28 May 2010

Solar System Tour: The Sun

Posted by Ryan Anderson

Everyone knows about the sun, it’s that really bright thing that rises every morning and sets every evening. Not everyone knows much about it though. For example, did you know the sun is actually a star? Ok, so maybe you knew that. But if you’re so smart, what’s it made of? The sun is almost entirely hydrogen, with a bit of helium mixed in and a tiny amount of all the rest of the elements, just to spice things up. You might think that a bunch of gas floating around in space wouldn’t hold together very well, but the sun is HUGE! It weighs 300,000 times as much as the earth! Because it is so big, gravity holds it together very nicely. In fact, gravity squeezes the center of the sun together so hard that the hydrogen atoms stick together to form helium in a process called fusion. Here’s a diagram of what happens during fusion:

It looks sort of complicated, but the important thing to know is, four hydrogen atoms get turned into one helium atom, and in the process a LOT of energy is released. All the energy being released in the center of the sun has to go somewhere, and it goes into heating up the center of the sun. The center of the sun is 15 million degrees Kelvin (that’s 27,000,000 Fahrenheit!!)!

At really high temperatures, matter is no longer a solid, liquid, or gas. It becomes a plasma, which means the electrons aren’t attached to the nucleus anymore, and both can just go flying around. Energy from the core of the sun makes its way toward the surface in two ways. The first is radiation, which means it travels as photons (light) through the dense plasma. After traveling by radiation for a while, the energy starts to travel by convection. Convection is what happend when you heat a blob of gas. Hot gas is less dense, so it starts to float upward. Once it floats to the top, the hot blob cools down and begins to sink again. This rising and sinking is called convection. Convection is what causes weather on earth, hot air rises and cooler air rushes in beneath it, making wind. On the sun, we can see convection happening right at the surface. The blobs of hot plasma are called granules. Each granule is about 1/10 the size of the earth! Check them out in this picture:

The bright parts in the center of the granules is hot gas rising, the dark edges are the cool gas sinking back down. This layer of the sun (with all the granules) that you can see is called the photosphere. It has an average temperature of about 5800 K. Most people think of the photosphere as the “surface” of the sun. Above the photosphere is a thin layer called the chromosphere. You normally can’t see this layer because the photosphere below it is much brighter, but sometimes when the moon passes in from of the sun and blocks most of the light (a solar eclipse), you can see the chromosphere glowing red. (Never look directly at the sun, even during an eclipse!!) The chromosphere is hotter than the photosphere, at around 7000K. At this temperature, hydrogen glows red, giving the chromosphere its color.

Above the chromosphere is a wispy layer that extends off into space called the corona. The corona is really hot (more than a million degrees!) but not very dense. It is made of gas and bits of atoms that are getting blown off the sun. These particles are called the solar wind. When they hit the earth’s atmosphere, they make the very highest layers of air glow, causing the northern and southern lights (also called aurora). Telescopes have shown that Jupiter and Saturn also have aurora, so the solar wind keeps going a very long way.

Spots and Loops and Flares, Oh My!

The sun isn’t just a boring ball of plasma, there’s some really amazing stuff going on up there. We said before that plasma is when the bits of atoms can move around however they want, but that’s not the whole story. When things that have a charge (like electrons and protons) start moving around, they create magnetic fields. Those magnetic fields force the plasma to follow them, and end up making some really interesting features on the surface of the sun. Sunspots are the most well known result. What happens is, a bunch of magnetic field lines cluster together and keep the blobs of gas from convection from rising up in that place. The gas that is in the sunspot gets stuck and cools off, so it looks darker. Check out this movie of sunspots forming! (Note: Unfortunately all the movies in this post are .mov format, which I can’t embed. But they are truly awesome, so I recommend clicking the links!)

The powerful magnetic fields on the sun force the plasma to follow them, and make beautiful glowing loops that extend into the corona. The loops can grow to be extremely large. When they are seen in front of the photosphere, they are called filaments, when they are seen at the edge of ths sun in front of space they are called prominences.

These loops contain a lot of energy stored in the magnetic fields. Sometimes the filaments “let go” and plasma goes flying either away from the sun, or is attracted to other magnetic fields on the sun. Check this out! A blob of gas goes flying off and hits some loops, making them shake back and forth. Sometimes, when the loops get crossed or twisted, a huge amount of energy is released in what is called a flare. During a flare, a patch of the sun heats up VERY quickly because of all the energy released. Sometimes this causes huge clouds of hot gas to go flying off the sun.

These clouds of gas are called Coronal Mass Ejections, and can be enormous! Check out the next picture, with a picture of the earth added for scale.