Ecliptic planes and Galactic Planes: Why?

Druk

Thanks to the wonder of the Internet, I've been checking out something that's pretty interesting. I always thought the disc-shaped solar system, and disc-shaped galaxies, were just representations of a likely spherical group of objects. My question is: why?

An hour-long Google session didn't really help. Mostly the only information I can find on the subject is definitions of what the ecliptic plane is, or just statements that basically amount to "it happens during formation of the systems", which isn't really an explanation.

I assume there's some sort of mathematical explanation for why massive groups of rocks in the universe tend to circle each other within a plane; what is it?

TychoCelchuuu

I Googled "why are galaxies flat" and I was about to link you a bunch of stuff I found but I think it's more efficient to have you sort through it. This thing is pretty, though.

Druhim

You're not going to get a simple mathematical explanation for something as complex as the orbits of all the bodies in a solar system or galaxy. And of course, not all orbits in the system are actually on the same plane, they just statistically tend to fall on or near a plane, but there are of course outliers. So it's a statistical representation, not an absolute fact for all bodies in orbit around a point.

RiemannLives

When you take a particle system operating under gravity (eg: gasses in a nebula, stars in a galaxy) and make them spin they will end up in a flat(ish) spinning disk shape.

It is the spin of a galaxy / star system that results in them being "flat".

edit: Try and find a copy of Neil Degrasse Tyson's series of lectures "My Favorite Univerise". One of them explains this very well.

Mojo_Jojo

Druk wrote: »

Thanks to the wonder of the Internet, I've been checking out something that's pretty interesting. I always thought the disc-shaped solar system, and disc-shaped galaxies, were just representations of a likely spherical group of objects. My question is: why?

An hour-long Google session didn't really help. Mostly the only information I can find on the subject is definitions of what the ecliptic plane is, or just statements that basically amount to "it happens during formation of the systems", which isn't really an explanation.

I assume there's some sort of mathematical explanation for why massive groups of rocks in the universe tend to circle each other within a plane; what is it?

They're not all on quite the same plane, but the general idea is that if you take these sphere of stuff and drop a line though its centre around which it rotates, then the mass gets thrown to the outermost points. which is the plane perpendicular to the axis at its centre.

It's hard to explain without a diagram.

Demerdar

Conservation of mass and angular momentum will answer your question.

Lykouragh

The same reason you are lighter at the equator than you are at a pole. If you take a globe of dust in space and spin it, it makes a disk.

Demerdar

^ if the rate of spin is sufficient enough to outweigh the force holding that mass together (mostly gravity). There's a reason why Earth (and pretty much most planets) are spherical and not disc shaped. In short, those galaxies are rotating really fast.

Smasher

Demerdar wrote: »

^ if the rate of spin is sufficient enough to outweigh the force holding that mass together (mostly gravity). There's a reason why Earth (and pretty much most planets) are spherical and not disc shaped. In short, those galaxies are rotating really fast.

Although even Earth is a few dozen miles shorter pole to pole than it is equator to equator.

Iceman.USAF

Take some pizza dough. Ball it up. Toss it in the air and spin. What happens?

Same idea, generally.

Beasteh

seconding looking up Neil Degrasse Tyson, I could (and often do) listen to that guy talk all day

he has more than enough material to keep you occupied for weeks on end, and I have personally never been bored by his lectures