New videos DAILY: https://bigth.ink
Join Big Think Edge for exclusive video lessons from top thinkers and doers: https://bigth.ink/Edge
Spinning discs are everywhere – just look at our solar system, the rings of Saturn, and all the spiral galaxies in the universe. Spinning discs are the result of two things: The force of gravity and a phenomenon in physics called the conservation of angular momentum.
Gravity brings matter together; the closer the matter gets, the more it accelerates – much like an ice skater who spins faster and faster the closer their arms get to their body.
Then, this spinning cloud collapses due to up and down and diagonal collisions that cancel each other out until the only motion they have in common is the spin – and voila: A flat disc.
Dr. Michelle Thaller is an astronomer who studies binary stars and the life cycles of stars. She is Assistant Director of Science Communication at NASA. She went to college at Harvard University, completed a post-doctoral research fellowship at the California Institute of Technology (Caltech) in Pasadena, Calif. then started working for the Jet Propulsion Laboratory’s (JPL) Spitzer Space Telescope. After a hugely successful mission, she moved on to NASA’s Goddard Space Flight Center (GSFC), in the Washington D.C. area. In her off-hours often puts on about 30lbs of Elizabethan garb and performs intricate Renaissance dances. For more information, visit
MICHELLE THALLER: Rick, you have noticed one of the most wonderful and consistent patterns in the whole universe. The universe is very good at making spinning discs. Our solar system is a disc, and all the planets go around in basically the same plane, and they all go around in the same direction. Why should that be?
There are discs all over the place. I mean, think about the rings of Saturn. The rings of Saturn are also very, very thin, and they all go around in the same direction. Galaxies, spiral galaxies are one big disc with everything moving around a common center. Discs seem to be something that the universe likes to make. And, in fact, that really is true. And it has to do with a number of things. It has to do with the force of gravity and something called the conservation of angular momentum.
Now, gravity is very good at bringing stuff together and bringing it together so it becomes denser and denser and begins to fall into the center. Our solar system formed out of a giant cloud of dust and gas about 4 and 1/2 billion years ago. It was actually many trillions of miles across at first, but it had to get much smaller in order for the densities to get high enough and the temperatures also to get warm enough inside to give birth to the Sun, actually ignite a star. So you have this collapsing cloud of dust.
OK, well, you can sort of understand that gravity wants to bring all that together, but why does it start to spin up? There’s something called the conservation of angular momentum. And that basically says that if anything has any spin at all, even just a little bit of motion, as gravity brings it together and makes it smaller, that spin is accelerated; it’s sped up. And probably the example most people know best of all – you can actually feel this if you want to do this – but an ice skater. If you’ve seen an ice skater do a spin, usually what they do is that they have their arms outstretched, and they’re spinning around relatively slowly. And then they bring their arms in, and they spin faster and faster. It’s kind of amazing that any person can keep their balance when they do that. That is an application of the conservation of angular momentum. You have an extended body, your arms are out, and you’re spinning slowly. In order to conserve the energy in that spin, as that body becomes smaller, the spin goes faster and faster. And so what happens in these clouds is that a cloud usually has just a tiny little bit of a drift velocity. It’s going around the galaxy or maybe a nearby star exploded, and it’s kind of all moving in one direction. The cloud itself has a little bit of velocity as a cloud, as a whole. Particles inside that cloud could be going any which way.
But as the cloud begins to come together under gravity, any little bit of spin gets accelerated, actually becomes faster. And so as the cloud collapses, any little directional drift becomes a spin, and the cloud itself begins to spin around. OK, so that gets you a spinning cloud. Why does it collapse down into a disc? And this is an interesting bit …
For the full transcript, check out https://bigthink.com/videos/why-are-so-many-objects-in-space-shaped-like-discs