Eli5 Why are galaxies flat instead of being spherical like stars and planets?
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Same reason dollar systems are flat – they are formed as an [accretion disk](https://en.m.wikipedia.org/wiki/Accretion_disk)
Galaxies and solar systems start as clouds of gas and dust. These particles are going in random directions and bounce into each other. When they bounce into each other they transfer momentum, sometimes they stick together. As they stick together they’ll form bigger clumps which will collide and make even bigger rocks, etc.
Throughout this whole process, the total momentum of the system does not change, it just transfers from particle to particle, Rick to rock.
Importantly, while the total momentum of the system is random (as it’s the sum of the momentum of the original randomly moving dust and gas particles) it is not 0, it will have some angular momentum – that is to say the particles at the start are imperceptibly orbiting the center of the overall system.
As the particles and rocks and planets and stars slowly accumulate mass, they will tend to be moving in the same orbital direction as they are made of particles that tended to be orbiting in that general direction.
They won’t all be moving in that direction though, some dust particles will be moving the opposite direction, some rocks will be orbiting perpendicular to the orbital plane, some random asteroid flung from a nearby system may be going the wrong way, but since most of the particles are moving in the same orbital plane, anything going the wrong way will eventually hit something going the predominant direction. If theyre similar sizes, their momentum cancels out and most of the debris from their collision will fall into the center of the system (in galaxies that’s a black hole, in solar systems that’s a star)
Eventually all that’s left in orbit is stuff that happened to have been going the same orbital direction as most of the other stuff in the system
Same reason dollar systems are flat – they are formed as an [accretion disk](https://en.m.wikipedia.org/wiki/Accretion_disk)
Galaxies and solar systems start as clouds of gas and dust. These particles are going in random directions and bounce into each other. When they bounce into each other they transfer momentum, sometimes they stick together. As they stick together they’ll form bigger clumps which will collide and make even bigger rocks, etc.
Throughout this whole process, the total momentum of the system does not change, it just transfers from particle to particle, Rick to rock.
Importantly, while the total momentum of the system is random (as it’s the sum of the momentum of the original randomly moving dust and gas particles) it is not 0, it will have some angular momentum – that is to say the particles at the start are imperceptibly orbiting the center of the overall system.
As the particles and rocks and planets and stars slowly accumulate mass, they will tend to be moving in the same orbital direction as they are made of particles that tended to be orbiting in that general direction.
They won’t all be moving in that direction though, some dust particles will be moving the opposite direction, some rocks will be orbiting perpendicular to the orbital plane, some random asteroid flung from a nearby system may be going the wrong way, but since most of the particles are moving in the same orbital plane, anything going the wrong way will eventually hit something going the predominant direction. If theyre similar sizes, their momentum cancels out and most of the debris from their collision will fall into the center of the system (in galaxies that’s a black hole, in solar systems that’s a star)
Eventually all that’s left in orbit is stuff that happened to have been going the same orbital direction as most of the other stuff in the system
Let’s start by assuming a galaxy starts off sphere-like where stars and planets are orbiting a black hole. Over time, there will be an imbalance as some planets and stars are clumped together in one region of this sphere as orbits go at different velocities. This means that a large of amount of mass is accumulated in that spot. What happens? Gravity happens. So even my miniscule amounts nearby planets and stars would be attracted to this mass. Over a long time this consolidates into a disk-like shape as this mass is concentrated in this equatorial region. Where it may deform is when there is another galaxy nearby, when it is in the process of merging, or post-merger.
The guy who mentioned pizza is fairly good making it easy to understand though.
Let’s start by assuming a galaxy starts off sphere-like where stars and planets are orbiting a black hole. Over time, there will be an imbalance as some planets and stars are clumped together in one region of this sphere as orbits go at different velocities. This means that a large of amount of mass is accumulated in that spot. What happens? Gravity happens. So even my miniscule amounts nearby planets and stars would be attracted to this mass. Over a long time this consolidates into a disk-like shape as this mass is concentrated in this equatorial region. Where it may deform is when there is another galaxy nearby, when it is in the process of merging, or post-merger.
The guy who mentioned pizza is fairly good making it easy to understand though.
Let’s start by assuming a galaxy starts off sphere-like where stars and planets are orbiting a black hole. Over time, there will be an imbalance as some planets and stars are clumped together in one region of this sphere as orbits go at different velocities. This means that a large of amount of mass is accumulated in that spot. What happens? Gravity happens. So even my miniscule amounts nearby planets and stars would be attracted to this mass. Over a long time this consolidates into a disk-like shape as this mass is concentrated in this equatorial region. Where it may deform is when there is another galaxy nearby, when it is in the process of merging, or post-merger.
The guy who mentioned pizza is fairly good making it easy to understand though.
The best answer I can give is that once an imbalance starts with something going around, it starts to flatten in a manner that mimics that. The winning pizza dough idea doesn’t really work because something outside the dough introduced the spin. Explaining the geometric formation of galaxies is more akin to pizza dough becoming a round disc when left to its own devices.
If you have a flat galaxy and something just appears above it, all of the mass is below it and pulling it down. It will fall through and pop out of the other side, but in doing so it will likely get close to other things that will help pull it in behind them. They dance around and something else pulls them down flatter, then something else, then something else, etc.
Just imagine that as it falls down, the way it falls in line is similar to a ripple throughout everything orbiting. Not so graceful at first but it’ll mellow out over time.
Now rewind the idea back as far as you can and you can see how it forms. It just takes a tiny bit of a random imbalance with angular velocity.
The best answer I can give is that once an imbalance starts with something going around, it starts to flatten in a manner that mimics that. The winning pizza dough idea doesn’t really work because something outside the dough introduced the spin. Explaining the geometric formation of galaxies is more akin to pizza dough becoming a round disc when left to its own devices.
If you have a flat galaxy and something just appears above it, all of the mass is below it and pulling it down. It will fall through and pop out of the other side, but in doing so it will likely get close to other things that will help pull it in behind them. They dance around and something else pulls them down flatter, then something else, then something else, etc.
Just imagine that as it falls down, the way it falls in line is similar to a ripple throughout everything orbiting. Not so graceful at first but it’ll mellow out over time.
Now rewind the idea back as far as you can and you can see how it forms. It just takes a tiny bit of a random imbalance with angular velocity.
The best answer I can give is that once an imbalance starts with something going around, it starts to flatten in a manner that mimics that. The winning pizza dough idea doesn’t really work because something outside the dough introduced the spin. Explaining the geometric formation of galaxies is more akin to pizza dough becoming a round disc when left to its own devices.
If you have a flat galaxy and something just appears above it, all of the mass is below it and pulling it down. It will fall through and pop out of the other side, but in doing so it will likely get close to other things that will help pull it in behind them. They dance around and something else pulls them down flatter, then something else, then something else, etc.
Just imagine that as it falls down, the way it falls in line is similar to a ripple throughout everything orbiting. Not so graceful at first but it’ll mellow out over time.
Now rewind the idea back as far as you can and you can see how it forms. It just takes a tiny bit of a random imbalance with angular velocity.
All galaxies are formed around a black hole, everything orbits this or leaves the galaxy one way or another.
The funny thing about black holes is they spin, and also create a current around themselves that other things also spin in. This force means that objects above or below their axes either join in the spinning or again are removed from the galaxy one way or another
All galaxies are formed around a black hole, everything orbits this or leaves the galaxy one way or another.
The funny thing about black holes is they spin, and also create a current around themselves that other things also spin in. This force means that objects above or below their axes either join in the spinning or again are removed from the galaxy one way or another
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