Suns / Moons
Suns are a very important part of a solar system, as they're what the planets orbit around. Our sun, in our solar system, is an average size star of an average temperature; scientifically, it's called a G-class star. Planets in our solar system rotate around our sun, which is at the center of our solar system. It provides the heat that warms our planets, especially those closest to the sun, through a process called nuclear fusion, where it combines atoms to create energy. Also, it's immense gravitational pull is what keeps the planets in orbit around it, though different planets orbit our sun at different rates. When a plane revolves around the sun once, it's known as a year. So, different planets' years are different length of time. Mercury has a very short year of only 88 Earth days, while Neptune has a year of 163 Earth years and 263 Earth days, which is a massive amount of time. The stars / suns planets orbit around can be very different from our own. Some are hotter stars (blue/blue-white), while others can be cooler (orange/red). Some planets even revolve around binary stars - two stars that either revolve around each other or a common object. In this case, the stars either have to be very close together or very far apart, as the planets need room for revolution or they'll be ejected from the solar system all together. Some planets even orbit neutron stars or pulsars, though we've only begun to recently discover these.
Moons are small bodies, quite a bit like dwarf planets, though our moon is larger than Pluto, one of the dwarf planets in our solar system. Unlike our sun, however, the moon doesn't produce light; it appears to be lit through reflection from the sun, as the sun's light shines on the moon and bounces back onto the Earth. Our moon also has a small gravitational pull on the Earth, which creates the tide our oceans have. Each solar system has many moons, including our solar system; so far, astronomers have found 146 moons orbiting our solar system. Another 27 are waiting to be confirmed as moons as well. Gas giants are the planets with the most moons; the first two terrestrial planets, Mercury and Venus, don't have any moons, while Earth has one and Mars has two. Jupiter, Saturn, Uranus, and Neptune, however, have numerous moons, accounting for most of the other moons in our solar system. Jupiter has over 60 and 17 awaiting confirmation, Saturn has 53 and 9 awaiting confirmation, Uranus has 27, and Neptune has 13. Moons have very little gravity, as they're much smaller than planets our, and they usually orbit the planets, not the actually sun of the solar system. At night, we can see the moon in the sky because it reflects light from the sun, creating the moonlight that many originally believed came form the moon itself. So, the moon is simply another celestial body, not an actual star.