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Why are stars and moons round? And comets and asteroids… not?

I’m puzzled as to why the planets, stars and moons are all round (when) other large and small objects such as asteroids and meteorites are irregular shapes?

— Lionel Young, age 74, Launceston, Tasmania

This is a fantastic question Lionel, and a really good observation!

When we look out at the Solar System, we see objects of all sizes — from tiny grains of dust to giant planets and the Sun. A common theme among those objects is the big ones are (more or less) round, while the small ones are irregular. But why?