In fact, galaxies come in many different shapes, but most of them have a spiral disk shape.
A galaxy is a large system of celestial bodies and matter bound together by gravity, including stars, stellar remnants, interstellar medium containing gas, cosmic dust, and dark matter, an important component that is not yet fully understood.
Galaxies exhibit a wide range of characteristics, from dwarf galaxies containing a few hundred million stars to giant galaxies containing hundreds of trillions of stars, with each star orbiting the center of the galaxy it belongs to.
In most people’s impressions, galaxies resemble spiral disks; however, beyond the vast universe, galaxies come in many different shapes. For example, the galaxy PKS 2014-55 resembles two boomerangs facing each other; the galaxy ESO 593-IG 008 looks like a hummingbird but is named “Tinker Bell Fairy”; the galaxy NGC 6240 has a shape similar to that of a giant sea turtle, and there are many other galaxies with unique shapes.
Galaxy PKS 2014-55 was captured by the MeerKAT radio telescope in South Africa. This radio image was taken 800 million light-years away from Earth, providing 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 located in the Sagittarius constellation, 650 million light-years from Earth. It was created by the merger of two spiral galaxies and an unusual dwarf galaxy.
NGC 6240, also known as the Starfish Galaxy, is a nearby ultraluminous infrared galaxy in the Ophiuchus constellation. This galaxy is the remnant of a merger between three smaller galaxies.
Main Shapes of Galaxies
The famous astronomer Edwin Hubble proposed a classification scheme known as the “Hubble Sequence” in 1926. Based on differences in the optical appearance of galaxy images on photographic plates, he divided galaxies into four basic types.
1. Spiral Galaxies
According to a 2010 survey by the Hubble Space Telescope Project, this type of galaxy accounts for about 72% of all galaxies observed by researchers.
Spiral Galaxy.
Is our Milky Way a spiral galaxy? Not long ago, the Milky Way, with its many spiral arms, was classified as a spiral galaxy, similar to Andromeda (the Andromeda Galaxy). Our Milky Way features a huge bulge at its center, making it look somewhat spherical.
However, a significant number of observations since the 1960s have indicated that the central bulge of the Milky Way does not appear spherical; instead, it has a somewhat square shape.
In 2005, infrared observational data of about 30 million stars at the center of the Milky Way from the Spitzer Space Telescope was finally confirmed. Our Milky Way is not merely a typical spiral galaxy. Its center contains a massive bar approximately 27,000 light-years long. Consequently, the Milky Way is classified as a barred spiral galaxy.
2. Elliptical Galaxies
Elliptical galaxies lack spiral arms and a rotation axis.
They appear completely different from spiral galaxies, having a spherical or elliptical shape rather than a disk form, with a bright center and fading edges. Unlike spiral galaxies, elliptical galaxies do not have spiral arms or a rotation axis.
The sizes of elliptical galaxies vary widely, ranging from a few hundred light-years to hundreds of thousands of light-years. Their shapes range from circular to extremely elongated ellipses, numbered from E0 to E7.
3. Lenticular Galaxies
This type of galaxy can be considered a disk galaxy without spiral arms.
This is a special type of galaxy that resembles a convex lens and can be considered a disk galaxy without spiral arms. Its center bulges more than that of a spiral galaxy.
4. Irregular Galaxies (Irr)
This type of galaxy develops in an irregular manner, lacking clear symmetry and a distinct central nucleus. About 20% of galaxies in the universe can be classified as irregular galaxies.
This type of galaxy develops in an irregular manner.
Why Most Galaxies Have a Disk Shape Instead of a Spherical Shape
The shape of a galaxy is determined by the state of the matter it contains. Here, matter refers not only to those luminous stars but also includes dust, gas, dark matter…
Theoretically, young galaxies form from a chaotic cloud of dust and gas, at which point they do not have a disk shape. Over time, gas and dust can collide (friction) in large quantities for extended periods. Their momentum will cancel each other out in the direction of their movement, but angular momentum is conserved, and gravity pulls everything toward the center of the young galaxy.
This means that as gas and dust collapse toward the center, causing the size of the galaxy to shrink after collisions and energy dissipation, the outer gas and dust will rotate faster.
As it spins faster, the collisions increase, and it begins to flatten out, gradually taking on a disk shape. Simply put, due to angular momentum, as the initial spherical shape it formed collapses and becomes denser, any objects outside the plane of the disk may be influenced or attracted by objects within the disk, ultimately forming a flat disk shape. Spiral galaxies tend to contain more gas and dust, which is why they have a disk shape.
Current data suggests that stable spherical galaxies are often small and sparse, with their matter distributed too widely to gather into a smaller ball and gradually take on a disk shape. As a result, over time, the number of spherical galaxies is minimal and they are very rarely detected.
