Many stars are massive celestial bodies made mostly of hydrogen and helium. They generate energy through nuclear fusion, which produces light and heat. Our own Sun is a star, and it is just one of the billions of stars in our Milky Way galaxy. Stars come in various sizes and colors, ranging from small, cool red dwarfs to massive, hot blue giants. They are often grouped into constellations, which are patterns that help us navigate the night sky. Over time, stars can evolve, change, and even explode as supernovae, enriching the universe with new elements.