The Sun
At the center of the Sun, the temperature and pressure are so great that as hydrogen atoms collide with each other, they stick together, or fuse. This reaction is called nuclear fusion. In this process, four atoms of hydrogen combine into one atom of helium. But interestingly, the helium atom weighs slightly less than the four hydrogen atoms. Some of the matter is actually lost, by being turned into energy.
According to Einstein's famous equation, E=mc2 (Energy = mass x speed of light squared). Since the speed of light is very large number (186,000 miles per second), the speed of light squared is a very large number. So a small amount of mass will produce a large amount of energy. All shining stars use fusion to create light. This is also the same basic process that is used in a hydrogen bomb. Every second, about 700 million tons of hydrogen is fused into 695 million tons of helium and 386,000,000,000,000,000,000 (386 quintillion) megawatts of energy are released. Even though that's a lot of matter being destroyed, the Sun is so massive that it will last for a very long time. In fact, we think the Sun (and the solar system) is about five billion years old and has enough fuel to last another five billion years.
On the surface of the Sun, there are huge storms that we call Sunspots. They look dark, but they are really very bright. They look dark in photographs because they are cooler than the surroundings. Their temperature is around 3800 deg C (6800 deg F), compared the to Sun's surface temperature of 6000 deg C (11,000 deg F).
Sunspots are created by interactions with the powerful magnetic field of the Sun, which extends beyond the orbit of Pluto (the sun is like a huge bar magnet, surrounded by a huge magnetic field). The number of Sunspots increases and decreases in a cycle of 11 years on average. The spots are caused by changes in the Sun's magnetic field, which has a 22 year cycle. There is evidence that sunspots can affect radio transmissions, magnetic storms, the auroras, and even the climate on Earth. Surrounding the sun is the corona, a layer of very hot gases. It extends into space for millions of miles and is over 1,000,000 degrees C. Its bluish-white light is readily visible during a total solar eclipse, extending away from the Sun in irregular shapes.
Tremendous eruptions called solar flares eject particles and emit radiation into space. These solar flares can last up to four hours. Temperatures within these flares can reach five million degrees C. These eruptions shoot charged particles into the solar system.
Some of these particles eventually enter the Earth's magnetic field. As they spiral down through the atmosphere toward the earth's north and south poles, they interact with the atmosphere to form glowing bands and curtains of light, known as the beautiful Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights). When it runs out of hydrogen, the outward pressure from the nuclear fusion reactions will no longer be there to balance the inward pressure of gravity. The Sun's outer layers will collapse inward until hydrogen in a layer outside the core ignites, fusing into helium and forcing the outer layers of the Sun to swell and cool until the Sun becomes a red giant star. As a red giant, the Sun will become so huge that it will swallow up Mercury & Venus. This inward compression will raise the temperature and pressure in the Sun's core, so that the helium atoms now begin fusing into carbon atoms. When the helium is used up, the Sun will not have enough mass to fuse carbon into heavier elements. The Sun will collapse down into a small, white dwarf, about the size of the earth. In this process it will blow off its outer layers in a colorful shell of gas called a planetary nebula. (Planetary nebulae have nothing to do with planets; they get their name because their round shape can resemble the round shape of a planet as seen through a small telescope.) No longer capable of fusion, our white-dwarf Sun will slowly cool down to a black dwarf, a cold dense object that no longer shines.