Understanding the Various Shapes of Galaxies: What Influences Their Appearance

Astronomers estimate that there are roughly 200 billion galaxies in the observable universe. However, many science-fiction films depict galaxies with the same general appearance as ours. This reflects a human-centered perspective. While several resemble the Milky Way, others exhibit incredibly unique shapes and structures.

Chances are, you've encountered depictions of our galaxy in science classrooms around the world. But did you know that those posters often overestimate the thickness of one of the galaxy's most important elements?

Indeed. Much like a fried egg, the Milky Way features a central bulge surrounded by a flat, thin disc. And when we say "thin," we really mean astonishingly thin. As physicist and Forbes correspondent Jillian Scudder highlights, the Milky Way's "disc" stretches across about 100,000 light-years in length but measures only 0.6 light-years in height. This makes it 30 times thinner than a standard sheet of printer paper, proportionally.

But before diving deeper into what makes the Milky Way so unique, let's take a step back and explore the common traits shared by all galaxies.

What Exactly Is a Galaxy?

Galaxies are intricate systems bound by gravity. They consist of gases, stellar dust, and millions—sometimes even billions—of stars, each surrounded by their own planets and asteroid belts.

Among the more than 200 billion galaxies, many exhibit a spiral shape (just like the Milky Way). There are other types as well, such as irregular galaxies, elliptical galaxies, and lenticular galaxies. Dwarf galaxies, which are smaller and contain fewer stars than spiral or elliptical galaxies, come in diverse shapes and sizes.

What Determines the Shape of Galaxies?

Each galaxy tells its own unique story, and its shape reflects its history. Scientists categorize galaxies based on their appearance. The Milky Way is classified as a spiral galaxy, which means it has a broad, flat disc shape with a slight bulge at its center.

This structure results from a combination of rotation speed, time, and gravity. To gain further insight, we spoke with astrophysicist Raja GuhaThakurta, Ph.D., a professor at the University of California Santa Cruz and an expert on galaxy evolution—a field rich in debate.

"The exact physics behind the formation of these structures is still not fully understood," says GuhaThakurta. Nonetheless, it is widely believed that spiral galaxies originate from spinning clouds of gas and dust. The rate of their rotation plays a crucial role. According to GuhaThakurta, large, rapidly rotating clouds are more likely to form spiral galaxies.

Gravity works to compress these spinning, shapeless clouds into flattened planes. As time passes, the clouds shrink due to gravitational forces and energy loss from friction. Additionally, the principle of angular momentum conservation dictates that as a spinning object contracts, its rotation accelerates.

You can observe this principle at play at your local ice skating rink. Skilled skaters know that by pulling their arms closer to their body, they can increase their twirl speed.

Much like a spinning ball of pizza dough, spiral galaxies form when shapeless clouds of gas and dust rapidly flatten out. The same physical forces also shape the spiral "arms" that encircle these galaxies.

"The nature of spiral arms is almost certainly linked to the rate of rotation," says GuhaThakurta. Faster rotating systems typically have tightly packed, small arms, while slower rotating systems tend to have longer, more loosely wound arms.

To understand this concept, GuhaThakurta suggests a simple experiment you can try at home: "Imagine stirring your coffee. Place a dollop of cream off-center. You'll notice that the cream forms a spiral pattern," he explains. Then, stir the coffee with a spoon. If you do this quickly, you'll see that the spiral arms become tighter and smaller.

The Enigma of the Bulge

Let's pause for a quick summary. We've discussed how spiral galaxies form and how their rotation affects the shape of their arms. But what about the bulges we mentioned earlier? At the heart of spiral galaxies lies a cluster of very old stars revolving around a central point—this is the bulge.

While the stars in the disc orbit in an orderly, horizontal plane, the stars in the bulge behave more erratically, like bees swarming around a hive. Astronomers are still trying to determine how these bulges form. Some believe they form before the rest of the spiral galaxy, while others think the opposite is true.

Now picture a galaxy that's entirely made up of a bulge. This galaxy would have no disc and would appear either as a large, rounded sphere or a giant American football. Within it, stars would orbit the central point in all directions. Congratulations, you've just visualized an elliptical galaxy.

It's crucial to remember that not all galaxies are either spiral or elliptical. There are also irregular galaxies, which lack bulges and can take on a wide range of shapes.

Galaxies in Collision

Scientists have observed ongoing galactic mergers. While these galaxies might one day form nice, well-rounded elliptical shapes, for now, they appear disordered and distorted as they come together.

There are also several documented instances of large spiral galaxies cannibalizing smaller ones that have ventured too close, with the smaller galaxy slowly being consumed over time. Experts predict that our Milky Way will eventually collide with the nearby Andromeda galaxy, merging the two spiral galaxies into a single elliptical galaxy.

This cosmic event is expected to begin around 3 billion years from now, with the process completed in another 4 billion years. Of course, none of us will be around to witness it. However, scientists have already named the future elliptical galaxy that will result from this merger: They call it "Milkomeda." A fitting portmanteau for the occasion.

Spiral Galaxies

A spiral galaxy, like the Milky Way, consists of a central bulge surrounded by spiral arms. These arms, made up of stars, gas, and dust, give the galaxy a distinctive pinwheel-like shape. Depending on the galaxy, the spiral arms can be tightly wound or more spread out.

Spiral galaxies are the most common type of galaxy, with barred spiral galaxies being the most frequent subtype. A barred spiral galaxy has a central bar-shaped structure, composed of stars, gas, and dust, extending outward from the galactic center. Scientists speculate that the presence of a bar indicates the galaxy has reached full maturity.

Elliptical Galaxies

An elliptical galaxy, characterized by its smooth, ellipsoidal or spherical shape, lacks distinct features like arms or disks. Primarily made up of older stars, these galaxies typically contain little interstellar gas or dust. Researchers suggest that elliptical galaxies might result from galactic collisions.

Giant elliptical galaxies, a subtype of elliptical galaxy, are some of the largest and brightest galaxies, often located at the heart of galaxy clusters — groups of galaxies bound together by their gravitational pull.

Lenticular Galaxies

Lenticular galaxies, blending features of both spiral and elliptical galaxies, have a central bulge, older stars, and a flat, disk-like structure. However, they lack distinct spiral arms. Some theories suggest these galaxies lost their arms over time, while others propose that lenticular galaxies formed through the merging of spiral galaxies.

Irregular Galaxies

As the term suggests, irregular galaxies lack a defined, symmetrical shape or structure. Their form may have emerged due to galactic collisions or the gravitational influence of a neighboring galaxy. These galaxies don't follow a clear pattern of stars, gas, and dust, and they can contain both young and old stars.