Download Luminosity

Chapter 8
The Sun and Other Stars
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Radius:
6.9  108 m
(109 times Earth)
Mass:
2  1030 kg
(300,000 Earths)
Luminosity:
3.8  1026 watts
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What is the Sun’s structure?
Insert TCP 6e Figure 14.3
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Core:
Energy generated
by nuclear fusion
~ 15 million K
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How does nuclear fusion occur in
the Sun?
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Fission
Fusion
Big nucleus splits into
smaller pieces.
Small nuclei stick
together to make a
bigger one.
(Example: nuclear
power plants)
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(Example: the Sun, stars)
High temperatures
enable nuclear
fusion to happen in
the core.
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The Sun releases energy by fusing four hydrogen nuclei into one
helium nucleus.
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IN
4 protons
OUT
4He nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower.
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Radiation Zone:
Energy transported
upward by photons
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How does the energy from fusion
get out of the Sun?
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Energy gradually leaks out of radiation zone in form of randomly
bouncing photons.
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Convection Zone:
Energy transported
upward by rising
hot gas
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Convection (rising hot gas) takes energy to surface.
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Bright blobs on photosphere show where hot gas is reaching the surface.
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Photosphere:
Visible surface of
Sun
~ 6000 K
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Chromosphere:
Middle layer of
solar atmosphere
~ 104–105 K
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Corona:
Outermost
layer of solar
atmosphere
~1 million K
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Solar wind:
A flow of
charged
particles from
the surface of
the Sun
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Gravitational
equilibrium:
Energy supplied
by fusion
maintains the
pressure that
balances the
inward crush of
gravity.
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Gravitational
contraction:
Provided the
energy that
heated the core as
Sun was forming
Contraction
stopped when
fusion began.
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How we know what is happening
inside the Sun?
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We learn about the inside of the Sun by …
• making mathematical models
• observing solar vibrations
• observing solar neutrinos
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Patterns of
vibration on the
surface tell us
about what the
Sun is like inside.
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What causes solar activity?
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Solar activity is like “weather”.
• Sunspots
• Solar flares
• Solar prominences
All these phenomena are related to magnetic
fields.
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Sunspots
Are cooler
than other
parts of the
Sun’s
surface
(4000 K)
Are regions
with strong
magnetic
fields
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Loops of bright gas often connect sunspot pairs.
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Magnetic activity
causes solar flares
that send bursts of
X rays and
charged particles
into space.
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Magnetic activity
also causes solar
prominences that
erupt high above
the Sun’s surface.
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The corona
appears bright in
X-ray photos in
places where
magnetic fields
trap hot gas.
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Charged particles streaming from the Sun can disrupt electrical power
grids and can disable communications satellites.
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Insert TCP 6e Figure 14.21a unannotated
The number of sunspots rises and falls in an 11-year cycle.
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Properties of Other Stars
• Luminosity
• Surface Temperature
• Mass
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How do we measure stellar
luminosities?
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Luminosity:
Amount of power a star
radiates
(energy per second = watts)
Apparent brightness:
Amount of starlight that
reaches Earth
(energy per second per
square meter)
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The amount of
luminosity passing
through each sphere is
the same.
Area of sphere:
4 (radius)2
Divide luminosity by
area to get brightness.
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The relationship between apparent brightness and
luminosity depends on distance:
Brightness =
Luminosity
4 (distance)2
We can determine a star’s luminosity if we can measure
its distance and apparent brightness:
Luminosity = 4 (distance)2  (brightness)
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Most luminous
stars:
106 LSun
Least luminous
stars:
10–4LSun
(LSun is luminosity
of Sun)
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Apparent Magnitude
•
•
•
•
Greek astronomer, Hipparchus
Brightest stars were magnitude 1
Faintest stars were magnitude 6
Quantitatively redefined by modern scientists:
– Difference of five “magnitude” = brightness ratio
of 100
– Star Vega = magnitude of zero
– Brightness in units of watts/m^2
– Logarithmic scale – mag 1 is 2.512 times mag 2
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Apparent Magnitude (concluded)
•
•
•
•
•
•
•
•
•
•
Sun
Full Moon
Venus (max)
Mars (max)
Mars (min)
M31 (Andromeda Galaxy)
Best naked eye
7x50 binoculars
My telescope
Hubble telescope
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-26.4
-12.92
-4.89
-2.91
1.84
3.44
7-8
9.5
?? 10-12
31.5
How do we measure stellar
temperatures?
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Every object emits thermal radiation with a
spectrum that depends on its temperature.
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Remembering Spectral Types
(Hottest)
O B A F G K M
(Coolest)
• Oh, Be A Fine Girl (Guy), Kiss Me
• Only Boys Accepting Feminism Get Kissed
Meaningfully
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Lines in a star’s spectrum correspond to a spectral type that
reveals its temperature.
(Hottest)
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O B A F G K M
(Coolest)
How do we measure stellar
masses?
Insert TCP 6e Figure 15.7 unannotated
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We measure mass using gravity.
Direct mass measurements are possible only for
stars in binary star systems.
42
p2 =
a3
G (M1 + M2)
M1 and M2 are the masses of the two stars
p = period
a = average separation
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Most luminous
stars:
106 LSun
Least luminous
stars:
10–4LSun
(LSun is luminosity
of Sun)
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Most luminous
stars:
106 LSun
Least luminous
stars:
10–4LSun
(LSun is luminosity
of Sun)
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Three Major Star Groups
• The Main Sequence
– Most follow the surface temp – luminosity
trend of red=cool and blues=hot
– Generate energy by fusing hydrogen in cores
– Lower tempertures and luminosities result in
longer lives
– The term “main sequence” will become selfevident later
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Main-Sequence Star Summary
High-Mass Star:
•
•
•
•
High luminosity
Short-lived
Larger radius
Blue
Low-Mass Star:
•
•
•
•
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Low luminosity
Long-lived
Small radius
Red
Three Major Star Groups
• Giants and Supergiants
– The bright red stars in previous slide
– Cooler temperature but greater luminosity than
our Sun
– To be brighter while being cooler means they
must have must greater surface area – giants
and supergiants
– Have run out of hydrogen fuel in core and
nearing end of life
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Three Major Star Groups
• White Dwarfs
–
–
–
–
Too dim to be seen in the previous slide
Very hot (white) but low luminosity
Must have a smaller surface area than our Sun
Embers of giants that have run out of fuel and
blown off outer layers
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Sizes of Giants and Supergiants
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Pioneers of Stellar Classification
• Annie Jump
Cannon and the
“calculators” at
Harvard laid the
foundation of
modern stellar
classification.
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What is a Hertzsprung-Russell
diagram?
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Luminosity
An H-R
diagram plots
the luminosity
and temperature
of stars.
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Temperature
Off the Main Sequence
• Stellar properties depend on both mass and age: Those that
have finished fusing H to He in their cores are no longer on
the main sequence.
• All stars become larger and redder after exhausting their
core hydrogen: giants and supergiants.
• Most stars end up small and white after fusion has ceased:
white dwarfs.
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Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of 10MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years  10/104
Life expectancy of 0.1MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years  0.1/0.01
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What have we learned?
• Why was the Sun’s energy source a major
mystery?
– Chemical and gravitational energy sources could
not explain how the Sun could sustain its
luminosity for more than about 25 million years.
• Why does the Sun shine?
– The Sun shines because gravitational equilibrium
keeps its core hot and dense enough to release
energy through nuclear fusion.
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What have we learned?
• What is the Sun’s structure?
– From inside out, the layers are:
•
•
•
•
•
•
Core
Radiation zone
Convection zone
Photosphere
Chromosphere
Corona
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What have we learned?
• How does nuclear fusion occur in the Sun?
– The core’s extreme temperature and density are
just right for nuclear fusion of hydrogen to
helium through the proton–proton chain.
– Gravitational equilibrium acts as a thermostat to
regulate the core temperature because fusion
rate is very sensitive to temperature.
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What have we learned?
• How does the energy from fusion get out of
the Sun?
– Randomly bouncing photons carry energy
through the radiation zone.
– Rising of hot plasma carries energy through the
convection zone to photosphere.
• How do we know what is happening inside
the Sun?
– Mathematical models agree with observations
of solar vibrations and solar neutrinos.
© 2010 Pearson Education, Inc.
What have we learned?
• What causes solar activity?
– Stretching and twisting of magnetic field lines
near the Sun’s surface cause solar activity.
• How does solar activity affect humans?
– Bursts of charged particles from the Sun can
disrupt radio communication and electrical
power generation and damage satellites.
• How does solar activity vary with time?
– Activity rises and falls with an 11-year period.
© 2010 Pearson Education, Inc.