ELi5 what makes the sun a star and not a planet? And why is the moon not considered a planet?
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Moons go around planets, and planets go around stars.
A star generates light through fusion, planets and moons do not.
The sun is the center of our solar system! Lots and lots of stars have planets that rotate around them, just like ours. Therefore, when you see a star, it’s likely supporting an entirely different solar system with its own planets. Moons orbit around planets—so the scale is smaller. Moons are often just floating aimless space rock that got caught in the orbit of a planet.
We consider the Sun a star and not a planet because it is big enough to sustain nuclear fusion. And we do not consider the Moon a planet because it orbits another planet instead of orbiting a star.
Lots of things, like what they’re made of and how they form.
But in the simplist way.
Planets orbit stars.
Moons orbit planets.
Stars have a life cycle that involves eating their planets before they die.
The sun is a star because it is a massive nuclear reaction and ball of plasma rather than rock or metal (which can be covered in a gas atmosphere).
The moon is not considered a planet because it orbits the Earth. Planets must orbit the sun and not a planet.
Planets must also be massive enough that their gravity pulls them into a sphere – smaller things are asteroids, comets, or dwarf planets. The moon meets that criteria.
A star is a very specific type of object, a massive body of hydrogen and helium gas that has become so massive that it has started to undergo nuclear fusion in its core.
They are usually by far the most massive object in their gravitational vicinity and dominate their systems, but not always. They emit a tremendous amount of energy from this core nuclear reaction.
The moon is considered a satellite because another larger object (the Earth) gravitationally dominates the orbit.
If Jupiter were to gain~13–20 times its current mass in hydrogen, it could become a brown dwarf star.
Basically being promoted from planet to star.
Planets orbit around stars. Moons orbit around planets.
The ancient greeks knew that there are different lights on the night sky and observed them. Those that didn’t move in relation to the other lights were called „aster“ or „star“ in english. What exactly it ment for the ancient greeks, I am not certain. However, those that DID move were called „planētēs“ or „planets“ in english. And that word means „wanderer“.
Now, the greeks didn’t know that they weren’t the same things but moving. But we, today, do.
Stars are currently (or previously) undergoing fusion, this is what gives the sun the energy to be the big bright ball in the sky. Stars are large because they rely on their own gravity to squeeze the hydrogen and cause the fusion. The smallest stars are about 10x larger than Jupiter.
Planets are things which orbit a star, are round, and have pretty much cleared out their orbit. This last bit is so that Ceres (biggest asteroid) isn’t a planet, otherwise it would be.
Dwarf planets are things that directly orbit a star, are round, but may not have cleared out their orbit. This is stuff like Ceres and Pluto and Eris and all the far out little Plutoids.
Moons are rocks which orbit a planet, that’s basically the end of their criteria list. Jupiter has moons which range in size from Ganymede (2.5% the mass of Earth) all the way down to Valetudo which is just 1km in diameter. If it orbits a planet and its natural, its a moon!
There is some science behind the idea that earths moon isn’t really a moon. If you follow some of the definitions of what a planet is, then the earth and the moon could maybe be considered a binary planet system. The moon orbits us, but we also orbit the moon as we are orbiting the sun. All the other answers you are getting about what makes a star a star are exactly right. There is some wonderful language in the definition of a planet about how it has cleared the space around it of all other bodies. We have done that as the earth, but the moon has also done that.
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