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Exam #2 Review
Looks “quiet in the visible”
• The Sun is a huge ball of
gas at the center of the
solar system
The Sun
– 100 times Earth diameter,
300,000 earth mass
– 1 million Earths would fit
inside it!
– Releases the equivalent
of 100 billion atomic
bombs every second!
– 1366 watts/square meter
at Earth
– 15 Tera watts in 62 sq mi
• Exists thanks to a
delicate balance of
gravity and pressure
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Knowledge of interior based on models which fit observables:
•Mass
•Radius
•Luminosity
•Surface Temperature
•Image details: granules, spicules, corona, chromosphere
The Photosphere
• The photosphere
is the visible
“surface” of our
star
– Not really a
surface, as the
Sun is gaseous
throughout
– Photosphere is
only 500 km thick
– Average
temperature is
5780 K
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Energy Transport in the Sun
• Just below the photosphere is the convection zone.
– Energy is transported from deeper in the Sun by convection, in
patterns similar to those found in a pot of boiling water (hot gas rises,
dumps its energy into the photosphere, and then sinks)
• Energy in the convection zone comes from the radiative zone.
– Energy from the core is transported outward by radiation
– Takes more than 100,000 years for a single photon to escape the
Sun!
The Solar Atmosphere
• Regions of the Sun above
the photosphere are called
the Sun’s atmosphere
• Just above the photosphere
lies the chromosphere
– Usually invisible, but can
be seen during eclipses
• Above the chromosphere is
the corona
– Extremely high
temperatures (more than
1 million K!)
– Rapidly expanding gas
forms the solar wind.
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The Ideal Gas Law
Pressure = Constant × Temperature × Density
The Sun’s Energy
• The Sun’s energy comes from fusion – the merging of
hydrogen nuclei into helium
• The reaction releases only a little bit of energy, but it
happens a lot!
• A hydrogen nucleus has less mass than the four protons
(hydrogen nuclei) that fuse
• This difference in mass is converted into energy:
E = m×c2
The Proton-Proton
Chain
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Sunspots
• Sunspots are highly
localized cool regions in
the photosphere of the
Sun
– Discovered by Galileo
– Can be many times
larger than the Earth!
– They contain intense
magnetic fields.
Solar Flares
Coronal Mass Ejection
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• When CME material
reaches the Earth, it
gets funneled by
Earth’s magnetic field
and collides with
ionospheric particles,
close to the poles
• The collision excites
ionospheric oxygen,
nitrogenand which
causes it to emit a
photon
• We see these emitted
photons as the
aurora, or Northern
Lights
The Aurora
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Measuring the
Distance to
Astronomical
Objects using
parallax
Just a little
Trigonometry…
1 parsec = 206265 AU
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Moving Stars
• The positions of stars are not
fixed relative to Earth
– They move around the center of
the galaxy, just as Earth does.
– This motion of stars through the
sky (independent of the Earth’s
rotation or orbit) is called proper
motion
– Over time, the constellations will
change shape!
• The speed of a star’s motion
toward or away from the Sun is
called its radial velocity
The Inverse-Square
Law
•
•
A star emits light in all directions,
like a light bulb. We see the
photons that are heading in our
direction
As you move away from the star,
fewer and fewer photons are
heading directly for us, so the star
seems to dim – its brightness
decreases.
•
The brightness decreases with
the square of the distance from
the star
– If you move twice as far from
the star, the brightness goes
down by a factor of 22, or 4!
•
Luminosity stays the same – the
total number of photons leaving
a sphere surrounding the star is
constant.
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You see this every day!
•
•
•
More distant streetlights appear
dimmer than ones closer to us.
It works the same with stars!
If we know the total energy output of
a star (luminosity), and we can count
the number of photons we receive
from that star (brightness), we can
calculate its distance
L
4
!
B
• Some types of stars have a known
d=
luminosity, and we can use this
standard candle to calculate the
distance to the neighborhoods these
stars live in.
Measuring
Temperature using
Wein’s Law
2.9 #106 K " nm
T=
!
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The Stefan-Boltzmann
Law
flux = "T 4
Flux is energy / unit area
Where, σ= 5.67×10− 8 W·m-2·K-4
L = flux • Area = "T 4 • 4 # r 2
•
!
The Stefan-Boltzmann Law links
a star’s temperature to the
amount of light the star emits
!
– Hotter stars emit more!
– Larger stars emit more!
•
•
A star’s luminosity is then
related to both a star’s size and
a star’s temperature
We need an organizational tool
to keep all of this straight…
Photons in Stellar
Atmospheres
•
•
•
Photons have a difficult time moving through a star’s atmosphere
If the photon has the right energy, it will be absorbed by an atom and raise
an electron to a higher energy level
Creates absorption spectra, a unique “fingerprint” for the star’s composition.
The strength of this spectra is determined by the star’s temperature.
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Classify:
1- variation in H line
strengths….
“Spectral types”
based on H lines strength:
A
B
C
D
E
F…
1901, Annie Jump Cannon > spectral classification
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Spectral Classification
• Spectral classification
system
– Arranges star
classifications by
temperature
• Hotter stars are O
type
• Cooler stars are M
type
• New Types: L and T
– Cooler than M
• From hottest to coldest, they are
O-B-A-F-G-K-M
– Mnemonics: “Oh, Be A Fine
Girl/Guy, Kiss Me
– Or: Only Bad Astronomers Forget
Generally Known Mnemonics
A convenient tool for
organizing stars
•
•
In the previous unit, we saw
that stars have different
temperatures, and that a
star’s luminosity depends on
its temperature and diameter
The Hertzsprung-Russell
diagram lets us look for
trends in this relationship.
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Stars come in all
sizes…
•
•
A star’s location on the HR diagram
is given by its temperature (x-axis)
and luminosity (y-axis)
We see that many stars are located
on a diagonal line running from
cool, dim stars to hot bright stars
–
•
–
–
•
The Main Sequence
Other stars are cooler and more
luminous than main sequence stars
Must have large diameters
(Red and Blue) Giant stars
Some stars are hotter, yet less
luminous than main sequence stars
–
–
Must have small diameters
White Dwarf stars
• So what’s going on here?
The Mass-Luminosity
Relation
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Stars come in all
sizes…
• L vs T
b=
L
4 "d 2
b=brightness,
d=distance
away
2.9 "10 6 K # nm
T=
$
!
!
Stellar Evolution on the
Main Sequence
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The Main-Sequence
of a Star
High-mass starsLifetime
10 mpg
Low-mass stars 60 mpg
•
The length of time a star spends fusing hydrogen into helium is
called its main sequence lifetime
– Stars spend most of their lives on the main sequence
– Lifetime depends on the star’s mass and luminosity
– More luminous stars burn their energy more rapidly than less luminous
stars.
– High-mass stars are more luminous than low-mass stars
– High mass stars are therefore shorter-lived!
•
•
Cooler, smaller red stars have been around for a long time
Hot, blue stars are relatively young.
A (temporary) new
lease on life
• The triple-alpha
process provides a
new energy source
for giant stars
• Their temperatures
increase temporarily,
until the helium runs
out
• The stars cool, and
expand once again
• The end is near…
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Evolution to red giant
phase
Fuel runs out
Core pressure drops
Gravity compresses
core
Core temperature rises
Shell burning
Pressure puffs outer layers
Core heats up more
Shell burning grows stronger
Atmosphere expands and
cools further
• The star is expanding and cooling, so its luminosity
increases while its temperature decreases
• Position on the HR diagram shifts up and to the
right…
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Helium Fusion
•
Normally, the core of a star is
not hot enough to fuse helium
– Electrostatic repulsion of the
two charged nuclei keeps them
apart
•
The core of a red giant star is
very dense, and can get to very
high temperatures
– If the temperature is high
enough, helium fuses into
Beryllium, and then fuses with
another helium nuclei to form
carbon.
Main Sequence
Turn-off
Main sequence
What are we looking at?
a) Stars of the same
mass?
b) Stars of the same
color?
c) Stars of the same
magnitude?
d) Stars of the same
age?
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Main Sequence
Turn-off
Main sequence
Mass at turn-off
What does the mass of the
Main Sequence-Turn-off
tell us?
a) The Mass of the cluster?
b) The Age of the cluster?
c) The Distance of the
cluster?
d) The Brightness of the
cluster?
The Life-path of the
Sun
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Formation of Planetary
Nebula
• As a red giant expands, it
cools
– Outer layers cool enough for
carbon flakes to form
– Flakes are pushed outward
by radiation pressure
– Flakes drag stellar gas
outward with them
– This drag creates a
high-speed stellar
wind!
– Flakes and gas form
a planetary nebula
White Dwarf Stars
• At the center of the
planetary nebula lies the
core of the star, a white
dwarf
– Degenerate material
– Incredibly dense
• Initially the surface
temperature is around
25,000 K
• Cools slowly, until it
fades from sight.
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The Lifespan of a
Massive Star
Layers of Fusion
Reactions
• As a massive star burns its
hydrogen, helium is left behind, like
ashes in a fireplace
• Eventually the temperature climbs
enough so that the helium begins to
burn, fusing into Carbon. Hydrogen
continues to burn in a shell around
the helium core
• Carbon is left behind until it too starts
to fuse into heavier elements.
• A nested shell-like structure forms.
• Once iron forms in the core, the end
is near…
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Stellar Corpses
• A type II supernova leaves behind the
collapsed core of neutrons that started
the explosion, a neutron star.
• If the neutron star is massive enough, it
can collapse, forming a black hole…
Azimuthal
Radial
velocity
velocity
•Most luminous EM events in
the universe
•Short (few sec) gamma ray
burst
•Longer optical afterglow
•2 kinds (short and long)
pressure
density
•(1)Massive star death
•(2)Collisions of dead stars
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Supernova Remnant
• The supernova
has left behind
a rapidly
expanding shell
of heavy
elements that
were created in
the explosion.
• Gold, uranium
and other
heavies all
originated in a
supernova
explosion!
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