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.

Description: a huge glowing ball of hot gas
Significance: As stars live and die, they produce almost all of the elements in the universe.
Observed: Astronomers use telescopes on Earth and in space to study all wavelengths of starlight.
Number in Milky Way Galaxy: More than 200 billion stars (200,000,000,000). Every star we can see with the naked eye is in the Milky Way.
Number in Universe: More than 40 sextillion stars (40,000,000,000,000,000,000,000)!

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