Why do galaxies have different shapes?

If you look up at the night sky, you can see the stars of hundreds of billions of galaxies. Some galaxies are swirling blue disks like our own Milky Way, others are red spheres or misshapen, lumpy mess, or something in between. Why the different configurations? It turns out that the shape of a galaxy tells something about the events in that galaxy’s ultra-long life.

At the basic level, there are two classifications for galaxy shapes: disk-shaped and elliptical. A disk galaxy, also called a spiral galaxy, is shaped like a mirror ice, said Cameron Hummels, a theoretical astrophysicist at Caltech. These galaxies have a more spherical center like the egg yolk, surrounded by a disk of gas and stars – the egg white. The Milky Way and our closest galaxy neighbor Andromeda fall into this category.

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Theoretically, disk galaxies initially form from hydrogen clouds. Heaviness pulls the gas particles together. As the hydrogen Atoms move closer, the cloud begins to spin and its collective mass increases, which also increases its gravitational force. Eventually, gravity causes the gas to collapse into a swirling disk. Most of the gas is in the rim, where it feeds star formation. Edwin Hubble, who confirmed the existence of galaxies beyond our own just a century ago, called disk galaxies late galaxies because he suggested that their shape meant that they formed later in the history of the universe. NASA sea.

Alternatively, elliptical galaxies – what Hubble called early-type galaxies – appear to be older. According to Robert Bassett, an observational astrophysicist who studies galaxy evolution at Swinburne University in Melbourne, Australia, stars in elliptical galaxies don’t move like rotating galaxies, but rather randomly. It is believed that elliptical galaxies are a product of galaxy fusion. When two galaxies of equal mass merge, their stars begin to pull together under gravity, disrupting the stars’ rotation and creating a more random orbit, Bassett said.

Not every fusion results in an elliptical galaxy. The Milky Way is actually quite old and large, but it retains its disk shape. It added to its mass by simply pulling in dwarf galaxies, which are much smaller than our home galaxy, and collecting free gas from the universe. However, Andromeda, our disc-shaped sister galaxy, is headed straight for the Milky Way, Bassett told LiveScience. In billions of years, the two spiral galaxies could merge and each of the duo’s star disks will offset the rotation of the other, creating a more random elliptical galaxy.

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These mergers are far from instantaneous. They take hundreds of millions, even billions of years. In fact, there are ongoing mergers that, in our view, move so slowly that they appear static. “They were basically in exactly the same state, unchanged for all of human civilization,” said Bassett. Hubble gave these galaxies their own classification – irregular galaxies. To look at them, “They’re usually a multi-component mess,” said Hummels. “Irregular galaxies look like a big train wreck,” added Bassett.

After all, lenticular galaxies, a less common shape, appear to be a mix between an elliptical and a disk galaxy. Bassett said that when a disk galaxy uses up all of its gas and cannot form new stars, the existing stars begin to interact. Pulling them together creates a shape that looks like a lens – a kind of elliptical, but still rotating disk.

What scientists have found so far about galaxies and their 3-D shapes has been derived using thousands of 2-D images and using other properties such as galaxy color and movement to fill in the gaps, Bassett said.

For example, the younger age of disk galaxies is confirmed by their blue color. Blue stars are generally larger and burn faster and hotter (blue light has a higher frequency and is therefore more energetic than red light). Meanwhile, elliptical galaxies are filled with older stars – called red dwarfs – it doesn’t burn quite as hot or quickly.

Despite everything we’ve learned about the massive sky structures around us, there is still so much we don’t know, Hummels said.

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

Originally published on Live Science.