Lecture Notes

Arny Chapter 11

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Chapter 11 - The Sun, Our Star
The Sun is a star.
The star that is closest to us, by far.
Our Sun is the star we know the most about.
It is an above-average star, bigger and brighter than the average star.

Section 11.1 - Size and Structure
Section 11.2 - How the Sun Works
Much of what we know about our Sun and other stars comes from spectroscopy (chapter 3).
From the spectrum of the sunlight we get from the Sun and physical models, the composition of the Sun is
3/4 hydrogen (71%), 1/4 helium (27%),
and 2% heavier elements ("metals").
We believe this is the composition the Sun started with and it still has basically the same composition today.

The Sun is huge, a million Earths could fit into its volume.
Its diameter is over a 100 times that of Earth, its mass 333,000 times as much, see Table 11.1 on page 324.
The Sun is far larger than any planet.

Temperatures and pressures within the Sun are enormous.
Surface temperature = 5800 K (similar to center of Earth).
Core temperature = 15 million K.
[Room temperature on the kelvin scale is 295 K, boiling water is 373 K.]
The Sun is entirely gaseous.

The "surface" of the Sun is called the photosphere.
The Sun's "surface" is not really a surface.
It is just the point at which the Sun's gases become so thick that we can't see any deeper, become opaque rather than transparent.
Roughly 400 km thick.
Temperature = 5800 K.

Density and pressure increase as one moves deeper into the Sun.
See figure 11.2 on page 326.
Heat (energy) flows from hot to cold.
There is a constant flow of energy from the center (core) of the Sun outwards to the surface and on into space.

Huge amounts of energy pour out of the Sun.
And this has been going on for billions of years.
Where does all this energy come from?
That is, what keeps the Sun so hot?

Chemical Energy?
Like energy released by fire.
Is the Sun "on fire"?
No, fires could never have burned so long.

Gravitational Energy?

Or maybe the Sun continues to contract, generating fresh gravitational energy.
No, that could work for millions of years, but not billions.
This was once thought to be the Sun's source of energy, and the Earth and Sun were thought to be about 10 million years old.

There must be some other source of the Sun's energy.

Radioactivity?
Also known as nuclear fission.
Radioactive materials keep the center of the Earth hot.
They're also used to make nuclear weapons, and it's the energy source for nuclear power plants.
But we know what elements are radioactive and which aren't.
Some metals are, hydrogen and helium are not.
The Sun does not contain enough radioactive material (by far) to explain its hot temperature.

Nuclear Fusion?
When conditions are right, certain light elements can combine together in an explosive reaction to create heavier elements.
This is called nuclear fusion (to fuse - to combine together).
Hydrogen atoms can fuse together to create helium and release energy.
This is the process used to make "H-bombs" and which may one day replace fission reactors with fusion reactors in power plants.
This is the Sun's source of energy.

Nuclear Fusion!
The Sun is entirely a gas, mostly hydrogen gas.
The hydrogen atoms in the core have been ionized into a "plasma" of loose protons and electrons.
Now protons all have positive electrical charge, they repel each other.
It is difficult to force them close enough together, but if you can smash them together you'll get a fusion reaction.

Deep within the Sun, temperatures are extremely high so the protons all move very fast.
Pressures and densities are enormous; collisions between protons occur frequently.
The conditions are right for nuclear fusion of the protons (hydrogens) to occur.


A series of nuclear reactions occur in the core of the Sun.
See figure 11.11 on page 332.
This set of reactions is called the proton-proton chain.

The overall reaction is the following:
4 H => He + 2 e+ + 2 v + 3 g

H = hydrogen nucleus = p = proton
He = helium nucleus = two protons and two neutrons together.
e+ = positron = anti-electron (anti-matter)
Yes, anti-matter exists.
And when matter and anti-matter get together they annihilate (just like taught in sf).
That's what happens to these positrons,
e+ + e- => 2 g
[A positron and electron annihilate to form two gamma-ray photons.]
The positron is the least important part of this reaction.
v = Greek letter "nu" = neutrino (an elusive little particle)
g = Greek letter "gamma" = gamma-ray = photon = light = electromagnetic radiation

This nuclear reaction releases energy.
Most of the generated energy is carried by the photons and neutrinos (as kinetic energy, energy of motion).

Why does this process release energy?
The mass of the initial state (protons) exceeds the final state (He).
Some mass has been converted into energy.
[Technically, mass has been converted into radiation, the total energy is unchanged.]
This fusion energy is sometimes called mass energy.
This conversion follows Einstein's famous equation E = m c^2
m = mass difference
E = energy released
c = speed of light = 3 x 10^8 m/s.

c^2 is big, so even a small change in mass gives a lot of energy.
The proton-proton chain converts 0.7% (.007) of the initial mass into energy.
The core of the Sun is filled with constantly exploding H-bombs which keep the Sun hot and shining.

In fact, the Sun converts 4.5 million (metric) tons of mass into energy every second!
Yet, even at this phenomenal rate of consuming itself, the Sun will easily live to be 10 billion years old.

Note that gravity never stops.
The energy generated by fusion in the core creates pressure, the pressure pushes the gases outward, away from the Sun.
The Sun has reached a balance, the outward pressure is equal to the inward pull of gravity, this is called hydrostatic equilibrium.
Without pressure, the Sun would collapse into a smaller, dense, solid ball.

The energy produced in the core of the Sun is carried off by photons (and to a lesser degree, neutrinos).
Let's consider these in more detail.

Photons
The photons (light, EM radiation) created by the fusion reactions in the core cannot simply fly out of the Sun.
The Sun is an extremely dense gas, "opaque" to light, not transparent.

The energy must work its way
slowly out of the Sun.
[See figure 11.3 on page 326.]
Radiative Zone: light reflects, bounces
from atom to atom (ion to ion).
On average getting nearer the surface
with each short trip.
Convection Zone: huge loops of gas
moving within the Sun.
Atoms absorb the photons.
The loops carry hotter atoms near the
core up towards the surface.
Hence the energy created in the core
moves to the surface.

It can take the energy a million years to
get from the core to the surface!

This brings us to the Sun's "surface".
The photosphere.
At this point energy flows away from
the Sun into space by radiation.

Section 11.3 Probing the Sun's Core
Neutrinos
Neutrinos are produced in huge numbers by the Sun.
Neutrinos interact only very weakly with matter.
Neutrinos can pass entirely through the Sun or Earth as if they weren't even there!
So neutrinos produced in the core of the Sun can fly right out of the Sun into space.

The number of neutrinos produced is mind-boggling.
Even at the Earth, far from the Sun, 60 million billion (6 x 10^16 ) neutrinos pass through the tip of your thumb every second!
And you never even notice.
[About 3.5 x 10^17 photons, 6x more, per cm^2 per second, (before the atmosphere, assuming all yellow photons).]

Neutrinos provide us with a view of what is going on in the core of the Sun.
Scientists have measured the flux of neutrinos coming out of the Sun.
This isn't easy to do considering the elusive nature of neutrinos.
[Requires giant underground pools monitored continuously.]

These measurements have given a surprising result.
Only about one-third as many neutrinos are detected as expected.
This is called the Solar Neutrino Problem (recently solved).

[May skip the following.]
There are three major explanations;

(1) Maybe the Sun goes through cycles.
Maybe we're in the middle of a lull right now.
Because it takes so long for light energy to work its way from the core to the surface, it might be possible that the Sun is currently in a nuclear fusion lull.
But no one knows why the Sun would go in cycles, and there is no other evidence that it does.

(2) Maybe the proton-proton chain is wrong.
Could we be wrong in how the Sun generates energy?

(3) This is the culprit that most scientists suspect:
It has long been known that there are actually three types of neutrinos.
It was suspected that neutrinos might be able to change type.
This could easily explain the solar neutrino problem because experiments were detecting only one type of neutrino but maybe the neutrinos were changing into the other types on their way from the Sun to the Earth.

This behavior is called "neutrino oscillations".
Recent experimental results have verified this idea.
To be able to "oscillate", the neutrinos must have a slight mass which has been discovered in the laboratory.
A neutrino detector was just built in Japan which can detect all types of neutrinos and is expected to detect three times more neutrinos coming from the Sun.

Section 11.4 Solar Magnetic Activity
We will skip this and the following sections (there's so little time...) but we will cover page 341, sunspots.

The Sun has an active surface with a "granulated" appearance.
See figure 11.4 on page 327.
Each granule is larger than the state of Texas.
The granules are the tops of the convection loops in the Sun's convection zone.

Sunspots are dark blemishes on the `surface' of the Sun.
See figures 11.4 and 11.14.
They are actually hot, bright areas but not as hot or bright as the rest of the Sun (4500K versus 6000K).
So sunspots merely appear dark in comparison.

[View sunspots using the Sunspotter.]

Sometimes the Sun has many spots.
Sometimes very few or even none.
But sunspots aren't entirely random.
The number of sunspots varies with a period of about 11 years.
See figure 11.22 on page 341.

When sunspot numbers are high, the Sun's solar wind is stronger, and the Earth is affected:

• radio communications can be disrupted.
• auroras will be larger and brighter.
• the Earth's magnetic field can become distorted (compasses can point in strange directions).
• astronauts in space are at risk from radiation.

From 1645 until 1715, the Sun showed almost no sunspots.
See figure 11.26 on page 344.
This is called the Maunder minimum in the sunspot cycle.
Why no sunspots? Nobody knows.
The same period of Earth is known as the Little Ice Age, a period of unusually cold weather on the Earth.
Coincidence? Or cause and effect? Uncertain.

[Because sunspots are cooler, darker regions, you might think a lack of sunspots means a warmer Sun.
Not so, because sunspots always come along with some other things which increase the Sun's output, more shortly.]

The rest of this chapter might be skipped.
The Chromosphere, the atmosphere of the Sun.
"Chromosphere" means "color sphere", so named because it is responsible for the absorption lines - the missing colors - in the Sun's spectrum.
It extends about 2000 km above the photosphere.

The Corona is a still more distant, hotter region.
Temperature close to 1,000,000 K.
The corona stretches throughout the solar system, the solar wind is considered to be part of the corona.
[See figure 11.7 on page 328.]


Prominences are huge loops of hydrogen gas thrown upwards by magnetic forces.
Can hover above the Sun for weeks or even months.
Associated with sunspots.
[See figures 11.17 and 11.18 on page 337.]

Flares are violent explosions of hot material occurring in regions around sunspots.
It is flares that cause disruptions here on Earth.
[See figure 11.19 on page 338.]

End chapter 11.