A star is a massive, luminous ball of plasma. Stars group together to form galaxies, and they dominate the visible universe . The nearest star to the Earth is the Sun, which is the source of most of the energy on Earth, including daylight. Other stars are visible in the night sky, when they are not outshone by the Sun. A star shines because nuclear fusion in its core releases energy which traverses the star's interior and then radiates into outer space . Without stars, life on Earth and most atomic elements would not exist.

Starlight comes from the several hundred billion stars in our single galaxy, as well as from the stars in the billions of galaxies in our universe. As it takes time to get to us, we see the sun as it was eight minutes ago, the nearest night stars as they were four years ago, and the Andromeda galaxy as it was 2 ½ million years ago. The light from some stars has not yet reached us.