Welcome back from break! Everyone tan, rested, and ready? I hope so!

First off, I'm glad that we weren't all wiped out by an asteroid! We're going to pretty much skip over the chapter (Chapter 12) that goes in detail into asteroids, comets, and the risk of something crashing into the Earth. An asteroid crashing into the area that's now Mexico may well have caused the extinction of the Dinosaurs 65 million years ago.

Here is a plot of where asteroids are in general. Note that most are between Mars and Jupiter (in green), that some are 60 degrees away from Jupiter (in the specially stable parts of its orbit--these are the Trojan asteroids), and the asteroids with red dots are the "Apollo" asteroids that go closer in than Mars and can approach Earth.

Here is a neat site that lets you see the orbits of the asteroids that may approach the Earth. The orbits are 3-d and you can rotate the viewing to look at them from different angles. The predictions aren't accurate over the long run though--as I mentioned in talking about orbital resonances, the three-body problem is very complicated, so it is hard to calculate the motion of an asteroid far into the future when the asteroid and Earth are both acted on by the Sun's gravity.

In 1908, an explosion rocked Siberia, knocking down a forest of trees. We now suspect that this was an asteroid crashing into the Earth, an asteroid about the same size as the one that just missed on March 8. No crater was formed, so the asteroid probably burned up and exploded before it reached the ground. There are some more speculative ideas about what may have caused the explosion, but an asteroid is probably the best explanation we now have.

Some of the things the Sun does are amazing. Of course it's responsible for all our energy here on Earth, but we'll get to the source of the Sun's nuclear energy in a bit. Then tomorrow I'll talk about sunspots and how they change over time, and how they affect our weather.

First, a fun tour of how the Sun looks:

An image of the Sun in visible light. The sunspots are actually not, but only appear dark against the greater light of the rest of the Sun. Click for more information on the photosphere.

Here is a link to a bigger image of the Sun. Note not only the sunspots but also that the Sun appears darker along the edges (called the limb of the Sun). Why's that? Because at the edge, we look through more of the darker outer layers.

(What do we mean by the edge of the Sun? We call its "edge" the photosphere. That's where we see the light from--it's the place where, statistically, the light was bounced around last. Light from the Sun typically bounces around something like 200,000 years before reaching the surface--this is something you'll explore in this week's lab--the flow of energy from the Sun.)

The Sun's surface actually shows details--granulation, as seen in these movies. Granulation is the result of convection in the outer layers of the Sun's interior.

The Sun's heat is made in its inside, and leaks to the outside as the photons keep bouncing around in a "random walk". So we expect the Sun to be hotter inside than outside. That's why the "limb darkening" makes sense. The higher up layers are cooler. They cause abosorption lines (remember, cool in front of hot blackbody will cause absorption?)

In its center the Sun is about 15 million degrees--very hot! At the surface it is only 5800 K.

But then slightly further up the temperature of the Sun starts to reverse. The photosphere is a few 100 km thick. Just outside is the chromosphere, in which the temperature rises to 6,000 to 20,000 K. We can see emission lines from the chromosphere, because in this case we have a hot gas in front of a cooler blackbody.

Click on the image for more information on the solar chromosphere.

Those dark lines you see in the chromosphere are called filaments, but when viewed from the side are called prominences. They may be tens of times bigger than the Earth. Click here for an amazing image of a solar prominence.

Even higher up, in the corona, the density goes down and the temperature rises to millions of degrees. Why? This was a puzzle for a very long time. Heat flows from hot to cold. The source of heat inside the Sun is near its core, where it is hot and dense enough for nuclear fusion. There's no fusion going on in the corona or chromosphere, so why are they so hot?

This is just a tour for now, so I'll come back and explain later! Once we talk about fusion, then we'll talk about how energy gets out from the inside of the Sun!

Between the photosphere and chromosphere is a very thin layer called the transition region. There are some beautiful movies made by the TRACE satelite (TRACE stands for Transition Region And Coronal Explorer) that you can view here.

But that merely heated up the inside enough to start the fusion going.

What is fusion? How does it give off energy? Why does it have to be hot for fusion to start?

Fusion is when the nuclei of two atoms come together to form a new single nucleus. Remember, atoms are made of nuclei (which contain positive protons and neutral neutrons) surrounded by negatively charged electrons. But in something as hot as the center of the Sun, the electrons are all kicked away from the nucleus by collisions (the gas is said to be ionized.) So you pretty much just have nuclei floating around.

But all nuclei are positively charged. Like charges repel. So it's hard to get them to come together. They come together in the center of the Sun because the high temperature gives the atoms so much velocity that they can overcome that electrical repulsion and get close enough for the nuclear force to pull them together.

Fusion can release a tremendous amount of energy. That's something demonstrated in a hydrogen bomb--a hydrogen bomb works by fusion, and a regular atomic bomb by fission (when heavy atom such as uranium breaks apart, it releases energy.)

Why is this? Why is energy released when fusion happens to light elements like hydrogen, but fission happens to heavy elements like uranium?

Well, in a nucleus there are 2 main forces vying for power. All the protons are positively charged, and positive charges repel. So a nucleus would blow apart, if it weren't for another force, the nuclear force (actually the strong nuclear force--there are 2 varieties) that keeps it together. But the nuclear force gets especially weak over longer distances. (If you want to be mathematical about it, the nuclear force is behaves similarly to the gravitational force, but contains a factor of e^-r/R, where r is the separation of the particles and R is a distance, called the range of the force. The mathematical form of the force is also called the "Yukawa potential".)

So you have a nucleus held together by this nuclear force that gets weak over long distances, and pushed apart by electricity, which stays stronger. Result: when you increase a small nucleus, you are also increasing how tightly it is bound together. In a small nucleus, each particle can feel each of the others, and increasing the number increases the cohesion. But in a large nucleus, so many particles are far away that they are beyond the range of the attractive nuclear force and only the repulsive electrical force adds up.

A graph of how much energy per proton or neutron each nucleus has holding it together. Click for more details (but it gets a bit mathy.) This is extra detail for those who like it: In a star like the Sun, fusion operates to convert 4 hydrogen nuclei into one nucleus of Helium. It does this in several stages (and in fact there are other entire "branches" by which this can happen, similar but slightly different.) This method is called the "p-p" chain, because it involves protons kicking into other protons.

First step: p+p-->Deuterium (a nucleus with one proton and one neutron, an isotope of hydrogen) and a positron (an antimatter version of an electron) and a neutrino. The positron can then bump into an electron--they destroy each other giving off energy

Second step: p+D-->He(3)+photon (He (3) is a version of Helium with 2 protons and 1 neutron)

Third step: He(3)+He(3)-->He(4)+p+p