Hundreds of billions of galaxies can be seen if you look up into the night sky. Like our Milky Way, some galaxies are spinning blue disks, while others are crimson spheres, clumpy messes, or anything in between. What’s the deal with all the various setups? It turns out that the shape of a galaxy may provide information about its ultra-long life’s activities.

At the most fundamental level, there are two types of galaxy shapes: disk and elliptical. According to Caltech theoretical astrophysicist Cameron Hummels, a disk galaxy, also known as a spiral galaxy, is formed like a cooked egg. These galaxies have a more spherical core, similar to the yolk, and are surrounded by a disk of gas and stars, similar to the white of an egg. This category includes the Milky Way and Andromeda, our closest galactic neighbor.

Disk galaxies are thought to have formed from hydrogen clouds at first. The gas molecules are drawn together by gravity. As the hydrogen atoms get closer, the cloud starts to spin, and their aggregate mass grows, increasing their gravitational attraction. The gas eventually collapses into a spinning disk as gravity takes its toll. The majority of the gas is concentrated around the rim, where it fuels the birth of stars. According to NASA, disk galaxies are named late-type galaxies by Edwin Hubble, who established the existence of galaxies beyond our own just a century ago. He assumed their shape showed they evolved later in the universe’s history.

On the other hand, elliptical or early-type galaxies seem to be older. According to Robert Bassett, an observational astrophysicist researching galaxy evolution at Swinburne University in Melbourne, Australia, stars in elliptical galaxies have more random movement than stars in disk galaxies, which rotate. A galaxy merger is assumed to be the source of elliptical galaxies. According to Bassett, when two galaxies of the equivalent mass combine, gravity pulls on their stars, interrupting their rotation and generating a more erratic orbit.

Every merger doesn’t form an elliptical galaxy. Even though the Milky Way is so ancient and huge, it has a disk form. It has been gaining mass by attracting dwarf galaxies, which are far smaller than our galaxy, and gathering free gas from the rest of the cosmos. But, according to Bassett of Live Science, Andromeda, our disk-shaped sibling galaxy, is on its way to collide with the Milky Way. In billions of years, the two spiraling galaxies may join, and each of their starry disks will counteract the rotation of the other, resulting in a more random elliptical galaxy.

These mergers aren’t going to happen right now. They take millions, if not billions, of years to complete. Some ongoing mergers seem to us to be static because they are progressing so slowly. “They’ve virtually remained in the same condition, unmodified throughout the whole of human civilization,” said Bassett. These galaxies are known as irregular galaxies, according to Hubble. “They’re generally a tangle with several components,” according to Hummels. “Irregular galaxies seem to be a huge train disaster,” Bassett noted.

Finally, lenticular galaxies seem to be a cross between an elliptical and a disk galaxy; however, they are less frequent. It’s possible, according to Bassett, that when a disk galaxy runs out of gas and is unable to generate new stars, the existing stars start to interact. Their gravitational pull forms a lentil-like shape that’s elliptical yet still rotates.

Thousands of 2D photos have been used to extrapolate what scientists have learned so far about galaxies and their 3D structures, and other attributes like galaxy color and velocity have been used to fill in the gaps, according to Bassett.

This galaxy, known as Mrk 820, is classified as a lenticular galaxy. Surrounding Mrk 820 is a slew of other galaxy types from elliptical to spiral. (Image credit: ESA/Hubble & NASA and N. Grogin (STScI), Acknowledgement: Judy Schmidt)

Disk galaxies, for example, have a blue tint that indicates a younger age. Blue stars are bigger and burn quicker and hotter than white stars (blue light has a higher frequency and is thus more energetic than red light). Meanwhile, elliptical galaxies are teeming with older stars known as red dwarfs, which don’t burn as brightly or as quickly as younger stars.

Despite all we’ve learned about the huge cosmic structures surrounding us, there’s still a lot we don’t understand. According to Hummels,

“One of the most fundamental unanswered topics in astronomy and astrophysics is how galaxies arise and evolve,” stated Hummels.

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