Transcript ASTR100 (Spring 2008) Introduction to Astronomy Our Star

ASTR100 (Spring 2008)
Introduction to Astronomy
Our Star
Prof. D.C. Richardson
Sections 0101-0106
Why does the Sun shine?
Luminosity
 Luminosity is light energy emitted per
unit time.
 Measured in Watts (Joules per second).
 Our Sun’s luminosity is 3.8 × 1026 W.
 That’s a lot of 100-W lightbulbs!!
Lifetime =
Total Energy
Luminosity
• This helps explain why the Sun shines!…
Is it on FIRE?
Is it on FIRE?
Chemical Energy Content
Luminosity
~ 10,000 years
Is it on FIRE? … NO!
Chemical Energy Content
Luminosity
~ 10,000 years
Is it CONTRACTING?
Is it CONTRACTING?
Gravitational Potential Energy
Luminosity
~ 25 million years
Is it CONTRACTING? … NO!
Gravitational Potential Energy
Luminosity
~ 25 million years
Is it powered by NUCLEAR ENERGY?
E=
2
mc
- Einstein, 1905
Is it powered by NUCLEAR ENERGY?
E=
2
mc
- Einstein, 1905
Is it powered by NUCLEAR ENERGY?
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years
E=
2
mc
- Einstein, 1905
Is it powered by NUCLEAR ENERGY? … YES!
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years
The Sun is in
gravitational
equilibrium…
The outward
push of
pressure
balances the
inward pull of
gravity.
Weight of upper
layers compresses
lower layers.
Gravitational
equilibrium…
Energy
provided by
fusion (heat)
maintains the
pressure.
Gravitational
contraction…
Provided
energy that
heated core as
Sun was
forming.
Contraction
stopped when
fusion began.
Quiz!
 Consider Schrödinger’s equation:
 Show that this equation is equivalent to:
Haha! April Fools!
What is the Sun’s structure?
Radius:
6.9 x 108 m
(109 times Earth)
Mass:
2 x 1030 kg
(300,000 Earths)
Luminosity:
3.8 x 1026 Watts
Solar wind:
A flow of
charged
particles
from the
surface of
the Sun.
Corona:
Outermost
layer of solar
atmosphere.
~1 million K
Chromosphere:
Middle layer of
solar atmosphere.
~ 104–105 K
Photosphere:
Visible surface
of Sun.
~ 6,000 K
Convection
Zone:
Energy
transported
upward by
rising hot gas.
Radiation
Zone:
Energy
transported
upward by
photons (light).
Core:
Energy
generated by
nuclear fusion.
~ 15 million K
How does nuclear fusion occur in the
Sun?
Fission
Fusion
Big nucleus splits
into smaller pieces.
Small nuclei stick
together to make
a bigger one.
(Nuclear power
plants)
(Sun, stars)
High temperature
enables nuclear
fusion to happen
in the core.
(Atoms move
faster when
they’re hotter.)
The Sun releases energy by fusing four hydrogen
nuclei into one helium nucleus in several steps.
The sequence of steps is called the proton-proton
chain.
IN
4 protons
OUT
4He nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower
E = mc2
Thought Question
What would happen inside the Sun if a
slight rise in core temperature led to a
rapid rise in fusion energy?
A. The core would expand and heat up
slightly.
B. The core would expand and cool.
C. The Sun would blow up like a
hydrogen bomb.
Thought Question
What would happen inside the Sun if a
slight rise in core temperature led to a
rapid rise in fusion energy?
A. The core would expand and heat up
slightly.
B. The core would expand and cool.
C. The Sun would blow up like a
hydrogen bomb.
Solar thermostat keeps burning rate steady
Solar Thermostat
 Drop in core
temperature causes
fusion rate to drop,
so core contracts
and heats up.
 Rise in core
temperature causes
fusion rate to rise,
so core expands
and cools down.
How does the energy from fusion
get out of the Sun?
Energy gradually leaks out of the radiation zone in the
form of randomly bouncing photons.
Convection (rising hot gas) takes energy to surface.
Bright blobs on photosphere are where hot gas reaches the surface.
How do we know what is happening
inside the Sun?
We learn about the inside of the Sun by…
 Making mathematical models.
 Observing “sunquakes”.
 Observing solar neutrinos.
Patterns of
vibration on
the surface tell
us what the
Sun is like
inside.
Results agree
very well with
mathematical
models of the
solar interior.
Neutrinos
created during
fusion fly
directly through
the Sun.
Observations of
these solar
neutrinos can
tell us what’s
happening in
core.
Solar neutrino
problem:
Early searches for
solar neutrinos
failed to find the
predicted number.
Solar neutrino
problem:
Early searches for
solar neutrinos
failed to find the
predicted number.
More recent
observations find
the right number of
neutrinos, but some
have changed form.
What causes solar activity?
Solar activity is like “weather”…
 Sunspots.
 Solar prominences.
 Solar flares.
 Coronal mass ejections.
All are related to magnetic fields.
Sunspots
Cooler
than
other
parts of
the Sun’s
surface
(4000 K).
Regions
with
strong
magnetic
fields.
Click me!
Charged particles spiral along magnetic field lines.
Click me!
Loops of bright gas often connect sunspot pairs (prominence).
Click me!
Loops trace magnetic field lines.
Magnetic activity
causes solar
flares that send
bursts of X-rays
and charged
particles into
space.
Click me!
Magnetic activity
also causes
solar
prominences
that erupt high
above the Sun’s
surface.
The corona
appears bright
in X-ray photos
in places where
magnetic fields
trap hot gas.
How does solar activity affect
humans?
Coronal
mass
ejections
send bursts of
energetic
charged
particles out
through the
solar system.
Click me!
Charged particles streaming from Sun can disrupt
electrical power grids and disable communications
satellites.
Energetic particles high
in Earth’s atmosphere
cause aurorae (e.g.,
Northern Lights).
How does solar activity vary with
time?
Click me!
Number of sunspots rises and falls in 11-year cycle.
Sunspot cycle related to winding and twisting of Sun’s
magnetic field.