Why are astronomical objects usually spherical?

14 March 2010

Question

As I stared at the perfect circle of a setting sun, I wondered why all stars, planets etc known to us (or me) are perfect spheres, rather than elliptical or totally irregular shapes. Or are there some out there which aren't spherical that I don't know about??? Eager to have an answer and just love the show which makes science accessible to all, including 40 something, middle-aged Mums like me!

Answer

Chris - It's a very good question and there's obviously some important science there for the simple reason that it doesn't matter what you look at, most of these objects are round, absolutely. The simple answer is it's down to gravity. If we take our own solar system as a really good example, you have the Sun in the centre and that's round, and the reason that's round is because a big ball of gas collapsed in on itself and squeezed hard enough to start fusion. Around that ball of gas would've been initially an envelope - like a shroud - of material comprising dust, debris and gas, which slowly condensed into a disk called a proto-planetary disk. Over time, all of this material, thanks to gravity - which is a function of mass - would've pulled this material slowly together. It would have accreted - or got together - and slowly would've built up planetessimals, miniature planets, and then they grew to make big planets as they hoovered up - under increasingly powerful gravitational fields - the rest of that residual material.Because gravity is pulling things together, everything that's being attracted wants to get us close to everything else as it can. The most effective way for that to happen is if objects are spherical. It's the same as a raindrop, because water wants to get us close to other water molecules as it can without being in contact with too much air. That's why raindrops form round blobs, not a flat sheet of water - this way, as many water molecules can get as close together and stick together as they can. That's also what's happening with these nascent planets, or other objects in space. The material squeezes together, and the way in which you can get as much material in as close a configuration as possible to other material is if it's a round shape.Now, if you go in at high resolution and look closely, obviously, there's not a perfect round surface because the Earth has mountains and things and so does Mars - it's got Olympus Mons, a giant volcano. But, to all intents and purposes, at low resolution, these things are round because gravity has made them that way.

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