A sun is a massive, luminous sphere of plasma held together by gravity, primarily composed of hydrogen and helium. It generates energy through nuclear fusion, where hydrogen atoms combine to form helium, releasing light and heat. This process is what makes the sun shine and provides the energy necessary for life on planets like Earth. There are billions of suns in the universe, and they are often referred to as stars. Each sun varies in size, temperature, and brightness, influencing the characteristics of the planets that orbit them. Our own sun, known as Sol, is a medium-sized star located at the center of our solar system.