The universe is mostly empty space. We are most interested in planets and stars (suns), because they make it possible for us to live. You are standing on a planet we call "Earth," and our Sun is a medium-sized star.
But there are many other things in the universe.
Moons and Planets
Moons and planets are very similar. They can be solid, liquid or gas, or a combination of two or three of these.
Planets orbit (revolve around) suns, while moons orbit planets. Read more...
More to Come...
Future articles will include "Suns and Stars," "Nebulas and Clusters," "Galaxies and Galactic Clusters," "Black Holes" and more.
Title image courtesy NASA. It shows the spiral galaxy, M51.