Stars are giant balls of plasma that create energy by squeezing atoms together.

Stars are mostly made of hydrogen and helium. In 1925, Cecilia Payne-Gaposchkin discovered this composition while studying the light from stars. The center of a star like our Sun reaches 15 million degrees Celsius. At this heat, atoms lose their electrons and become plasma, which is the fourth state of matter.The Sun turns 600 million tons of hydrogen into helium every second. This process is called nuclear fusion. It creates an outward pressure that stops the star from collapsing. Gravity pulls inward while fusion pushes outward to keep the star stable. This balance is called hydrostatic equilibrium.The Sun holds 99.8% of all the mass in our solar system. Blue stars are the hottest and can reach 30,000 degrees Celsius. Red stars are the coolest at about 3,000 degrees Celsius. Most stars in our galaxy are small, cool stars called red dwarfs.Energy created in the core takes over 100,000 years to reach the surface of the Sun. Once it leaves the surface, it travels at 299,792 kilometers per second. This light can travel for millions of years before it reaches our eyes on Earth.