So what is the nearest star to Earth? If you answered Alpha Centauri, you're wrong. And if you're smugly thinking that Proxima Centauri is closer, your right (it is closer) but it's still not the closest star to Earth. That honor goes to...the Sun. Our star is a rather ordinary star in many ways. It is special to us because it is so close. Nearly all life on Earth depends on the Sun. But what makes the Sun shine?
What Makes the Sun Shine?
People have probably wondered about this as long as there have been people to wonder. By the middle of the 1800s, the size of the Sun was known well enough to estimate how long the Sun could burn. The first idea was what if the Sun was made of nothing but pure carbon (the chemical found in coal and gasoline which makes them burn) and oxygen? In this case, the Sun would burn for only about 1,500 years.1 Another idea was that the Sun was giving up it's energy from gravitational collapse. When an object is compressed (squeezed) together, it gets hot. Gravity can do the squeezing. In 1847, a man named Hermon von Helmholtz calculated that if this were what caused the Sun to shine, it would last for about 20 million years. That was the best idea anyone had for about the next 50 years.
Fusion just means joining things together to form a new thing. The glue that is used to attach parts of a plastic airplane model melt the plastic parts a little so they are fused together like one part. For the Sun, fusion is what makes it shine, but the parts that get fused are atomic nuclei (nuke-lee-eye, the plural form of nucleus), specifically, the nuclei of hydrogen.
Hydrogen is the simplest atom. It has one proton, a positively charged particle in its center, which we call the nucleus. It has one electron, a negatively charged particle, orbiting around the proton. The center of the Sun is very hot and the pressure is also very large. This high pressure and temperature squeezes the protons and electrons very close together. Sometimes they get close enough that they can fuse together to form a helium nucleus. A helium nucleus is made of two protons and two neutrons. Where did the neutrons come from? Well, it turns out that sometimes, when you're squeezing electrons and protons close together, they can combine to form a neutron. It's a little more complicated than that, but that's the basic idea. The recipe in the middle of the Sun is something like this: take four protons and four electrons, squeeze them together really hard, and sometimes you end up with a helium nucleus (composed two protons and two neutrons all stuck together) and two left-over electrons.
If you add up the mass of the original four protons and four electrons and compare them with the final mass of the helium nucleus and two electrons, there's are tiny bit of mass missing. Where did it go? Albert Einstein had the answer: E=mc2. The missing mass ("m" in his equation) was converted into energy ("E" in his equation). The process is called nuclear fusion. All stars "burn" by this process, converting hydrogen to helium, and the helium into other elements all the way up to iron while producing energy that makes them get very hot and shine.
A Very Hot Onion
It is only in the very center of the Sun that the temperature and pressure are high enough to cause nuclear fusion. A little further out, the Sun is still very hot, but not quite hot enough for fusion. Each of the regions on the inside of the Sun that all behave a little differently.
First there is the core, the central part where nuclear fusion is really taking place. This region is the source for all the energy that has to works its way out of the core to the outer edge where it escapes into space to provide us with light and warmth.
Second is the radiative zone. This is where the energy radiates or spreads outward by having the atoms of hot gases (mostly hydrogen) bump against each other and emit electromagnetic radiation that is captured by the next atom over.
Third is the convective zone. This is where there the hot gases churn and move around much like the water in a boiling point. Instead of the heat moving outward by electromagnetic radiation, it is moving outward because the hot gases themselves are moving.
Fourth is the photosphere. The prefix photo just means "light." This is the layer of the sun that produces the light we see. This layer can be thought of as the Sun's "surface;" but the Sun doesn't really have a surface like Earth since it is not really solid. Sunspots, which are places where the Sun is actually a little cooler, appear on the photosphere. This part of the Sun is about 5,800ºC. The sunspots can be much "cooler" at a mere 3,700ºC!
Fifth is the chromosphere. The prefix chromo just means "color." This region can only be seen here on earth during an total solar eclipse. During a total eclipse of the sun, around the edge of the moon you can see the sun's colorful red edge sticking out. There is a good picture of the Sun's chromosphere (http://apod.nasa.gov/apod/ap040611.html) taken during the last time Venus appeared to cross in front of the Sun.
The sixth and last region is the corona. The word corona means "crown" and you can think of this as the part that sits on "top" of the other layers like a crown sits on top of a king or queen. The corona can also only be seen during a total eclipse of the sun as rays or streamers that seem to pour out from the Sun just above the chromosphere. There is a good picture of the Sun's corona (http://apod.nasa.gov/apod/ap010408.html) on the Astonomy Picture of the Day website.