The sun formed from a giant, spinning cloud of gas and dust called a nebula. Today it is a star at the center of our solar system. Its gravity holds the solar system together. The sun is a yellow dwarf, a medium-size kind of star. There are billions of stars like it in the Milky Way galaxy. It is a huge ball of gas. By mass, about three quarters of the sun is hydrogen, and helium makes up almost all of the rest. It is not a solid ball, and you could not stand on its surface.

Now let's look at how big the sun is and how far away it is. The sun is about 100 times wider than Earth. It would take 1.3 million Earths to fill it. It is also about 93 million miles from Earth, which is roughly 150 million kilometers. Its energy travels to Earth at the speed of light as electromagnetic radiation. The light takes about 8 minutes to get here. Besides light, the sun gives off heat. Its light and heat warm Earth and make life possible. That is a very long trip for light to make.

Next, let's look at how the sun is built. Deep in the core of the sun, nuclear fusion turns hydrogen into helium. This makes the sun's energy. The core is about 27 million degrees Fahrenheit. The inside of the sun has three parts: the core, the radiative zone and the convection zone. The part of the sun commonly called its surface is the photosphere. It is roughly 4,500 to 6,000 degrees Celsius, and NASA gives about 5,500.

The outside of the sun holds a puzzle. The sun has an atmosphere made of the chromosphere and the corona. You might expect the surface to be the hottest place on the sun. But the corona is much hotter than the surface. Scientists are still studying why the corona is hotter than the layers just below it. The sun also has sunspots, which are dark, cooler spots. The number of sunspots goes up and down in a cycle of about 11 years.

The sun sends out a steady stream of charged particles called the solar wind. It fills the heliosphere, a huge bubble of charged particles around the solar system. Solar storms and fast solar wind can cause a geomagnetic storm, a disturbance in Earth's magnetic field. Auroras happen when charged particles collide with the upper atmosphere of Earth. Never look directly at the sun without special eye protection made for looking at the sun. Nothing could live on the sun, but its energy is vital for most life on Earth.

Words to know

• nebula: gas and dust
• photosphere: the sun's surface
• corona: outer atmosphere
• heliosphere: a huge bubble