1. Universe: The largest entity, encompassing everything that exists, including all galaxies, stars, planets, and matter.

2. Galaxy: A massive collection of stars, gas, dust, and dark matter held together by gravity. Our own galaxy is the Milky Way.

3. Star: A giant ball of hot, glowing gas that generates its own light and heat through nuclear fusion. Our Sun is a star.

4. Planet: A celestial body that orbits a star, is large enough for its own gravity to make it round, and has cleared its neighborhood of other objects. Earth is a planet.