Universal spin

04 December 2018


Why is everything spinning in the universe? And if black holes are in the centre of galaxies, and spinning very fast, are black holes breaking up nearby objects and ricocheting them away?


Chris Smith put this question from Shawn to astronomer Carolin Crawford...

Carolin - Well let's leave black holes out things for a minute. Because yes a lot of stuff is spinning in the universe, right on the scale of not just planets and moons and solar systems and stars, but also whole galaxies are spinning. And it’s really a question of something called angular momentum. And this dictates that if you’ve got a body that’s spinning it’s telling you what rate it's spinning in and what direction, and angular momentum is a conserved property. This means that once the body has got angular momentum you can't get rid of it, unless you kind of apply an external force, and the way angular momentum goes if you have something that's very large that is spinning just a little bit and it shrinks down, then it spins faster.

So you've got this conservation, and the nice thing about astronomy and space is that things in space are made from gravity - so you start with a big gas cloud, then a tiny little tug gravity to one side to the other: it's got a very lazy spin. But as it collapses down to form a star or even a big scale galaxy, it starts spinning up faster and faster. So that's the basic reason why so much spins in the universe, because it's more likely that the original gas cloud is going to have the tiniest torque on it than not. So when you get to black holes, we think every black hole spins because they form from massive stars and massive stars are formed from these gas clouds. So if you have a star that’s spinning and then collapses down to a black hole it's going to spin even faster, and black holes can spin incredibly fast.

So going back now to the original question - can they break up nearby objects? Well they're going to break up nearby objects because of their gravity, not because of their spin. And as we discussed earlier it's not very good if you go over the event horizon of a black hole, you get what’s known as spaghettified! You know so things get broken up because of the gravity they're going to get pulled onto the black hole. They're not really going to get ricocheted away.

Chris - In the quiz there was the question that you answered for me about Venus, and you made the point that actually it's turning excruciatingly slowly so a day is almost longer than a year. How did it end up going so slowly then?

Carolin - I mean Venus is basically spinning at walking pace  - the Earth spins about a thousand kilometers an hour, Venus is literally walking pace, not just that it's also going in the wrong direction. This is really cool!

Chris - The sun rises in the west on Venus doesn't it?

Carolin - That's right - rises in the west sets in the east. And so if you look down on all our solar system, the sun is spinning in one direction, all the planets are going round the sun in that direction, all the planets are spinning that direction, just Venus is going the wrong way. So, we don't think it formed different from all the rest of the planets because we assume they just inherit the same rotational motion from the original kind of nebula that collapsed to form the sun and all the planets. There’s something that's happened to Venus subsequently that has slowed down its rotation, even just turned it the other way and it could be due to it's really thick atmosphere. You've got the gravity the sun creates - what we call tidal bulges, it kind of pulls this thick atmosphere in the direction of the sun. And if that is rotating at a different rate than the planet’s underneath rotating friction between the atmosphere and the planet, it just slows it down. We’ve got the same effect on earth, you know between the moon pulling the oceans round to form tides. That's breaking the Earth's rotation slightly every year, but in Venus it's much more extreme. You got this really dense atmosphere and so that is probably the most likely reason for it to have been slowed down and then just slightly started spinning the other way.

Chris - But for the reason you've outlined, something else will have inherited that angular momentum from Venus and something else will be spinning the equivalent amount won't it?

Carolin - Yeah it could be, and it could well be transferred to the sun. So whatever’s perhaps slowing down the angular momentum - you'd treat Venus and the sun as a closed system maybe it's going to spin the sun up slightly.


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