Wikijunior:Solar System/The Sun
What is the Sun?
The Sun is a star — the closest star to Earth. It is a large ball of very hot gas in a (plasma) state. The air we breathe and the helium in a balloon are both gases. It is over 5,500 °C at the surface, and much hotter at the center, about 15 million °C. The Sun is made of mostly hydrogen (70%) and helium (28%). It turns many tons of hydrogen into helium every second, thus creating heat and light.
The Sun makes light and heat that warms the surface of the Earth and allows plants to grow. We can get food from plants, and we can burn wood and other parts of plants to cook, warm our houses, and make cars go. Without the Sun there could not be any life on Earth.
How big is the Sun?
The Sun is very big — much, MUCH bigger than the Earth! Even the mighty planet Jupiter is small by comparison. It is more than a million km (109 Earths) across and contains about 99.86% of the Solar System's mass. If you could stand on the surface of the Sun, you would weigh 28 times as much as you do on Earth because the Sun has more mass and therefore more gravitational pull than earth.
More than a million Earths could fit beneath the surface of the Sun. It doesn't look that big from Earth, though. That's because the Sun is so far away. Compared to other stars, the Sun is about average-sized. There are much bigger stars, and much smaller stars.
A very thin solar wind of charged particles blows from the Sun all the way to the edge of the Solar System. When it gets there, the gases mix with those coming from other stars.
What is the surface like?
The Sun doesn't have a crust like the Earth that you can stand on. The whole Sun is made out of gases, fire, and plasma. The gas becomes thinner as you go farther from the center of the Sun. The outer-part we see when we look at the Sun is called the photosphere, which means "ball of light". We call it the surface of the Sun because that's where most of the light we see comes from. There is actually a lot of material from the Sun above the photosphere, and some of the gas is even blasted away to great distances.
How does the sun make light and heat?
The Sun is the main source of energy for the Earth. This energy is made deep inside the Sun in a process called nuclear fusion. Four hydrogen atoms are fused together to make one helium atom. Some of the leftover matter turns into energy. This is the same way energy is released in a hydrogen bomb.
|The Solar System|
Core: The center of the Sun is very dense. It's about 12 times as dense as lead. That means that a gallon of the gas from the core of the Sun would weigh half a ton. It's also very hot — about 15,000,000 °C. This region is where most of the nuclear reactions are taking place.
Radiative zone: In this zone the light and heat produced in the core fight their way out towards the surface. The gases that make up the zone are very dense and keep absorbing and emitting the rays. Have you ever tried to run through water? That's what it's like for light waves in this region of the Sun. Light can't go very far at all before it runs into something. Then it bounces off in a different direction. The light doesn't get very far this way. It can take a single ray of light a million years to get out of this zone.
Convection zone: Have you ever seen the air shimmer above a fire? Perhaps you've been told it's because heat rises? Actually, it is the hot air that is rising. Hot gases get less dense and rise. Cold gases get denser and sink. In this zone, the gases are less dense. They behave like air in a fireplace. Gas at the bottom of the zone gets heated up from below. It rises to the surface, gives off its heat to space, and sinks again. The gas in the convection zone forms currents like those in Earth’s oceans and atmosphere. The currents are called convection cells.
What are sunspots?
Sunspots are dark spots on the Sun, but they are still brighter than lightning. Sunspots look darker than the rest of the Sun because they are a little cooler. Even though sunspots are cooler than the rest of the Sun, they are still hot — about 4000 °C (7000 °F). Sunspots are caused by changes in the Sun’s magnetic field. The magnetic field stops convection, which causes the sunspot areas to cool off and become darker. Sunspots usually form in groups which are carried around the Sun as it rotates.
The number of sunspots we see goes up and down in a cycle of average length about 11 years.
What is the Solar Atmosphere like?
Above the photosphere, the Sun’s gases are not very dense at all. There are two layers that we can see with special telescopes. Above that, gases stream out as solar wind that reaches to the edge of the Solar System.
Prominences and solar flares
If you have a telescope with special filters, you can see bumps around the edge of the Sun. Each one of these is called a prominence. They look like volcanoes erupting. They are hundreds or thousands of kilometers long. Some are bigger than the Earth. They often seem to come from sunspots. Sometimes they get so far away from the Sun that they fly away from it. Then they are called solar flares.
Chromosphere means "color ball". It is just above the photosphere. It is not as bright as the photosphere, and you can't normally see it. But you can see it just before a solar eclipse (only with special filters!). It looks like a flash of all the colors of light. Surprisingly, the Chromosphere is even hotter than the photosphere, at some parts over 20,000 °C.
Corona means "crown." That is what pictures of the corona look like. It is just above the chromosphere. It is hotter than the photosphere, and it glows. It is made of thin gases, and blows away as solar wind. It shifts and changes, but it is hard to see, even with special telescopes.
At the top of the corona, some of the gas blows out as solar wind. It blows fast — about 60 km per second (more than 100,000 miles per hour). But there isn't very much of it. The solar wind is strong enough to push dust and gas away from a comet to make a tail.
The solar wind can even push big things. In 1960, the satellite Echo I was put into orbit. It was a large balloon. Since it was so large and light, the solar wind pushed it around in its orbit. In the future, some space craft may use the solar wind to travel between planets using solar sails similar to the way sailboats use the Earth's wind in their sails to cross the ocean.
thumb|left|200px|Heliopause: where the solar wind hits the edge of the Solar System
Heliopause is where the solar wind hits the wind from other stars. Near here, the solar wind slows down suddenly. In May 2005, the Voyager I spacecraft went through this region and felt a big bump. It is now just inside the heliopause. Because this happens so far from Earth, it is hard to study!
What is solar weather?
Did you know the Sun has weather? Earth weather is what is going on in Earth’s atmosphere. Solar weather is what's going on in the Sun’s atmosphere. Solar weather affects us on Earth. Solar weather (also called space weather) includes sunlight and the solar wind.
Solar flares shoot a lot of very hot gas out from the Sun. If a solar flare is aimed towards Earth, protons — subatomic particles with positive electric charge — might be shot at Earth at high speed, and a solar storm could result. That could cause electrical blackouts or block radio signals. It could damage satellites in orbit. Radiation from a bad solar storm could be very dangerous for astronauts, so they must be protected. The Earth’s magnetic field and atmosphere usually protect us from flares.
Solar flares can also cause an aurora. Auroras look like beautiful curtains of shimmering light. They are called Northern Lights (Aurora Borealis) if they are near the North Pole. They are called Southern Lights (Aurora Australis) if they are near the South Pole. Solar weather affects other planets, too. We have pictures of auroras on every planet except Mercury and Pluto.
Just like we can get Earth weather forecasts, we can get Solar weather forecasts. Forecasters study the Sun to figure out when flares will happen. They try to tell when solar storms will hit Earth. They also try to tell when solar storms will go to other parts of the Solar System.
The life cycle of a star
A forming sun's life starts out as a nebula; this is a cloud of gas made up mostly of hydrogen. Over time the nebula contracts; its core becomes more and more compact and hotter until it can fuse hydrogen to helium. This fusion becomes its power source. After this, the star will look very much like our sun. They fuse hydrogen atoms together to make helium atoms.
After billions of years, this star will die. The star will exhaust its supply of hydrogen in its core, so that there will no longer be any source of heat to support the core against gravity. The core shrinks, and the hydrogen starts to burn in a shell around the core. The outer layers of the star swell and cool, creating a red giant. This is all with an ordinary star and will be the life cycle of our sun, although the process may vary from star to star. When our sun becomes a red giant it will consume Mercury and Venus, and possibly the Earth. The sun will then begin a cycle of expansion and contraction, puffing away its outer atmosphere as the solar system becomes a planetary nebula. Once the sun can not longer sustain fusion, it will collapse down and condense into a white dwarf star. At this stage the light it emits is the left over energy of our dead sun, radiating the residual heat until eventually the white dwarf cools and darkens.
Exploration of the Sun
Early space probes designed to collect information about the sun were NASA's Pioneers 5 through 9 and the Helios 1 and 2. Those were in the 1950s, 60s, and 70s. They collected a lot of data about the Sun. Other projects observed the Sun from Earth orbit, such as a Japanese satellite launched in 1991 called Yohkoh, which means Sunbeam in English. It told scientists more about solar flares, and activity on the sun's surface. Thanks to Yohkoh, they knew more about how to classify solar flares, and whether or not they will cause electrical disturbance on Earth or not. Two important missions to study Sun are called the Solar and Heliospheric Observatory (SHO) and the Solar Dynamics Observatory (SDO). They have taken many pictures of the Sun, along with discovering many comets near the Sun. All of these observations were across the Sun's equator. The first space probe to observe the Sun's poles was named Ulysses, after a famous Greek king who traveled on a very, very long voyage. The Ulysses probe went all the way to Jupiter before approaching the Sun. It took a famous picture of the comet Shoemaker-Levy-9 colliding with Jupiter.
NASA. The Chromosphere — 
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