Transcript A105 Stars and Galaxies

A100
The Sun
Read Chapter 11
Homework and Quiz 9 this
week on Friday
Last Solar Lab on TOMORROW
at NOON
Today’s APOD
The Sun Today
Basic Facts
•
radius
•
•
distance
•
•
The Sun
7 x 105 km
about 100 x Earth’s radius
mass = about 300,000 x Earth’s mass
•
•
•
1 AU, 8 light minutes
1.5 x 108 km
about 100 x Sun’s diameter
• Temperature
• about 6000 Kelvin (10,000 F) at the surface
• about 15 million Kelvin inside
• Composition
• 90% of atoms are hydrogen
• 10% of atoms are helium
WHY WE STUDY THE SUN
Helioseismology
With helioseismology,
we can measure
temperature, pressure
and motion inside the Sun
from sound waves that
traverse the Sun’s interior.
Helioseismology
Listen to the Sun
blue = inward
motion
red = outward
motion
The Sun oscillates in complex patterns over the
whole interior and surface. The frequency and
location of the oscillations give us a detailed
picture of the inside of the Sun.
Basic
Structure
INSIDE
Core
Radiative Zone
Convection Zone
OUTSIDE
Photosphere
Chromosphere
Corona
Solar Wind
Conditions
inside the
Sun
Temperature peaks in the
core and drops off at the
outside edge of the Sun
Why does
the Sun
Shine?
The Sun is a “cooling ember”
The Sun is burning like coal or wood
The Sun is contracting due to gravity
NONE OF THESE PRODUCES ENOUGH ENERGY
NUCLEAR REACTIONS
produce enough energy
E =
2
mc
- Einstein, 1905
Nuclear Potential Energy (core)
Luminosity
~ 10 billion years
What produces nuclear energy?
Fission:
Big nucleus splits into
smaller pieces
Fusion:
Small nuclei stick
together to make a
bigger one
The Sun releases energy
by fusing four hydrogen
nuclei into one helium
nucleus
Nuclear fusion requires
high temperatures and
high density
The Sun’s Energy
Comes from Nuclear Fusion
The
Conversion
of hydrogen
into helium
Proton-proton chain fuses hydrogen into helium
IN: 4 protons
OUT:
4He
nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower
•The missing mass is converted to energy
•Rate of nuclear fusion depends on temperature
How do we know nuclear reactions are going
on in the Sun?
•Neutrinos created during fusion
fly directly out of the Sun
•These neutrinos can be
detected on Earth
The Sun’s
Lifetime
Estimating the
Sun’s Lifetime
How much fuel does the Sun have?
How fast is that fuel being consumed?
When will it run out?
How much fuel?
The Sun “burns” hydrogen to helium in a
nuclear reaction
E = mc2
How many hydrogen atoms in the Sun
2 x 1033 grams of hydrogen
6 x 1023 atoms in each gram
12 x 10 56 hydrogen atoms
How Much Energy Can
the Sun Produce?
E=
2
mc
For each helium nucleus created, the Sun
produces 5 x 10-12 joules of energy
the Sun can burn half its hydrogen
FOUR hydrogen atoms are needed to make
each helium atom
½ x ¼ x 12x1056 x 5x10-12
= 7 x 10
44
joules
The Solar
Constant
1400 watts =
 How Much Energy Each Second? 14 100-watt light bulbs
 At the distance of the Earth, the
Sun radiates 1400 watts (1400
joules per second) in each square
meter on the surface of a sphere
with a radius equal to one AU
 How many square meters are on
that sphere?
What is a joule?
Each second a 100-watt light bulb
produces 100 joules of energy
1 meter
Sphere with radius of 1 AU
Surface area
of a sphere
= 4pr2
How many square
meters on a sphere
with radius one AU?
The Sun produces
4 x 1026 watts
Area times energy
per square meter
per second equals
total energy output
per second
Solar Energy
How long will the
Sun shine???
From the conversion of hydrogen
into helium by nuclear reactions…
Total energy available = 7 x 1044 joules
Radiating energy at 4 x 1026 joules per second
Lifetime =
7 x 1044 joules
4 x 1026 joules per second
= 2 x 1018 seconds = 6 x 1010 years
E = mc2
Solar Energy
How long will the
Sun shine???
In fact, the Sun will only shine
for about 10 billion years, twice
its present age.
Not all the Sun’s hydrogen is in
regions hot enough for hydrogen
fusion reactions to occur.
E = mc2
How does the energy from fusion
get out of the Sun?
• Radiation
• Convection
How does the energy from fusion
get out of the Sun?
Radiation: In the inner regions
of the solar interior, energy
gradually leaks upward in form
of randomly bouncing
electromagnetic waves.
As the energy propagates
outward, the electromagnetic
energy shifts from gamma rays
and X-rays to ultraviolet and
optical light
Energy Transport
In the outer regions of the
Sun’s interior, energy flows
outward by convection; heat is
carried upward by bubbling hot
gas.
time lapse movie
The convection zone gives the surface the
appearance of boiling liquid
Balancing Gravity
Gravitational
contraction:
Provided energy that
heated core as Sun was
forming
Contraction stopped
when fusion began
Gravitational equilibrium:
Energy provided by fusion maintains the pressure
The Solar Thermostat
Decline 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
Summary:
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.
Dates to
Remember
Ch 11 – The Sun
 Quiz, Homework on Friday
Solar Lab tomorrow at NOON