A star is a luminous sphere of plasma held together by its own gravity. The Sun is a star. So is every point of light in the night sky, and every star in every galaxy. Stars are the factories that made the elements in your body. They are also the furnaces that light the universe.

Stars form from collapsing clouds of gas and dust. When the core gets hot enough, hydrogen fusion begins, and the star ignites. It spends most of its life on the main sequence, fusing hydrogen into helium. The balance between gravity pulling inward and fusion pushing outward keeps it stable. When the hydrogen runs out, the star evolves, and its fate depends on its mass.

Stars are classified by temperature and luminosity. The spectral types, from hottest to coolest, are O, B, A, F, G, K, and M. The Sun is a G-type star, yellow and middle-aged. O-type stars are blue and massive, living fast and dying young. M-type stars are red dwarfs, small and dim, but they live for trillions of years.

There are more stars in the universe than grains of sand on all of Earth's beaches. Most are red dwarfs. Most have planets. Some may harbor life.