The Sun is a giant self-luminous ball of hydrogen and helium gas that radiates light and energy through the process of nuclear fusion. The Sun is a star. It is the nearest star. It is so incredibly powerful that it banishes nighttime and turns it into day. When the Sun goes on the other side of the Earth, night falls, and the other stars become visible. We can see the Moon and other planets in our solar system because the light from the Sun reflects off them. We see other stars because they generate their own light. Stars are Suns All of the other stars in the night sky are like the Sun, but so far away they appear only as points of light. Some are much larger than the Sun, but most are smaller. They all produce light in the same way as the Sun. Stars are born out of the gas and dust in nebulas. This matter contracts under the influence of gravity, and as it gets denser, it heats up. Eventually it is dense enough, and hot enough, for nuclear fusion to ignite, and the star emits light. Stars and Their Neighbors
Stars usually form in clusters. As they live their lives over billions of years, sometimes they move out of the clusters and live alone in space, like our own Sun. Sometimes stars form into larger groups called globular clusters where the stars are very densely packed in the core of the cluster. Stars and clusters and nebulas are also part of a gigantic assembly called a galaxy. A galaxy like the Milky Way, the one our own Sun belongs to, contains hundreds of billions of stars. There are hundreds of billions of galaxies in the universe.
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