Types of Galaxies: Disks, Ellipses, and More

KOMPAS.com – The Milky Way Galaxy is only one of the galaxies in this universe.

The galaxy is described as having the shape of a rotating blue disk. But interestingly, it turns out that there are various forms of galaxies, not just disk-shaped.

Kinds of galaxies

Quoted from Live Science, Monday (3/7/2023) at a basic level, there are two classifications of galaxy shapes, namely disks and ellipses.

According to Cameron Hummels, a theoretical astrophysicist at Caltech, disc galaxies, also called spiral galaxies, are shaped like a fried egg.

The galaxy has a more rounded center, like an egg yolk, surrounded by a disk of gas and stars. The Milky Way and Andromeda, which are our closest galactic neighbors, fall into this category.

In theory, disk galaxies were originally formed from hydrogen clouds. Gravity then holds the gas particles together.

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As the hydrogen atoms approach, the cloud begins to rotate and its collective mass increases which causes its gravitational force to increase as well. Eventually, gravity causes the gas to collapse into a spinning disk.

Most of the gas is at the edges, feeding star formation.

Edwin Hubble, who confirmed the existence of galaxies outside our galaxy, said that disk galaxies are late-type galaxies because they formed later in the history of the universe.

Meanwhile, the elliptical galaxy that Hubble calls an early-type galaxy looks older.

Instead of rotating like disk galaxies, stars in elliptical galaxies have a more random motion. Elliptical galaxies are thought to be the product of galaxy mergers.

When two galaxies of the same mass merge, their stars begin to attract each other with gravity, disrupting the stars’ rotation and creating more random orbits.

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Galactic merger

However, not every merger produces elliptical galaxies. The Milky Way is actually quite old and big, but it has retained its disk shape.

Galaxies increase their mass simply by attracting dwarf galaxies which are much smaller than the Milky Way and collecting free gas from the universe. So billions of years from now, two spiral galaxies could merge and create an elliptical galaxy.

However, this merger is far from instant. It takes hundreds of millions or even billions of years. In fact, there are merges in progress that move so slowly they appear static.

“They were in basically the exact same, unchanged conditions for all of human civilization,” said Robert Bassett, an observational astrophysicist who studies the evolution of galaxies at Swinburne University in Melbourne, Australia.

Finally, there is a less common form of galaxy called a lenticular galaxy. It appears to be a mix between an elliptical and a disk galaxy.

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According to Bassett, when a disk galaxy uses up all its gas and is unable to form new stars, the existing stars begin to interact.

Their gravitational pull on each other creates a shape that looks like some sort of ellipse but is still a rotating disc.

The shapes of the galaxies are then deduced using thousands of 2D images by relying on other properties such as the color and motion of the galaxies. For example, the age of younger disc galaxies is enhanced by their blue color.

But despite all that has been learned about these structures, there is still much we do not know.

“The formation and evolution of galaxies is one of the biggest open questions in astronomy and astrophysics,” added Hummels.

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2023-07-03 13:00:00
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