Stars and galaxies

Science

Definition

Stars are massive balls of hot gas that produce energy through nuclear fusion, primarily converting hydrogen into helium. Galaxies are enormous collections of stars, gas, dust, and dark matter bound together by gravity, ranging from thousands to trillions of stars.

Examples

  • The Sun, a medium-sized main sequence star, fusing 600 million tons of hydrogen per second
  • The Milky Way, a barred spiral galaxy containing roughly 200–400 billion stars
  • The Andromeda Galaxy, the nearest large galaxy to the Milky Way at 2.5 million light-years away
Key Fact

Hertzsprung-Russell diagram classifies stars by luminosity and temperature; main sequence stars fuse hydrogen into helium

Related Terms

Solar system Fission and fusion Electromagnetic spectrum Nuclear physics

Study This Concept

Practice stars and galaxies with free review games in these units:

Earth Science Unit 8: Astronomy and Space