For thousands of years humans have wondered about the beautiful lights in the dark night sky. Over time astronomers began to understand what they were seeing. Today we know the universe contains more galaxies than there are people on Earth. And each galaxy contains hundreds of billions of stars. Some of the stars are so enormous that it would take years for a spacecraft just to get from one side to the other. Some stars are no wider than a small city. Our Sun is about midway between these extremes.
The sun is a star still in its youth. Stars do not exist forever. Like people, they are born and they die. Astronomers have discovered that the entire Universe has not existed forever either. About 15 billion years ago, an enormous explosion called the "Big Bang" created as expanding bubble of energy. In the first few minutes after the Big Bang, with temperatures of at least 18,000,000 degrees Fahrenheit, nuclear reactions converted part of the hydrogen into helium. After one half million years, radiation and matter separated. Galaxies then began to form from the gas. Later, stars were born within the galaxies. One of those stars was the Sun. Astronomers estimate the sun was born 4 ½ billion years ago. Earth and the other planets of the solar system were formed along with the Sun from leftover material.
When you look up into the night sky with the naked eye, everything you see is part of the Milky Way Galaxy, our own star system. The dust within the disk of our galaxy obscures our view of other galaxies and beyond. In all other directions it is just distance that makes them faint. The galaxies are spread very thinly through the emptiness of space. They are swirling, dynamic systems in which matter comes together to form stars, planets, satellites, asteroids, and comets.
Galaxies come in different shapes and forms, most of them being elliptical. Like the Milky Way, most of the galaxies contain thousands of millions of stars. They usually have a concentration of stars towards the center, forming a bright "nucleus." Spiral galaxies have great trailing, spiral "arms" radiating from the center; the spiral arms are made up of millions of stars. Our Sun is such a star, situated in a spiral arm toward the outer edge of the Milky Way Galaxy.
The most massive stars develop a core of iron. When the core begins to cool down, the center collapses. Following the collapse, there is a rebound, which forces a massive, "supernova" explosion that flings the star's matter out into space.
The birth and death of stars is the process that creates the chemical elements that make up everything in our world-including us. Stars operate like factories, creating these elements in the furnaces of their own contractions.
A supernova explosion can have the brightness of a million suns. It can also produce elements like uranium. All of these star-born elements eventually end up floating in space as part of gas clouds. They will in turn be the beginnings of future stars and maybe planets as well. The Big Bang produced hydrogen and helium. All other elements were created subsequently inside massive stars, and also by supernova explosions which dispersed this material throughout the universe. These processes continue to date.