The Sun King of all planets

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The Sun is our nearest star. Because of that, we can study it better than any other star.

And when you do that, you see that the Sun isn't as boring as it may look from earth!

The center of the Sun
The center of the sun is very hot (about 15 million degrees Celsius) and the pressure is immense (about 100 billion times the airpressure here on Earth). Because of that, atoms come so close to eachother that they fuse.

In every second, the Sun spends 700 billion tons of protons (or: Hydrogen) in this way. And only a small fraction (0.7 percent) is turned into light.

Right now, about half of the amount of Hydrogen in the core of the Sun has been fused into Helium. This took the sun about 4,5 billion years.

The photosphere is the thing you see when you look at the sun. The chromosphere lies just outside the photosphere, and is almost completely transparant.The temperature in the chromosphere rises slowly as you go further out; from about 4300 degrees to 8300 degrees at the edge. At that point, the temperature rises sharply.

The corona begins at the top of the atmosphere. Because the corona is so thin, you can only see it during a solar eclipse, when the glare of the Sun itself (its photosphere) is blocked by the moon. The temperature of the corona is very high: About a million degrees. It is so hot that it emits light at X-ray wavelengths; the picture to the right is an `X-ray photo' of the sun.

It still isn't well known how this can be: How can the surface of the sun be `only' 6000 degrees, while the area about it is much hotter? It probably has to do with the complicated magnetic fields of the sun, but the last word hasn't been said about that.

Between the Sun and Earth - Solar wind
A constant stream of particles flies from the sun, with a temperature of about a million degrees. They fly with a velocity of about 450 kilometers per second, and this solar wind reaches out to at least the distance of the Voyager 2 space probe, and that is beyond Pluto's orbit at about 5914 million kilometers.

The origin of the solar wind lies in the Sun's corona. The solar wind blows gas and dust oozing out of a cometary nucleus backward, creating the characteristic `tails' of comets. To study the solar wind, the European space probe Ulysses has been launced. Due to its unique orbit, it can for the first time study the north and south polar regions of the Sun. It has shown that the solar wind is very irregular, and much faster near the polar regions of the sun than at equatorial latitudes.

The sun's influence on earth
Besides the fact that the sun heats up the Earth, there are a few other clear influences.

As we saw already know the number of solar spots isn't constant in time. In fact, between 1640 and 1700 there were virtually no sunspots at all. That period coincides with the `little ice age': It was much colder in that era. This might be coincidence, but it might also be causally related. This is yet unknown.

The Skylab satellite has measured how much energy the Sun radiates in total. That too appeared not to be constant, but variable with a few tenths of percentage points. This can have an influence on earth's climate.

The charged particles of the solar wind are responsible for the northern and southern polar lights, when they hit earth's atmosphere at high speed and make it shine (a bit like a TL-light). This shows that the Sun has a number of influences on earth. This, however, is nothing compared to what will happen in a few billion years...

The future of the Sun (and of the Earth)
In this diagram you can see how the temperature and the luminosity (that's the amount of light radiated, or brightness) changes during the life of the Sun. Astronomers use these types of diagrams a lot, and they are called Herzsprung-Russell diagrams after the two astronomers who invented them.

A At point A, the Sun starts nuclear fusion in its core. This can be called the `birth' of the Sun.
B At this time, about half of the supply of Hydrogen has been used up. This is the situation right now.
C The Hydrogen in the core of the Sun has all been used, and the fusion of hydrogen in a shell around the core starts. This makes the sun swell up: Its radius becomes about 40% larger and its luminosity twice as bright.
D One and a half billion years later, the surface of the Sun is 3.3 times the size it is now, and its temperature about 4300 degrees. As seen from earth, the Sun will look like a big orange disk. The problem however is that the temperature on earth has increased by about 100 degrees because of this. So all the seas will have evaporated by that time. It can be seriously doubted if any people would survive to see this spectacle... Within 250 million years, the Sun will grow 100 times larger than it is now, and 500 times more luminous. From the lava-seas of the then molten earth, the Sun will take up almost half of the sky.
E The core temperature of the Sun has risen so high that in one bang, all the rest of the helium will fuse into carbon. By this explosion, probably one third of the solar envelope will be thrown out into space,

What happens after that is not well known at this time. The sun will become brighter, and the outer layers will be blown out into space in the form of a very dense solar wind. This is what is called a planetary nebula, and an example of this is seen to the right.

After that, only a white dwarf remains, with a mass of about half of the mass of the current sun, but with an enormous density of 2 tons per cubic centimeter (imagine compressing a car or two into a thimble.. ). This white dwarf will slowly cool down. This is the end of the solar system: A black dwarf, with the ashes of the planets circling around it. But that's a long time from now!!

The Sun - King of all planets