We’ve all seen the classic picture of a galaxy. It’s a large spinning disk, sometimes with spiral arms, surrounding a brighter core with a supermassive black hole at its center.
The question that has brought you here today is ‘why are galaxies so flat?’.
Look at the picture of NGC 891, found in the Andromeda constellation, below. It’s an incredibly thin galaxy, and typical of the spiral-type.
Most spiral galaxies are what we call ‘planar’, i.e. the majority of their stars lie on a single thin plane. Like a DVD, they are wide but thin disks.
In the case of the Milky Way, our home galaxy, it is thought to be about 100,000 light-years across, but only 1,000 light-years thick. That’s incredibly thin! It’s the equivalent of a 12″ (308mm) pizza being just 3mm thick.
Why are galaxies like this, wide and thin, and not just a big ball of stars?
We’ll answer that question in this article, but we’ll begin by acknowledging that professional astronomers, even today, do not have an agreed-upon model of galaxy formation.
How Do Galaxies Form?
Galaxy formation research is still very active today. Various groups are theorizing and testing different simulations to achieve better matches with our observations. As they do, the theories set out here may evolve.
Scientists still do not have a definitive explanation for the evolution of galaxies but there are presently two competing theories of galaxy formation: top-down and bottom-up.
Top-Down Galaxy Formation
The top-down theory of galaxy formation says that galaxies spring to life from truly enormous fields of gas that slowly coalesce and condense over millions of years to form stars.
This is an older theory that was originally developed in the 1960s and is largely out of favor today because it matches observations less convincingly than the bottom-up model.
Bottom-Up Galaxy Formation
The bottom-up theory says that star formation came first, probably in fields of stars that we still see as globular clusters today. Over millions and billions of years, these smaller groups collide and combine to form larger and larger galaxies.
This better matches the observed situation, especially since there are fewer large galaxies than small ones.
We also see real-life examples of colliding galaxies, such as the Mice Galaxies in the Coma Berenices constellation, and know that our home galaxy the Milky Way will collide with the giant Andromeda galaxy in about five billion years. This, we believe, will bring about their merger.
This bottom-up model is the more favored explanation today and the one we’re going to assume for the rest of this piece.
Find out if galaxies are still being formed today (opens a new tab)
Why Is the Milky Way Flat?
Now we understand a little about galaxy formation, let’s consider the big question: why are galaxies flat? For that matter, why do we see so many disks in nature? We have the rings of Saturn, planets orbiting a star like our solar system, as well as spiral galaxies.
All of these are large, flat, narrow disks. And you’d be right to assume that they all form from the same basic principles of physics. The key one we need to understand to grasp all of it is the conservation of angular momentum.
Angular Momentum in a Cloud of Gas
This can be a tricky subject to get your head around, so we’re going to take it steady and explain it one step at a time using a truly enormous cloud of interstellar gas.
Gas clouds in space are rarified. They might contain just a few atoms or molecules for every square meter. They are so thin that we would class them as a vacuum on Earth!
Within that cloud, the atoms, molecules, and bits of space dust and debris are moving around in random directions and at different speeds. But, and this is the crucial bit, after you add up all of the directions and speeds, you’re left with a net direction and speed of spin, better known as angular momentum.
For example, if our cloud had just three particles, one traveling up, one going down, and one going left, we could say that the up and down particles cancel each other out and the net angular momentum of the cloud was left.
We can’t calculate this for billions of individual, microscopic particles, but in any body such as our huge gas bubble, there is a net direction and speed of spin.
We say that the cloud has an angular momentum which is measured as a rate of spin in one plane about an axis passing through the center of gravity.
Our gas cloud’s angular momentum is set in the very early days of its formation and does not change afterward.
Over time this net angular momentum is made more visible because particles not already on it tend to collide and dissipate their energy. This gradually moves those particles out of ‘free space’ leaving only a flat spinning plane. This disk has to keep spinning due to the law of conservation of angular momentum, which we’ll look at next.
This is not an easy concept to understand. If you’re scratching your head a bit, watch the video below from Minutephysics that does a fantastic job of visualizing the creation of a disk from a gas cloud.
What is the Conservation of Angular Momentum?
The law of conservation of angular momentum says that unless an external force acts on it, the angular momentum of a body remains constant.
Angular momentum can be thought of as a combination of speed and overall distribution of mass. When the mass is spread out, the speed of rotation is low, and when the mass is in a smaller area, the speed of rotation is faster.
This is why a spinning ballerina speeds up when she pulls her arms in. Her mass has become more concentrated at the axis of rotation (because her arms have moved closer to it) but her angular momentum is conserved, so she has to spin more quickly to compensate.
There’s a lot of math behind this but we don’t need it for our explanation. Just know that due to the conservation of momentum, an object will spin faster when its mass is less spread out.
Okay, let’s go back to our gas cloud.
Conservation of Angular Momentum in an Interstellar Gas Cloud
Since it is spread over truly phenomenal regions of space, measuring light-years in all directions, our cloud has barely enough gravity to pull itself together and have some momentum.
However, gravity does act on it slowly but surely.
Steadily, the particles in the cloud are drawn towards a common center of gravity. As they do so, the volume occupied by the mass of the cloud shrinks, and, as we just learned, that means it begins to spin faster in the plane of its angular momentum.
This state of affairs continues until an equilibrium is reached.
Left to its own devices, with no angular momentum to worry about, gravity would keep pulling the gas into a tighter and tighter area, eventually leading to the formation of a star or planet.
However, our gas cloud was so vast that, as it condenses, its spin becomes fast enough to overcome the attractive force of gravity. Centrifugal forces resulting from the spin are trying to fling gas outwards as gravity is trying to pull it inwards.
A balance is reached in the plane of rotation between gravitational and centrifugal forces. This equilibrium is what causes the disks we see to form.
The Effect of Gravity on Disk Formation
Angular momentum forces only act at right angles to the axis of rotation. You can think of the axis of rotation like the center hole in a DVD.
Outside of the spinning plane, gravity is still the dominant force within the gas cloud.
Gravity continues to pull gas molecules towards the center of mass of the cloud, and towards the ever-denser spinning disk. Eventually, all the matter inside the gas cloud will settle into the disk or coalesce at the center of mass where centrifugal forces are weakest.
This is the reason there is more of a bulge at the center of a galaxy than at its extremities. It is also why we find the oldest stars at the center of galaxies and, we believe, why that’s where we find a supermassive black hole and its accretion disk.
Oh, and it’s also why galaxies are brighter at their center.
We’re coming to the end of our explanation now and you’ve read through some pretty complex topics.
Before we move on to look at how this ultimately leads to galaxies and solar systems being flat, take a look at the Space Time video below because it delivers a neat visual explanation of the whole process.
Why Galaxies and Solar Systems are Planar
We now have our answer to our original question:
Galaxies are flat and disk-shaped because a disk is the natural equilibrium state between the spinning forces of angular momentum trying to eject material outwards and gravitational forces trying to draw it inwards.
Spiral galaxies are flat because their stars formed within the rotating gas plane after it was already created.
The exception to this is halo stars. These are generally very old and formed before the disk existed, which is why we find these off the galactic plane inside clusters of stars.
Are All Galaxies Flat?
We’ve answered the question of why a galaxy is flat but not all of them are.
We also mentioned earlier, in the bottom-up theory of galaxy formation, the possibility that Globular Clusters are the original seeds of galaxies, and they are definitely not flat.
They are not flat because they contain less gas and therefore experience fewer collisions between their component parts. Since collisions are what lead to the revealing of the planar disk (watch the Minutephysics video above for the graphical explanation), a mass without them tends not to collapse into a disk but remains in a roughly amorphous ball shape.
We’ve been very careful to talk of spiral galaxies in this article but there is another whole group of galaxies called ellipticals.
Elliptical galaxies are much less flat than spirals. This is thought to be because ellipticals are the result of collisions between older galaxies that contain old stars and relatively little gas and star-forming regions.
Just like star clusters, the lack of gas and dust in these mergers mean there are far fewer off-axis collisions (stars are so far apart in galaxies that they don’t tend to collide, even as galaxies merge). Fewer collisions mean less likelihood of collapse into a disk.
Summary
Nature appears to love a disk. We see them in the rings of Saturn, the accretion disks of black holes, in the formation of solar systems, and, at the largest scales, the creation of spiral galaxies.
We have learned that the disk is an equilibrium state between gravity pulling matter towards the center of mass and the spin force wanting to throw it out into space.
The disk is revealed when there are many collisions between particles which dissipates their energy leaving them prone to succumbing to the net angular momentum of the cloud.
The galaxies we see today are flat because their stars formed within the disk of spinning gas after it was created.
