Astronomy 1 – Fall 2014
Lecture 9, November 4, 2014
Reminder: You Are Invited!
•
Astro 1 Observing Session (Optional, Just for Fun)
• WHEN: November 13th at 7-9pm
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•
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Same time Nov. 20th if the 13th is cloudy
WHERE: Broida rooftoop; take elevator to 6th floor. Turn
right as you exit the elevator. Go up the stairs. A TA will
great you.
WHAT: 3 Celestron C8 Schmidt-Cassegrain telescopes and
one C11.
•
•
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Andromeda Galaxy, Uranus, Mars, Crab Nebula, Pleides, etc.
Help identifying the Celestial Equator, NCP (& Polaris), and the
Ecliptic
Constellations: Pegasus, Taurus, Summer Triangle, Orion, etc.
• Will you be able to see the moon?
Announcement
• Guest Lecturer this Thursday 11/06/14
– Dr. Tucker Jones (SoCal Center for Galaxy
Evolution Studies Fellow)
– Professor Martin will be observing with the Keck I
telescope on Mauna Kea. Wish her clear skies and
a working laser guide star!
Previously on Astro-1
• What is the Moon made of?
• How did the Moon form?
• Tidal forces
• What they are
• Affect on Earch
• Affect on Moon
Today on Astro-1
• The Sun: From its inner depths to the Earth
– What holds the Sun up?
• Internal structure
– What is the Sun made of?
– Why does the Sun shine?
• How long will it shine?
• What does it produce?
• What is the solar neutrino problem?
– Does the Sun have a surface?
• Sunspots and the sun cycle
• The corona
• The solar wind
The Sun is Heavy! Why Doesn’t It Collapse?
The Earth is a Solid Made of Rock
It’s Size is Set by the Physical Size of Atoms.
The Sun is Made of Gas
… And the Earth’s Atmosphere Is Made of Gas
A Pressure Difference Holds
Up the Atmosphere
What Do You Feel When You
Dive Under Water?
Pressure Difference Balances Gravity
How Many Forces Act on the Fish?
A Pressure Gradient Holds the
Sun in Hydrostatic Equilibrium
Why is the Sun Hot?
The Sun is hot because …
(iClicker Question)
A. It is made of fire.
B. Nuclear reactions make it hot.
C. The average density is low, so the average
temperature must be high to produce enough
pressure to hold up its weight.
D. It is hot due to the contraction of the pre-solar
nebula.
E. Both C & D.
Model of the Sun
Note: We’ve said nothing about nuclear reactions!
Energy Conservation
• If we push on a gas to compress it, it will heat
up because we’ve done work on it.
• A gas must do work on its surroundings in
order to expand, so it will cool down.
• You can demonstrate this with a bicycle pump.
– The tube heats up as you compress the air.
– Press the valve on the inner tube with finger. As
the air expands, what do you feel?
Protostellar Contraction
Generated Heat
• Why did the solar nebula collapse?
–
–
–
–
Right, gravity!
Gravity beat the pressure gradient.
The cloud collapsed.
Gravity did work on the gas, so it heated up as it
collapsed.
– The pressure increased as the density and
temperature rose.
– Contraction stopped when the hydrostatic
equilibrium was reached.
The Sun is hot because …
A. It is made of fire.
B. Nuclear reactions make it hot.
C. The average density is low, so the average
temperature must be high to produce enough
pressure to hold up its weight.
D. It is hot due to the contraction of the pre-solar
nebula.
E. Both C & D.
How Long Will the Sun Shine?
A Paradox.
• In the mid-1800s, Lord Kelvin and Hermann von
Helmholtz calculated how long it would take the
Sun to lose the heat generated from its
contraction.
• The Sun loses its heat through radiation at its
surface.
– Time = (Work done by gravity) / (Solar Luminosity)
– Time = 25 million years
• Why is this timescale a problem?
Can chemical reactions fuel the Sun?
(iClicker Question)
Hint: Chemical reactions release about 10-19 J per atom.
And, the luminosity of the Sun is 3.9 X 1026 Watts.
A. Yes, the Sun has roughly 1057 H atoms to burn, so
this will take a really long time.
B. Yes, the Sun is roughly 2% carbon, so it burns like
coal for a very long time.
C. No, at a burning rate of 4 x 1045 atoms/s, all the
Sun’s mass would burn in a fraction of a second.
D. No, at a burning rate of 4 x 1045 atoms/s, all the
Sun’s mass would burn in just 10,000 years.
Can chemical reactions fuel the Sun?
(iClicker Question)
Hint: Chemical reactions release about 10-19 J per atom.
And, the luminosity of the Sun is 3.9 X 1026 Watts.
A. Yes, the Sun has roughly 1057 H atoms to burn, so
this will take a really long time.
B. Yes, the Sun is roughly 2% carbon, so it burns like
coal for a very long time.
C. No, at a burning rate of 4 x 1045 atoms/s, all the
Sun’s mass would burn in a fraction of a second.
D. No, at a burning rate of 4 x 1045 atoms/s, all the
Sun’s mass would burn in just 10,000 years.
What Keeps the Sun Hot Over
Billions of Years?
• Great mystery of the 19th century.
• Sun must be roughly 4.5 billion years old.
– Moon rocks from most heavily cratered regions are
4.5 billion years old.
– Most meteorites are 4.54 billion years old.
– Oldest rocks on earth are also close to 4.5 Gyr.
• In the early 20th century, ideas from relativity
and nuclear physics led to an understanding of
why the Sun can shine so long.
The Sun’s energy is produced by hydrogen fusion:
4 hydrogen nuclei  1 helium nucleus + energy
Potential Energy
How do Protons Fuse?
High Temperatures are Required to Overcome the
Repulsion of Two Positively Charged Particles.
Separation ~ 10-15 m
The Proton-Proton Chain
How do the electron and the neutrino differ?
A. There is no difference between them.
B. The neutrino has no charge, a much smaller mass
than the electron, and interacts weakly with matter.
C. The neutrino has no charge, a mass the same as the
electron, and interacts weakly with matter.
D. The neutrino has the same charge and mass as an
electron, and interacts weakly with matter.
E. The neutrino has no charge, a much smaller mass
than the electron, and interacts strongly with matter.
Q16.4
How do the electron and the neutrino differ?
A. There is no difference between them.
B. The neutrino has no charge, a much smaller mass
than the electron, and interacts weakly with matter.
C. The neutrino has no charge, a mass the same as the
electron, and interacts weakly with matter.
D. The neutrino has the same charge and mass as an
electron, and interacts weakly with matter.
E. The neutrino has no charge, a much smaller mass
than the electron, and interacts strongly with matter.
A16.4
Prediction: Solar Neutrinos
About 1014 neutrinos must pass through every square meter
of Earth each second
Detect the flashes of light they make when they interact with
a big tank of water deep underground.
Where Does the Sun’s Energy Come From?
(iClicker Question)
A. Nuclear fusion in the center of the Sun creates new
energy. Energy is not conserved.
B. Nuclear fusion converts mass into energy in the
center of the Sun. A 4He nucleus is a bit less
massive than four H nuclei.
C. Nuclear fusion converts energy into mass in the
center of the Sun. A 4He nucleus is a bit more
massive than four H nuclei.
D. Nuclear fission in the center of the Sun
E. Nuclear fission near the surface of the Sun.
Where Does the Sun’s Energy Come From?
(iClicker Question)
A. Nuclear fusion in the center of the Sun creates new
energy. Energy is not conserved.
B. Nuclear fusion converts mass into energy in the
center of the Sun. A 4He nucleus is a bit less
massive than four H nuclei.
C. Nuclear fusion converts energy into mass in the
center of the Sun. A 4He nucleus is a bit more
massive than four H nuclei.
D. Nuclear fission in the center of the Sun
E. Nuclear fission near the surface of the Sun.
Energy Production in the Sun
Thermonuclear reactions can only occur in the Sun’s core —
that’s the only place where pressures and temperatures are
high enough
Failed Stars
• A gas cloud less massive than the Solar Nebula
collapses until the density becomes so high that the
particles are touching. Then the particles hold up the
object in a manner similar to a solid, and no further
collapse occurs.
• The resulting object is hot. Its surface radiates like a
blackbody at 8000 to 1800 K.
• The central temperature can be as high as a million
degrees. Hot, but not hot enough to burn H by the
proton – proton chain.
• We call these objects Brown Dwarfs. They would
have become stars if they’d been slightly more
massive.
Does the Surface of the Sun Shine in Gamma Rays?
What wavelengths does the photosphere emit?
The energy produced in the central core of the Sun is
transported to the surface
A. by radiation in the layers just outside the central core and
by convection in the outer layers.
B. by convection in the layers just outside the central core and
by radiation in the outer layers.
C. by convection from just outside the central core all the way
to the surface.
D. by radiation from just outside the central core all the way
to the surface.
E. only by convection.
Q16.5
The energy produced in the central core of the Sun is
transported to the surface
A. by radiation in the layers just outside the central core and
by convection in the outer layers.
B. by convection in the layers just outside the central core and
by radiation in the outer layers.
C. by convection from just outside the central core all the way
to the surface.
D. by radiation from just outside the central core all the way
to the surface.
E. only by convection.
A16.5
Heat Transport in the Sun
It takes light about
200,000 years to
get from the core
to the surface (then
8 minutes to get to
us)!
Convection
transports heat in
the outer layers of
the Sun.
Granules are convection
cells about 1000 km
(600 mi) wide in the
Sun’s photosphere.
Inset: Rising hot gas
produces bright
granules. Cooler gas
sinks downward along
the boundaries between
granules; this gas glows
less brightly, giving the
boundaries their dark
appearance. This
convective motion
transports heat from the
Sun’s interior outward
to the solar atmosphere.
Scale of granules
Supergranules display
relatively little contrast
between their center
and edges, so they are
hard to observe in
ordinary images. But
they can be seen in a
false-color Doppler
image like this one.
Light from gas that is
approaching us (that is,
rising) is shifted
toward shorter
wavelengths, while
light from receding gas
(that is, descending) is
shifted toward longer
wavelengths
This series of photographs taken
in 1999 shows the rotation of the
Sun. By observing the same
group of sunspots from one day to
the next, Galileo found that the
Sun rotates once in about four
weeks. (The equatorial regions of
the Sun actually rotate somewhat
faster than the polar
regions.) Notice how the sunspot
group shown here changed its
shape.
Umbra
Penumbra
The Sun’s 22-year cycle is NOT uniform
Number of sunspots versus year, 1610-present
Maunder
minimum
The number of sunspots on the Sun varies with a period of about 11
years. The most recent sunspot maximum occurred in 2000, next
one will be in 2013 (predicted).
The surface (photo
sphere) of the Sun is
about 5800K, but the
corona above it is
about a million
degrees. How?
Variations in the Sun’s magnetic field drive
the activity on and above the solar surface
Rearrangements of
the magnetic field
cause eruptions and
flares
(an ultraviolet
movie)
Summary
• The Sun’s energy is produced by hydrogen fusion (E=mc2),
occuring at 107 K in the nucleus of the sun.
• The standard model of the Sun
–
–
–
–
–
–
hydrogen fusion within 0.25 solar radius.
a radiative zone extending to about 0.71 solar radius
opaque convective zone
Photosphere (5800 K blackbody)
Chromosphere (hotter)
Corona (very hot; powered by magnetic fields)
• Neutrino’s escape from the Sun’s core and reveal neutrino
oscillations
Summary
• The Sun’s cycle is 22 long: Its magnetic field increases,
decreases, and then increases again with the opposite
polarity, creating a 11 year cycle in Sun spot activity
• The magnetic-dynamo model suggests that many features of
the solar cycle are due to changes in the Sun’s magnetic
field. These changes are caused by convection and the
Sun’s differential rotation.
• A solar flare is a brief eruption of hot, ionized gases from a
sunspot group. A coronal mass ejection is a much larger
eruption that involves immense amounts of gas from the
corona.
Homework – Due 11/10/14
• On your own: answer all the review questions
in chapter 16
• To TAs: answer questions16.31, 16.32, 16.40,
16.48