The answer to your question is mostly about the
names that scientists decided to give things.
We say a big object in outer space is a star
if it is shining light from nuclear fusion.
This is a really crazy thing to do--it's a nuclear
reaction, not an ordinary chemical reaction. It
takes hydrogen as fuel, puts it under really high
pressure at a really high temperature, and turns
it into a different element, helium. This gives
the star so much energy that it releases a lot of
it by radiating light in all directions.
We say a big object in outer space is a
planet if it is not doing nuclear fusion, and it
is orbiting a star. Planets do not shine light,
they just reflect light that comes from stars.
If you could turn off the light from the stars, it
would be dark and we wouldn't be able to see the
So stars and planets are always separate
things. Why didn't we decide to say "stars are
planets, too"? I don't know for sure, but I
guess it is because stars and planets behave so
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