Why does the Sun shine? Is it a ball of fire?
A lump of burning coal or wood?
Ancient people had no way of knowing how far away the Sun is, so they had no idea of how much energy the Sun puts out.

By the mid-1800’s, the scale of the solar system was known reasonably well, so scientists knew that conventional burning could not possibly supply the energy
•
Every square meter at 1 AU receives 1360 W from sunlight
(1360 J/sec)

By the mid-1800’s, the scale of the solar system was known reasonably well, so scientists knew that conventional burning could not possibly supply the energy
•
Every square meter at 1 AU receives 1360 W from sunlight
(1360 J/sec)
•
The total area of a sphere of radius
1 AU is 2.8 x 10 23 m 2

By the mid-1800’s, the scale of the solar system was known reasonably well, so scientists knew that conventional burning could not possibly supply the energy
•
Every square meter at 1 AU receives 1360 W from sunlight
(1360 J/sec)
•
The total area of a sphere of radius
1 AU is 2.8 x 10 23 m 2
•
Total energy output from the Sun is
1360W x (2.8 x 10 23 ) = 3.8 x 10 26
W

By the mid-1800’s, the scale of the solar system was known reasonably well, so scientists knew that conventional burning could not possibly supply the energy
•
Every square meter at 1 AU receives 1360 W from sunlight
(1360 J/sec)
•
The total area of a sphere of radius
1 AU is 2.8 x 10 23 m 2
•
Total energy output from the Sun is
1360W x (2.8 x 10 23 ) = 3.8 x 10 26
W
For comparison, total energy usage in the world in 2012 was ~ 4 x 10 20 J
The Sun puts out 1,000,000 times more every second!

Contraction is a possibility: gravitational potential energy released through contraction could power the Sun for about 25 million years…
…about 1/400 th of the Sun’s 10 billion year lifetime. No good.

When Einstein published his Special Theory of Relativity – specifically the relationship
E = mc 2 – scientists immediately realized that the Sun had more than enough mass to power billions of years of sunshine, IF it could transform mass into energy .
By the end of the 1930s, we understood nuclear fusion well enough to know that it’s the source of the Sun’s energy
We’re going to look at the overall structure of the Sun before getting into the nuclear fusion going on in its core.

How long could the Sun shine if it were powered by gravitational contraction?
A) 10,000 years
B) 250,000 years
C) 1,000,000 years
D) 25,000,000 years
E) 100,000,000 years

How long will the Sun shine powered by nuclear fusion?
A) 5 billion years
B) 10 billion years
C) 13.8 billion years
D) 50 billion years
E) 100 billion years

Heat generated in the core allows thermal pressure
(outward pressure of the gas due to being hot) to balance the inward force of gravity
Pressure and Temperature in the Sun
“Gravitational Equilibrium”
(thermal pressure)
As you look deeper in the Sun, you find:
•
More weight above pushing inward
•
Higher temperatures
•
Greater thermal pressure pushing outward

Energy balance
All the energy generated in the core by nuclear fusion eventually ends up radiating away into space
This means the body of the Sun is not growing hotter or cooler
- which would upset the gravitational equilibrium

Structure of the Sun
Exterior layers
(the atmosphere):
1.
Photosphere (visible surface)
2.
Chromosphere
3.
Corona
Interior layers:
1.
Core
2.
Radiation zone
3.
Convection zone

Chromosphere
Temperatures around 10,000 K
This region emits most of the
Sun’s ultraviolet
Corona
Temperatures up to 1,000,000 K!
This region emits most of the
Sun’s X-rays

Photosphere
The visible “surface”
The gas here is much thinner than the Earth’s atmosphere at sea level!
Average temperature is
5800 K
This is where you find sunspots, filaments, and granules

The layers of the Sun’s atmosphere, in order from coolest to hottest, are
A) Photosphere, Corona, Chromosphere
B) Corona, Photosphere, Chromosphere
C) Chromosphere, Photosphere, Corona
D) Corona, Chromosphere, Photosphere
E) Photosphere, Chromosphere, Corona

The Core: Nuclear Fusion
Fission
Breaking a large nucleus into smaller nuclei
This is how the U.S. generates about 20% of its energy
Fusion
Combining two smaller nuclei into one larger one
This is how the Sun generates all of its energy
(We’d like to, but the technology is still beyond us!)

The Core: Nuclear Fusion
This is what you need to know
This is what is actually going on

A) 1
B) 2
C) 3
D) 4
E) 5

A) 1
B) 2
C) 3
D) 4
E) 5

+ neutrinos
Very important!

The Sun converts mass into 3.8 x 10 26 J of energy every second.
These have more mass… …than this.
+ neutrinos
That lost mass becomes these.
How much mass does that take? E = mc 2

The Solar Thermostat
Nuclear fusion is very sensitive to temperature

That allows the core to contract, heating it up, and increasing the rate of fusion.
Theoretical models suggest that the core has shrunk enough to increase its fusion rate and the
Sun’s luminosity by 30% over its
4.5 billion years.
Through fusion in the core, FOUR hydrogen nuclei turn into ONE helium nucleus.
That one nucleus takes less space than the four H nuclei.

What would happen if the temperature of the Sun's core increased suddenly?
a) It would continue to increase in a runaway fusion reaction.
b) It would reach equilibrium at its new higher temperature with a higher rate of fusion.
c) The rate of fusion would increase causing the core to expand and cool back to its original temperature.
d) The Sun would undergo continuing oscillations in temperature and fusion reactions.

The Radiation Zone
In this way, it can take hundreds of thousands of years for a single photon to get out of the radiation zone!
Those photons of energy generated in the core don’t get very far before they scatter off an electron.
They are scattered in a random direction.
They are soon scattered by another electron in another random direction.
This is called a “random walk”

The Convection Zone
In the convection zone, the photons are actually absorbed, heating the plasma.
Because hotter gases are less dense, they rise. This is called convection.
The spots where the hot gases reach the surface are brighter. They are called granules.
Individual granules last only a few minutes.

A) Radiation
B) Conduction
C) Convection

1) Four protons go into the “proton-proton chain” in the Sun.
Other than one Helium nucleus, what else comes out?

2) Rank the layers of the Sun’s atmosphere from coolest to hottest:
Photosphere
Corona
Chromosphere

3) What evidence is there that the Sun’s outermost interior layer is convective?

4) True / False
The Sun is actually brighter now than it was when it first formed.