In this topic you will learn about the billions of stars of which the Sun is one.

A star is a large, hot ball of gas, which is held together by gravity, and gives off its own light. Stars are actually at different distances from Earth.

How do scientists know a star's distance? If a star is close, they view it from different points in Earth's orbit. The star appears to shift its location compared with other stars farther away. This apparent shift of an object's location when viewed from two positions is called parallax. The closer a star is, the greater the shift. By measuring the shift, scientists can tell how far the star is.

Astronomers use a unit called the light-year to describe distances in space. A light-year is the distance light travels in a year, or about 9 trillion kilometers. The easiest way to find a star is by looking for its constellation. A constellation is a number of stars that appear to form a pattern. Rigel is a star in the constellation Orion.

Some stars are brighter than others. Sirius, in the constellation Canis Major, is the brightest star in the winter sky. It appears brighter than Rigel. However, Rigel is actually a brighter star than Sirius. The reason Sirius appears brighter is that it is closer to us than Rigel. The word magnitude is used to describe the brightness of a star. It can describe the actual brightness of a star, the absolute magnitude. It can also describe the apparent magnitude, the brightness of a star as you see it in the night sky. That depends on how much light it gives off and how far away it is.

A star begins its life as a nebula, an enormous cloud of gas and dust in space. As time goes by, gravity causes the nebula to contract. As the cloud shrinks, it heats up. Eventually the center reaches a temperature of several million degrees. At these temperatures nuclear reactions occur. Four hydrogen atoms fuse and form one helium atom. Energy is released. During its life, a star may become unstable; it may develop into a supernova—a star that explodes. A large, massive star that becomes a supernova may form a black hole. A black hole is an object whose gravity is so strong that light cannot escape it.

The Sun is the closest star to Earth. It contains 99.8 percent of all the mass in the solar system. The Sun is made of mostly hydrogen. The hydrogen atoms fuse together in the Sun forming helium atoms. This nuclear fusion produces the heat and light energy that Earth needs to sustain life.