In fact, galaxies come in a wide variety of shapes, but most of them tend to have a spiral disk shape.
Galaxies are vast systems of celestial bodies and matter held together by gravitational forces, including stars, remnants of stars, interstellar environments containing gas, cosmic dust, and dark matter, a crucial but not fully understood component.
Galaxies exhibit diverse features, ranging from dwarf galaxies with a few hundred million stars to giant galaxies containing hundreds of billions of stars, each of which orbits around the galaxy’s central mass.
While most people are impressed by galaxies that resemble spiral disks, the universe beyond is filled with galaxies of various shapes. For instance, PKS 2014-55 galaxy looks like two opposing boomerangs, ESO 593-IG 008 resembles a firefly but is named “Tinker Bell Fairy,” NGC 6240 has a shape akin to a massive sea turtle, and there are many more galaxies with unique shapes.
The galaxy PKS 2014-55, as captured by the MeerKAT radio telescope in South Africa, is located at a distance of 800 million light-years from Earth. This radio image provides detailed information about two powerful radio jets, each as long as ten galaxies, formed under the gravitational influence of a central binary black hole.
ESO 593-8, simply known as ‘The Bird,’ is a group of interacting galaxies situated in the Sagittarius constellation, approximately 650 million light-years away from Earth. It was created through the merger of two spiral galaxies and an irregular dwarf galaxy.
NGC 6240, also referred to as the Seagull Galaxy, is a super luminous infrared galaxy nearby in the Ophiuchus constellation. This galaxy is the remnant of the merger of three smaller galaxies.
The primary shapes of galaxies are classified based on Edwin Hubble’s famous classification scheme known as the “Hubble sequence” proposed in 1926. This classification is based on the optical appearance of galaxies on photographic plates. Hubble divided galaxies into four fundamental types.
Spiral Galaxies
According to a 2010 survey by the Hubble Space Telescope, this type of galaxy accounts for approximately 72% of the total number of galaxies observed.
Our Milky Way Galaxy used to be classified as a spiral galaxy not too long ago, with many spiral arms, similar to Andromeda (the Andromeda Galaxy). Our Milky Way Galaxy has a massive bulge in the center, giving it a somewhat spherical appearance.
However, numerous observations since the 1960s have indicated that the central bulge in the Milky Way doesn’t appear spherical, but rather somewhat boxy.
In 2005, infrared observation data of around 30 million stars in the center of our Milky Way Galaxy from the Spitzer Space Telescope was finally confirmed. Our Milky Way Galaxy is not just a regular spiral galaxy. Its center has a massive bar, about 27,000 light-years long, resulting in a classification of barred spiral galaxies.
Elliptical Galaxies
These galaxies look quite different from spiral galaxies, lacking a disk shape and instead having a spherical or elliptical shape with a bright center and gradually fading edges. Unlike spiral galaxies, elliptical galaxies do not have prominent spiral arms or a clear central nucleus.
The sizes of elliptical galaxies can vary significantly, ranging from a few hundred light-years to hundreds of thousands of light-years. They can take on various shapes, from being nearly spherical to being extremely elongated, numbered from E0 to E7.
Lenticular Galaxies
This type of galaxy resembles a convex lens and can be considered a disk galaxy without prominent spiral arms. Its center is more bulged than that of a typical spiral galaxy.
Irregular Galaxies
Irregular galaxies do not have a regular, symmetrical shape and lack a clear central nucleus. Roughly 20% of the galaxies in the universe can be classified as irregular galaxies.
Why Do Most Galaxies Have a Disk Shape Instead of a Spherical One?
The shape of a galaxy is determined by the state of the matter it contains. Matter here doesn’t just refer to the visible stars; it also includes dust, gas, dark matter, and more.
In theory, young galaxies are formed from chaotic clouds of dust and gas, lacking a disk shape. Over time, gas and dust can collide (friction) with a large number over an extended period. Their momentum will cancel each other out in the direction of their motion, but angular momentum is conserved, and gravity will pull everything toward the galaxy’s central mass.
This means that as the gas and dust collapse toward the center and consume their energy, the size of the young galaxy becomes smaller after the collision and energy depletion. The gas and dust outside will rotate faster.
When it rotates faster, the collision rate increases, and it begins to flatten and gradually transform into a disk. Simply put, also due to angular momentum, as the initially spherical mass created by the collapse becomes smaller and denser, any objects outside the plane of the disk can be influenced or attracted by objects within the disk, ultimately forming a disk-shaped structure. Spiral galaxies tend to contain a lot of gas and dust, which is why they take on a disk shape.
From the current data, it’s apparent that long-term stable spherical-shaped galaxies are usually smaller and less dense, with their matter distributed too widely to coalesce into a smaller sphere, instead evolving into a disk shape. Over time, the number of spherical galaxies is minimal, and they are rarely discovered.
Source: Earthlymission; Nature; NASA