Q1: temp photosphere Q2: spots Q3: corona Q4: limb Q5: next solarmax Q6: climate Q7: neutrinos Q8: Homestake Q9: Homestake
Lecture slides


The Sun is the largest, brightest, and most important object in the Solar System. All other objects in the Solar System (including the Earth) are bound to the Sun by gravity.

The Sun is a star, the closest one to us by many factors of ten.

We begin with the appearance of the Sun.

In ordinary visible light, the Sun looks like this:

This is the photosphere, the Sun you usually see (except that you should NEVER EVER look directly at the Sun!).

When we have a solar eclipse, we can see the corona:

From the picture of the photosphere, we can figure out a LOT about the Sun. But first, we need a quick digression.

You can find the temperature of an object from the color. Higher energy light = bluer = higher temperature. You know this already, from working with bunsen burners in high school chemistry class.

Here is the blackbody curve, which shows this relationship. The amount of energy released by the object in a given color is plotted vs. color on the x-axis. This is depicted for several different temperatures:

So, looking at the two pictures of the Sun, we find IMMEDIATELY, that the temperature of the surface of the Sun is 5800 K. The corona is hotter. Sunspots are cooler. AND, the temperature of the photosphere increases as you move towards the center. Does this seem weird? Yes. Because the temperature of the Sun cools down as the radius increases, and then suddenly increases again! Why? We don't really know. Could be lots of different things, but no one has yet been able to make a good description that holds up under scrutiny.

Let's take a closer look.




What about the interior?

A pictogram of the inner structure of the Sun looks like this:

In the deep core, the temperature is extremely high (measured here in Kelvins, a particular temperature scale, like Celsius or Fahrenheit. Once you are talking about temperatures this high though, it doesn't really matter which scale you are using. The interior of the Sun is HOT!). The pressure is also very high, compressing the mass to a density of 150,000 kg/m3. This is 150 times the density of water, and about 30 times the density of rock. You would never be able to move through material this dense.

As you move out through the Sun, the temperature and the density fall smoothly. Once the temperature drops below 8 million K, the energy produced can travel more easily out through the star by radiative diffusion.

The energy that we see leaving the Sun has traveled a tortuous path to get out. Here we broke for a few minutes to have a demonstration of radiative diffusion. The styrofoam ball did not travel from the interior of the Sun to the exterior in any sort of sensible way, and it took a very long time. In the Sun, the problem is extreme because of the high densities there. Each photon (bit of light) that wants to come out of the Sun travels only about 10-6 m before being absorbed by an atom, which 'spits it out' in a random direction. This means that it can take about 170,000 years for an individual photon to find its way out of the Sun. Some are faster, and some are slower, but on average, this is how long it takes. The point is that the energy you see now leaving the Sun actually was produced from mass 170,000 years ago, before man even existed. So we can't look at the surface of the Sun and know what's happening inside NOW. At least not by looking at the light.

Once the temperature falls to 1.5 million K, convection can develop. This is the boiling pot method of transferring energy.

How do we know this? Obviously, I didn't go there, and take a sample... We have figured this out from a few key pieces of evidence, plus a lot of thinking hard about what makes energy. First, we know that the Sun has been shining for billions of years. How do we know this? Because the fossil record shows that there's been life on Earth for at least 3.5 billion years. Presumably, that life could not have existed without the Sun, about as bright as it is now. Only one known source of energy can make as much energy as the Sun gives off for as long as it's been shining: NUCLEAR FUSION.

Nuclear fusion is the process of taking light atoms, and smashing them together to make heavier ones. In particular, stars like the Sun shine by fusing hydrogen into helium. The primary method of doing this is by the proton-proton chain:

Or, an animated version:

The Beta particle in these images is sometimes written +, and is more properly called a positron. A positron is an electron with a positive charge, and belongs to a class of particles called 'anti-matter'. Anti-matter. No kidding. As you know from Star Trek, when matter meets anti-matter, both particles are destroyed, and light comes out. So inside of the Sun, positrons are being created, which then run into electrons, and both are destroyed, producing a gamma ray (see below).

The neutrino, , is a teeny-tiny particle, with very little mass, that travels very close to the speed of light. It has no charge, and barely interacts with ordinary matter. A typical can travel through 3 light years of lead as though it wasn't even there. There are 1016 (10,000,000,000,000,000) neutrinos passing through your body every second, and you don't even know it. They go in one side and out the other, and don't even slow down.

A Gamma Ray is really just a high energy bit of light, like an X-ray, only with even more energy.

So you can see that a lot of energy comes out of this reaction. There's the energy to make the neutrino, and send it flying away, and the energy in the two gamma rays (one resulting from the positron-electron annihilation). Where did this energy come from?

The helium atom which we end up with is actually less massive than the four hydrogen atoms that we began with! All this talk about mass and energy should make you think immediately (ok, maybe not immediately!) of the most famous formula ever:


In this equation, E stands for energy, m stands for mass, and c is the speed of light, 3X108 m/s. What it says is that mass is just another form of energy, and that you can turn energy into mass, and mass into energy, according to the formula. This was one of Einstein's greatest contributions to humanity, figuring out that mass and energy are really just different forms of the same thing.

A Gamma Ray is really just a high energy bit of light, like an X-ray, only with even more energy.

So you can see that a lot of energy comes out of this reaction. There's the energy to make the neutrino, and send it flying away, and the energy in the two gamma rays (one resulting from the positron-electron annihilation). Where did this energy come from?

How much energy is produced? About 25 MeV (Mega electron-Volts). This is 1/10,000,000 the amount of energy needed to lift one drop of water one cm. You use more energy than this just sitting there. It's not much energy at all. So what does this tell you? It tells you that there must be an unimaginably large number of hydrogen atoms in the center of the Sun, all participating in this reaction all the time, in order to make the Sun as bright as it is!

The Sun consumes 6X1011 kg of hydrogen every second. Once again, we are in a region where it doesn't really matter if you have a good intuition for what the units are, 1011 is a lot of anything.

There is enough hydrogen in the Sun to keep it burning at this rate for 100 billion years at this rate. But you've probably heard at some time that the lifetime of the Sun is 10 billion years, and we're halfway through it... How can this make sense? Well, not all of the Sun will be fused into helium. Only about 10% of the hydrogen in the Sun will reach temperatures and pressures high enough to be fused. So the Sun will live for approximately 10% of 100 billion, or 10 billion years.

Recall those neutrinos. They could pass through three light years of lead without even noticing. Do you think they care about a few light seconds of star stuff? No. They don't. It takes a typical neutrino only 2 seconds to get out of the Sun, and about 8 minutes to cross the distance between the Earth and the Sun. Neutrinos are the best probe of what's happening inside the Sun NOW. But there's a problem. If neutrinos don't care about ordinary matter, how can we catch them? They won't interact with photographic film, or digital cameras, or eyes. So what do we do? Homestake experiment.

600,000 gallons of cleaning fluid buried underground (I suspect I kept saying 600,000 tons. No matter. It's a lot.). If 1016 neutrinos pass through you every second, even more of them pass through all this cleaning fluid. Once every 12 hours, a neutrino interacts with a chlorine atom, and turns it into an argon atom. Every 60 days, some poor slob grad student has to count 'em up.

Only about 1/3 as many neutrinos were found as were predicted. Two possible solutions to this disagreement between theory and experiment:

  1. We don't understand the Sun. Perhaps, for example, the Sun turns on and off, and so is only producing neutrinos 1/3 of the time. But we can't think of any reasonable way to make this happen. But maybe we're just missing something. Or maybe, it's solution number 2:
  2. We don't understand neutrinos. Turns out that this was true. There are actually three kinds of neutrinos, and Homestake could see only one of these, but the Sun produces all three kinds. Their names are 'electron', 'muon', and 'tau' neutrinos.
A second experiment 'Super-Kamiokande' was invented to look for these other kinds. It can see 'electron' and 'muon' neutrinos. But this experiment found slightly less than 2/3 of the predicted number of neutrinos. This may be because the 'muon' and 'tau' neutrinos are two forms of the same neutrino, and oscillate back and forth between types. As a neutrino travels through space, it spends some of its time as a muon neutrino, and some of its time as a tau neutrino.

By the way, Super-Kamiokande also could detect the direction from which the neutrinos were coming, which made it possible to figure out which ones came from the Sun, and which ones came from somewhere else. This lowered the error in the experiment significantly. All of the following movies and images were taken with SOHO, a space-based telescope placed between the Earth and the Sun at exactly the point where their gravitational pulls cancel out. This means the satellite will stay there, in that one spot indefinitely, always looking at the Sun, always transmitting cool pictures back to Earth!

SOHO actually exists for a practical purpose. Eruptions from the Sun can disrupt or destroy satellites, such as spy satellites, weather satellites or communications satellites. It's important to know what the 'weather' is in space, so that we know to turn off the satellites if they are at risk.


Concept Question 1:
What is the temperature of the photosphere of the Sun?

  1. 58 K
  2. 580 K
  3. 5800 K
  4. 5,800,000 K

Concept Question 2:
Are sunspots hotter or cooler than the photosphere?

  1. hotter
  2. cooler

Concept Question 3:
Is the corona hotter or cooler than the photosphere?

  1. hotter
  2. cooler

Concept Question 4:
The Sun is a round ball of gas. Near the center, you see more deeply into the Sun than near the edge. Does the photosphere increase or decrease in temperature as you go deeper into the Sun?

  1. increase
  2. decrease

Concept Question 5:
In 2001, the Sun was at maximum. When will aurorae next peak?

  1. 2006
  2. 2009
  3. 2012
  4. 2023

Concept Question 6:
Why do some astronomers claim the solar cycle is connected to the Earth's climate?

  1. Every 11 years we get hotter weather.
  2. Every 11 years we get cooler weather.
  3. Unusually low cycles match unusually cold climate.
  4. Unusually high cycles match unusually cold climate.

Concept Question 7:
Nothing can travel faster than light. Yet, neutrinos beat photons out of the Sun. How?

  1. Neutrinos are a special case. They DO travel faster than light.
  2. Neutrinos tunnel through other dimensions.
  3. Neutrinos do not travel through space.
  4. Neutrinos travel in a straight line through the Sun, unlike light.

Concept Question 8:
About how many Argon atoms does Homestake expect to find after 60 days?

  1. ~10
  2. ~100
  3. ~1000
  4. ~10,000

Concept Question 9:
About how many Argon atoms did they actually find?

  1. ~3
  2. ~10
  3. ~30
  4. ~300

Jan 10: the corona

Jan 10: ultraviolet (prominences)

The Sun 'at night'

Two comets
CMEs and p+ showers (Apr, 01)
Solar Wind Interaction with Earth

Wide field view of an eruption.

Another of the same.

Another big eruption.
An Earth-directed CME And in the wide-field UV
A 'Sun-Quake'
Aurora movie
eruption movie prominence sequence
Flare with field lines from TRACE Positron-electron annihilation SOHO-MDI_views_the_sun, 3 months in 2001 Spinning sunspot Sunspot number and UV Flux
The Sun at different temperatures A very cool eruptive prominence in 1998 MPEG Movies: