Why are galaxies different shapes? - Live Science

At the very basic level there are two classifications for galaxy shapes: disk and elliptical.

A disk galaxy, also called a spiral galaxy, is shaped like a fried egg, said Cameron Hummels, theoretical astrophysicist at Caltech.

These galaxies have a more spherical center, like the yolk, surrounded by a disk of gas and stars — the egg white.

Edwin Hubble, who confirmed the existence of galaxies beyond our own only a century ago, called disk galaxies late-type galaxies because he suspected their shape meant they formed later in the history of the universe, according to NASA. .

Alternatively, elliptical galaxies — what Hubble called early-type galaxies — appear to be older.

Instead of rotating, like disk galaxies, stars in elliptical galaxies have more random movement, according to Robert Bassett, an observational astrophysicist who studies galaxy evolution at Swinburne University in Melbourne, Australia.

Elliptical galaxies are thought to be a product of a galaxy merger.

When two galaxies of equal mass merge, their stars start to tug on one another with gravity, disrupting the stars' rotation and creating a more random orbit, Bassett said. .

Hubble gave these galaxies their own classification — irregular galaxies.

Finally, a less common shape, lenticular galaxies seem to be a mix between an elliptical and a disk galaxy.

What scientists have uncovered so far about galaxies and their 3D shapes has been inferred using thousands of 2D images and by relying on other properties, such as galaxy color and motion, to fill in the blanks, Bassett said. .

Meanwhile, elliptical galaxies are filled with older stars — called red dwarfs — that aren't burning quite as hot or fast