ASTR-1010
Planetary Astronomy
Day - 38
Course Announcements
Homework Chapter 11: Due Friday April 23.
Homework Chapter 12: Due Wednesday April 28.
Homework Chapter 21: Due Wednesday April 28.
-this is extra credit.
Exam 4 and “Final”: Friday, April 30 – 1030 am
Chapter 14
A Run-of-the-Mill G Dwarf: Our Sun
Forming a star & solar system
Except for
hydrogen &
helium, the
“stuff” of the
Solar System
is made in
stars
The sun
formed from
the
gravitational
collapse of a
giant cloud of
gas & dust
Our Star, the Sun
The Sun is the Largest Object in
the Solar System
• The Sun contains more than 99.85% of
the total mass of the solar system
• If you put all the planets in the solar
system, they would not fill up the volume
of the Sun
• 110 Earths or 10 Jupiters fit across the
diameter of the Sun
How big is the Sun?
Size of the Sun
• We’ve looked at the size of the Sun
compared to things we know within our
solar system. But, how does it stack up to
other stars?
• Star Sizes
• As big as our Sun is, it’s a rather small star.
The HR
Diagram
The Sun goes through periods
of relative activity and inactivity
Structure of the Sun
•
•
•
•
We only see the outer layers of the Sun.
Physics tells us about the interior.
Key idea: hydrostatic equilibrium.
At each point there’s a balance:
– Outward pressure = inward force of gravity.
– Rate of energy emitted = rate produced in the
core.
• Density, temperature, pressure increase
towards the center.
The Sun’s
interior has three
layers:
(1) core
(2) radiative zone
(3) convective
zone
Energy generated in the core of the Sun propagates outward
through these different layers, and finally, through the
atmosphere of the Sun. This process takes tens of thousands
of years or more.
Where does the Sun get its
energy?
To the ancients who believed the
Earth was the center of the
universe, the Sun was made of
quintessence, an element whose
property was to glow. The
concept of “energy” wasn’t even
invented until the late 1600’s.
By the 1700’s the best ideal for
the source of the Sun’s energy
was chemistry
If Sun was highest quality
chemical fuel it would exhaust
the fuel in less than 10,000
years!
Later ideas for source of Sun’s
energy: Gravitational Collapse
Kelvin-Helmholtz Contraction
Whenever anything shrinks it
heats up.
This could produce the
observed solar output for about
25 million years. This was the
original source of energy as the
Sun was forming.
In 1905 Einstein proposed
a new way to get energy:
from matter
The
answer
came
from his
famous
equation:
E = mc2
The Sun gets’ its energy from
matter – energy conversion via
Thermonuclear Fusion
The same source of energy as the
hydrogen bomb
Powering the Sun
• The Sun has been around a long time:
about 4.6 billion years.
• The Sun must therefore generate a lot of
energy over a long time.
• Source: fusion (joining) of hydrogen to
helium in the central core.
• Fusion is often called hydrogen burning.
• This happens for all main sequence stars.
Overall Proton – Proton Cycle
4
1H

4He
+
+
2e
+ 2g + 2n
Releases 4.3x10-12 Joules per helium atom produced
The Sun converts 600,000,000 tonnes of H into 596,000,000 tonnes of
He every second! The difference in mass is the energy produced
according to E = mc2. This is only a 0.67% efficient conversion!
The Sun has enough hydrogen in its’ core to last another 5 billion years
before it runs out
Energy is only produced in the core region where the temperature and
pressure are high enough
Fusion and Energy
• Mass of He nucleus is smaller than that of
original four H nuclei.
• Difference in mass is called the mass
deficit.
• Relativity: mass and energy are
equivalent: E = mc2 or
m = E/c2
• Mass of He is smaller because binding
energy is released during fusion.
• Binding energy = (mass deficit)  c2.
Heat Transfer: Energy can
move by one of three methods
Conduction: atomic & molecular
vibrations in solids.
Example…cast iron skillet
Convection: large scale motions in
liquids and gasses
Example…boiling water
Radiation: electromagnetic radiation
Example…heat lamp
Energy Transport
• Mechanisms of moving energy:
– Radiative transfer (photons).
– Convection (rising/falling of hot/cool gas).
• Inner part of the Sun: radiative zone.
• Outer part: convective zone.
• Surface: radiation emitted into space.
• Energy from the core takes 105 years to
reach the surface.
Concept Quiz – Fusion
Where does nuclear fusion take place in the Sun?
A. In the core.
B. In the radiative zone.
C. In the convective zone.
D. In the corona.
The Solar Interior
How do we know about the
interior of the sun?
GONG
Solar Surface Oscillations
Different ways sound
bounces around inside sun
Internal Structure from
Surface Waves
Solar Sound Waves
• Sound waves move through the Sun.
• This makes surface and interior waves.
• Doppler shifts give the speed of wave
motion.
• Speeds depend on the Sun’s composition
and the depth of the convection zone.
• Observations precisely agree with models.
• This is called helioseismology.
Internal Differential Rotation
Spectrum of the Sun
Nigel Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
The Sun’s atmosphere also has
three layers…
• Photosphere - the layer we see, 5800 K
• Chromosphere - the red layer observed
using a hydrogen filter, 10,000 K
• Corona - the incredibly thin outer
atmosphere, 1,000,000 K
Structure of the Atmosphere
The Photosphere
The “surface”
is a layer of
gas that is
only about
four hundred
kilometers
thick.
The Photosphere
• Photosphere: layer where light is emitted.
• Average temperature 5770 K.
• Temperature declines outward in the
photosphere, increases inward.
• This produces a complex absorption
spectrum.
• Cooler outer layers absorb some of the light
from hotter, deeper layers.
Solar Granulation
Each granule is ~1000
km across and lasts a
minute or two
Granulation Cells are
Convection Cells
Sunspots
NASA/SOHO/ESA
Note the decrease in
temperature in the photosphere
The “cooler” temperature at the top of
the photosphere is where all the
absorption takes place in the solar
spectrum
Colored Card Question
At the photosphere, the dominant mechanism
for heat movement is
A) Radiation changing over to conduction.
B) Radiation only.
C) Convection changing over to radiation.
D) Convection only.
E) Conduction only
Another Colored Card Question
The photosphere of the Sun is like
A) The surface of the Earth, you could stand on it if you could survive the
heat.
B) The surface of the ocean, you couldn’t stand on it but can clearly
detect differences above and below it
C) An apparent surface, you would notice very little change as you go
through it, like flying through a cloud.
D) The surface of a trampoline, you could land on it but the intense
pressure would push you away again.
Magnetic Fields
These are involved in many aspects of the
active Sun.
• The solar wind: charged particles flowing
away from the Sun.
• Coronal loops: rising and falling coronal gas.
• Prominences: hot rising gas in the
chromosphere.
• Sunspots: cooler areas in the photosphere.
• Structure: dark umbra with surrounding
penumbra.
The Active Sun
 NASA/Science Photo Library/Photo Researchers
 NASA/SOHO/ESA
Sunspots are the most well known feature in
the photosphere. Monitoring sunspots
reveals the Sun’s rotation.
Concept Quiz  Sunspots
Sunspots have temperatures about of 4000 K.
Why do sunspots appear to be dark?
A. They don’t emit any light.
B. They emit light, but mainly in the
ultraviolet.
C. They emit less energy per square meter
than the photosphere (Stefan’s law).
D. Light can’t pass through magnetic fields.
The
movement
of sunspots
reveals that
the Sun’s
rotation
takes about
…
4 weeks
The annual change in numbers of sunspots reveals that
the Sun experiences an 11-year Sun Spot cycle
Maximum
number
Minimum
number
Sunspot Cycles
• Sun shows an 11-year sunspot cycle.
• Solar maximum: the most sunspots, other activity.
• In alternate cycles, the direction of the Sun’s
magnetic field flips.
• The Sun is slightly (0.07%) brighter at solar
maximum.
• Previous low sunspot numbers (the Maunder
minimum) may have been connected with cooler
Earth weather.
Solar Luminosity and Sunspots
Solar magnetic fields also create
other atmospheric phenomena
• prominences
Solar magnetic fields also create
other atmospheric phenomena
• prominences
• solar flares
Solar magnetic fields also create
other atmospheric phenomena
• prominences
• solar flares
• coronal
mass
ejections
(CMEs)
http://www.spaceweather.com/
http://www.spaceweather.com/images2002/18mar02/cme_c3_big.gif
The most powerful solar flare in 14 years, ..
erupted from sunspot 486 in late October of
2003. The explosion hurled a coronal mass
ejection almost directly toward Earth, which
triggered bright auroras when it arrived on
Earth.
Outer Atmosphere
Chromosphere:
• Located above the photosphere.
• Higher temperature than the photosphere.
• Gives off an emission-line spectrum.
Corona:
• Above the chromosphere.
• Very hot: T = 1 to 2 million K.
• Emits X-rays.
The Chromosphere
Spicules
The
temperature in
the
chromosphere
climbs slowly
but then jumps
up in the
corona
The corona, the outermost part of the
Sun’s atmosphere, is characterized by
its high temperature and low density
The Sun also
ejects a stream
of charged
particles into
space known
as the solar
wind
The corona is heated by
coronal loops
Note the size of Earth for scale
Concept Quiz – Regions of the Sun
From hottest to coolest, the layers in the
Sun are:
A. Corona, photosphere, chromosphere.
B. Corona, chromosphere, photosphere.
C. Photosphere, chromosphere, corona.
D. Photosphere, corona, chromosphere.
Colored Card Question
The temperature from the photosphere up
A) Decreases uniformly as you get higher
above the photosphere.
B) Increases uniformly as you get higher
above the photosphere.
C) Increases slowly in the chromosphere
then jumps rapidly in the corona.
D) Decreases slowly in the chromosphere
then jumps rapidly in the corona.
Another colored card question
The corona is a
A) uniform spherical shape like the
photosphere.
B) wavy surface like giant tsunami waves
rolling around an otherwise spherical
surface
C) highly irregular shape due to the solar
wind and magnetic field of the sun
D) shaped like the field around a bar magnet
The Earth’s magnetic field produces a
magnetosphere that deflects and traps
particles from the solar wind protecting Earth
Relevance of Earth’s protective
magnetosphere
• Protects against Solar Flares - violent
explosions on the Sun releasing large burst of
charged particles into the solar system
• Protects against Solar Wind - dangerous
stream of charged particles constantly
coming from the Sun
• Northern Lights (Aurora Borealis)
Northern Lights (Aurora Borealis)
As the charged particles from the Sun interact
with the magnetic field around Earth, the
particles collide with the nitrogen and oxygen
atoms in the atmosphere and excite those
atoms to emit light.