The universe is a place of many wonders. It inspires many questions regarding the myriad of objects contained in it. One question that you may ponder is why are galaxies flat.
In this article, we will explore this question and look at why other objects are spherical. We’ll also investigate other exciting information about galaxies.
When smaller objects form, they form a sphere. This is true for objects with a diameter larger than 375 miles (600km) if rocky or 250 miles (400km) if composed of ice. They do so because there is a minimum mass that an object needs to be to generate strong enough gravity. The bodies form a sphere due to their own gravity.
This is true for planets and meteors but not galaxies. Galaxies may be flat and take the form of a disk rather than a sphere.
In this article, we will provide an answer as to why galaxies tend to be flat.
What Are Galaxies?
From a distance, a galaxy can be considered a cloud of stars. With closer inspection, we find that planets orbit the stars. Also contained in galaxies are clouds of dust and gas. It is from these clouds that new stars form. Most, if not all, galaxies have a black hole at its center.
Galaxies usually have an internal structure and rotate.
How Do Galaxies Form
When using tools like the Hubble Space Telescope, we look back in time. As some of the galaxies found with this telescope are up to 11 billion light-years away, we see them as they were 11 billion years ago. James Webb Space Telescope is looking further back in time.
The galaxies at these distances are much smaller than those we see closer. Also, they are much closer together.
As a result of this observation, scientists believe that modern galaxies result from collisions and mergers of smaller galaxies.
You may think that these are violent events, but they are not. Due to the large distances between objects collisions between objects are rare. These interactions are therefore called collisionless interactions.
However, gas clouds are compressed during collisions. This results in a flurry of star creation.
As a given galaxy evolves, it will take on different shapes. In most cases, they will form a spiral. In others, they will form an elliptical shape.
Just to be clear, most galaxies form a disk. In about 10% of cases, they will appear spherical.
Are Galaxies Flat?
Yes, galaxies can be flat! Galaxies form a flat shape due to rotation. Almost every object in the universe is in a state of constant movement.
The shape of galaxies is governed by the balance between angular momentum (caused by the rotation) and the inward pull of the collective mass of the galaxy.
As a galaxy develops so does the central black hole. The black hole will also inherit the angular momentum, which will rotate in the same direction as the rest of the galaxy.
Our galaxy the Milky Way rotates and as a result forms a flat shape. It takes out solar system 225 – 250 million years to complete a single orbit of the galaxy center. This is called a cosmic year.
Galaxy Shape and Classification
Edwin Hubble devised the classification scheme that we use for galaxies. He constructed the scheme based on the shape that galaxies appear from Earth.
Hubble’s scheme can be represented as a diagram of what looks like a tuning fork (see below). On the left are the elliptical galaxies with classes from E0 to E7. To the right are the spiral galaxies. One end of the ‘fork’ contains the normal spirals and barred spirals on the other. Separating the elliptical and spiral galaxies is what are called lenticular galaxies. Sitting off the ‘fork’ (and not shown in the diagram below) are the irregular galaxies.
Below is an illustration of the Hubble’s galaxy classification scheme. While it may appear so, do not make the mistake of thinking that the diagram shows the progression of galaxy development.
Elliptical Galaxies
The elliptical galaxies have a spherical to ellipsoidal shape. The most spherical galaxies are classed as E0 to E7. E0 are the most spherical while E7 are the most ellipsoidal. These galaxies have much less structure than the spiral galaxies. The ellipticals are almost devoid of interstellar gas and dust. As this is the material that stars are formed from there is no new star formation. They are therefore composed of very old small stars.
Spiral Galaxies
A normal spiral galaxy contains a dense central bulge surrounded by a flat disk containing arms. The stars in the bulge are old stars so the bulge is typically yellowish or reddish in color. The arms contain interstellar gas and dust and, consequently, contain a proportion of newer stars. These newer stars give the arms a bluish tinge. The galaxies are further divided based on the arms’ character and the bulge’s relative size (Sa to SC). The structural differences is related to the amount of interstellar gas and dust.
The barred spiral galaxies are the most common galaxy structure in the modern universe. They are much like normal spirals except that a long bar structure replaces the central bulge. Like the normal spirals, they are further divided based on the same criteria (SBa to SBc). The Milky Way is a barred spiral galaxy.
It is believed that the internal structure of galaxies results from gravitational forces within the galaxy.
You may ask why most galaxies are barred, and others do not. To our best knowledge, this is due to the amount of dark matter the galaxy has. Galaxies with significantly more dark matter tend to form barred structures.
Lenticular and Irregular Galaxies
Lenticular galaxies are intermediate between the ellipticals and spirals. They are believed to be former spiral galaxies that have used up their supply of gas and dust. With no new star formation, the arm structures have been lost.
As the name suggests, irregular galaxies lack shape and an internal structure.
Galaxy Rotation Anomaly
We mentioned earlier that galaxies rotate around the center of it. However early studies found that they did not rotate as expected.
In our solar system, planets farther from the Sun take longer to complete an orbit. This is expected when most of the mass in the system is concentrated in the center.
In a galaxy, it is also expected that objects farther from the center should take longer to orbit. However, it will be moderated somewhat by the any mass outside the object’s orbit. The more mass outside of an object’s orbit results in it orbiting quicker.
Early studies found that objects orbited a lot faster than expected towards the edges of galaxies. This indicated that substantial mass existed outside of the visible galaxy.
These observations and conclusions were one of the first evidence for dark matter.
While we do not yet know what dark matter is, it appears it is a significant component of galaxies.
Is Our Solar System Flat?
Our solar system is flat, and it rotates on the same plane. This is until we pass Neptune where things change.
Our solar system is flat for the same reasons that galaxies are flat. This is due to the conservation of momentum.
This physics concept keeps the moons rotating around the planets and the planets rotating around the sun.
However, not all objects are in the same plane. This is because, earlier on in our solar system’s life things were much more erratic.
Beyond Neptune is the dwarf planet Pluto. Pluto’s orbit is inclined by about 15 degrees to the plane that the planets orbit in. Outside of Pluto’s orbit is the Oort Cloud. This body of small objects forms a sphere around the solar system. Incidentally, the Oort cloud is a source of comets.
Final Thoughts
So, there you have it!! You should now have all the information you need to answer the question of why galaxies are flat from this ultimate guide.
To summarize, we have established that galaxies rotate in the same direction as the black hole at their core. The rotation is a product of the collisions that formed it. it is this rotation that is responsible for a galaxy being flat.
The internal structure of a galaxy is caused by gravitational forces and the amount of gas and dust it contains. It is also influenced by the amount of dark matter it is composed of.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
