A star is a huge glowing ball of hot gas. Deep inside its core, hydrogen atoms smash together, forming helium and releasing huge amounts of energy that heats the gas. This is called nuclear fusion, and it's why a star shines. As the hot gas pushes outward, it opposes the inward pull of gravity. This balance of forces is what makes a star, a star. It holds the star together and keeps it at a steady temperature for most of its life.

Image credits: main image, © AMNH; Mordecai-Mark Mac Low: courtesty of AMNH / D. Finnin.