c = speed of light, E = energy
• Mass and energy are equivalent.
• Why does that help ?
– Speed of light is a large number !
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Thermonuclear Fusion
• Turning mass into energy :
– Transforming Hydrogen into Helium
– Mass balance : 4 1H atoms 6.693 x 10-27 kg
1 4He atom 6.645 x 10-27 kg
-----------------------------------Mass lost : 0.048 x 10-27 kg
• Extremely efficient process :
– Fusing 1 kg of Hydrogen releases same energy as
burning 20,000 metric tons of coal !
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Thermonuclear Fusion
• How much hydrogen must be converted to
give solar luminosity ?
– 600 million metric tons per second !
– Sounds enormous, but…..
– Sun has enough fuel for at least 9 billion years
• Solution to energy crisis !
• Problem : How does fusion work in detail ?
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Thermonuclear Fusion
• ‘nuclear’ – regarding nuclei of atoms
• ‘fusion’ – putting together (NOT fission)
• ‘thermo’ – need enormous temperatures as
nuclei do not fuse easily due to electric
repulsion
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Thermonuclear Fusion
• Introducing the Proton-Proton Chain

p  p H  e  e (twice)
2
3
H  p He   (twice)
3
3
4
He  He  He  p  p
2
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neutrino
positron
photon
2H
3He
4He
3He
2H
positron
neutrino
photon
Models of the Sun
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Solar surface temperature ~ 5800 K
Temperature required for fusion ~ 10 million K
Fusion can only occur at core of the Sun !
Can we understand and describe the conditions
inside the Sun ?
• Use theoretical models !
– Start with well-known principles and laws of physics
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Models of the Sun
• Observational Fact 1 :
– The Sun does not change size over long periods
of time, keeps its shape quite well.
• Principle of Hydrostatic Equilibrium
– Pressure and Gravity maintain a balance.
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Hydrostatic Equilibrium
Gravity
Gas Pressure
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Models of the Sun
• Conclusions :
• Pressure increases
with depth
• Temperature
increases with depth
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Models of the Sun
• Observational Fact 2 :
– The Sun does not heat up or cool down over
long periods of time, keeps its temperature
quite well.
• Principle of Thermal Equilibrium
– Energy Generation and Energy Transport
maintain a balance.
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Models of the Sun
• Modes of Energy Transport :
– Heat conduction  very inefficient for gases
– Convection : ‘circulation of fluids’, upwelling
of hot gases
– Radiative diffusion : Photons carrying away
energy
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Convection
Hot blob rises
cooler
water
sinks
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Photon Random Walk
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Models of the Sun
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Models of the Sun
• Construct computer model to describe state
of solar material from core to atmosphere
• How can we test those models ?
• Do they describe the interior well ?
• How can we ‘see’ into the Sun ?
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Testing the Models
• Test 1 : Helioseismology
– Sun vibrates (‘rings like a bell’)
– Use waves to test interior structure
– Same Principle as Geologist/Geophysicists with
Earthquakes
– Nice analogy : Ripeness of melons
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Astronomers probe the solar interior using
the Sun’s own vibrations
• Helioseismology is
the study of how the
Sun vibrates
• These vibrations have
been used to infer
pressures, densities,
chemical
compositions, and
rotation rates within
the Sun
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Testing the Models
• Test 2 : Catching solar neutrinos
– By-products of fusion
– Unlike photons, neutrinos escape solar interior
very easily
– ‘Ghost-like’ particles : very, VERY hard to
detect
• 1014 solar neutrinos every second through 1m2 on
Earth
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Catching Neutrinos
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The Solar Atmosphere
• Core of Sun hidden because gases become
opaque
• Outermost layers show remarkable
structures (‘atmosphere’)
• Tiny and much less dense than interior
• Nonetheless most important for life on our
planet
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The Solar Atmosphere
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The Photosphere
•Almost all of
the visible light
emanates from
that small layer
of gas.
• Temperature
decreases
upwards in
photosphere.
• Sun darker at
the edge ?
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The Photosphere
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The Photosphere
• Cooler layers further out  Notice
absorption lines in spectrum !
• Cool ?
– Still about 4400 K at the upper edge of the
atmosphere.
– Comparison : Siberian winter night to hot
tropical day in Hawaii.
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The Photosphere
• Solar Granulation : Convection cells of gas
in photosphere
• 4 million granules cover the solar surface
• Each granule covers area ~ Texas &
Oklahama combined
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The Solar Chromosphere
• Above the
photosphere is a
layer of less dense
but higher
temperature gases
called the
chromosphere
• Spicules extend
upward from the
photosphere into the
chromosphere along
the boundaries of
supergranules
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The Solar Chromosphere
• Spectrum : Emission lines !
•  Temperature must rise again :
– Top of chromosphere : 25,000 K
• Approximately 300,000 spicules exist at
one time, each last about 15 minutes
• Covering ~ 1 % of solar surface
• Phenomenon related to Sun’s magnetic field
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The Solar Corona
• The outermost layer
of the solar
atmosphere, the
corona, is made of
very hightemperature gases at
extremely low density
• The solar corona
blends into the solar
wind at great
distances from the
Sun
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The corona ejects mass into space to form the solar wind
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Activity in the corona includes coronal mass ejections and coronal holes
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Sunspots are low-temperature regions in
the photosphere
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Sunspots are produced by a 22-year cycle
in the Sun’s magnetic field
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The magnetic-dynamo model suggests that many
features of the solar cycle are due to changes in
the Sun’s magnetic field
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The future of the Sun
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Equilibrium holds due to energy generation
What happens when Sun runs out of fuel ?
Drama at the End of the Solar Life
A story for another day….
When will that happen ?
– ~ 4.0 – 5.0 billion years from now
– Puuuh, we are safe !
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