Our Sun is a star, just like the stars that twinkle in our night sky. But because the Sun is much closer to Earth than any other star, it appears very large and bright. Without the Sun's light and heat, there would be no life on Earth.
The Sun is a huge ball of gases -- mostly hydrogen and helium, the two simplest and lightest chemical elements. These gases are so hot that they make the Sun glow. The Sun does not "burn" in the same way that a fire burns. Instead, heat and pressure inside the Sun cause atoms of hydrogen and helium to "fuse," or join together, producing nuclear energy.
The Sun and planets formed about 4.5 billion years ago from a cloud of interstellar gas. This cloud gradually condensed to form a "protostar" -- a ball of gas that grew hotter and hotter as it was squeezed by its own gravity. Finally, the temperature at the heart of the young Sun reached 18 million degrees Fahrenheit (10 million degrees centigrade) -- hot enough for nuclear reactions to occur -- and the Sun was born. Enough hydrogen remains in the Sun's core to keep our star shining for another five billion years or more.
The Sun consists of a series of layers. From outside to inside, they are.
Corona - The outer atmosphere Although it is extremely hot, gas in the corona is spread very thin, so the only time we can see the corona with our eyes is during a total solar eclipse, when the Moon completely covers the Sun's disk. The Sun's powerful magnetic fields may be responsible for heating the corona to millions of degrees. Chromosphere - The inner atmosphere Huge flares and loops of hot gas often shoot into the chromosphere, extending tens of thousands of miles above the Sun's surface. These flares shoot electrically charged particles into the solar system; when they reach Earth, the particles can disrupt radio and television signals and cause colorful displays known as the "aurorae" -- the northern and southern lights.
Photosphere - The Sun's visible surface Almost all of the visible light from the Sun comes from the photosphere. Although it is very hot (about 10,000 degrees F; 5,500 C), the photosphere is much cooler than the Sun's inner layers. Cooler, dark blotches called sunspots sometimes appear in the photosphere. Most of these giant magnetic storms are larger than our Earth. The number of sunspots increases and decreases every 11 years, although astronomers are not sure why that is so.
Convective zone - The Sun's "mixing pot" The process of convection -- the same process that makes a pot of soup boil -- carries energy from the Sun's core to its surface. Detailed pictures of the photosphere, which is at the top of the convective zone, show large bubbles of hot gas welling up from deep inside the Sun.
Radiative zone - The first step outward Energy from the core "radiates" outward through this layer of hydrogen and helium gas into the convective zone.
Core - The Sun's power plant Hydrogen inside the core is packed so tightly, and the temperature is so hot, that individual atoms ram into each other, forming heavier helium atoms and releasing energy in the process. This energy takes many thousands of years to make its way to the photosphere and out into space.