Some galaxies resemble blue spiral disks, much like our Milky Way, while others appear as reddish spheres or undefined shapes. So why do galaxies come in various shapes? As it turns out, the shape of a galaxy reveals insights about the events occurring throughout its incredibly long lifespan.
Galaxy classification and formation
Galaxies are often classified based on their shapes, with two fundamental categories: disk and elliptical.
According to Cameron Hummels, a theoretical astrophysicist at the California Institute of Technology, disk galaxies resemble fried eggs in shape. These galaxies have a more spherical bulge, akin to the yolk, encircled by a gas and star-studded disk – the egg white. Our Milky Way and Andromeda, our nearest neighboring galaxy, fall into this category. In theory, disk galaxies initially form from hydro clouds. Gravitational attraction draws gas particles together. As hydrogen atoms come closer, the cloud begins to spin, accumulating collective mass, leading to increased gravitational force. Eventually, gravity causes the gas to collapse into a rotating disk. American astronomer Edwin Hubble, who confirmed the existence of galaxies beyond the Milky Way just a century ago, once speculated that disk galaxies formed later in cosmic history, while elliptical galaxies seemed to form earlier.
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The Sculptor Galaxy – a disk-shaped galaxy
According to Robert Bassett, an astrophysicist at Swinburne University of Technology in Melbourne, Australia, stars within elliptical galaxies have random motion, rather than the rotation seen in disk galaxies’ stars. Elliptical galaxies are thought to result from the merging of galaxies. Bassett adds that when two equally massive galaxies merge, their stars are drawn together by gravitational force, disrupting the stars’ rotational motion and creating a more random orbit. Not every merger leads to an elliptical galaxy, though. Our Milky Way, although large and ancient, still maintains its disk shape. It’s steadily gaining mass by drawing in dwarf galaxies and free gas from the universe. However, Bassett notes that the Andromeda Galaxy is actually headed straight for the Milky Way. Thus, over billions of years from now, the two spiral galaxies could merge, compensating for each other’s rotations and creating a more random elliptical galaxy.
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The giant elliptical galaxy ESO 325-G004
Other Galaxy Forms
Galaxy mergers are not instantaneous; they can take hundreds of millions or even billions of years. In fact, some mergers are happening so slowly that they appear nearly static. Hubble categorized these galaxies as “irregular galaxies.”
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NGC 1427A, an example of an irregular galaxy.
Finally, lenticular galaxies are a blend of elliptical and disk galaxy shapes. Bassett suggests that when a disk galaxy exhausts its gas, unable to form any new stars, its existing stars begin to interact. Their mutual gravitational attraction results in a shape resembling a lentil – an elliptical-like form that still maintains some rotation.
Scientific Inferences Bassett explains that what scientists have discovered about galaxies and their 3D shapes thus far has been deduced from thousands of 2D images and relies on other characteristics like color and galaxy motion.
For instance, disk galaxies appear younger, evidenced by their bluish hue. Blue stars tend to be larger, brighter, and hotter (blue light has a higher frequency and thus more energy than red light). Conversely, elliptical galaxies harbor more “senior” stars – known as red dwarfs, which are dimmer and cooler.
However, alongside what humanity has unveiled about the colossal structures surrounding us, much remains unknown. Hummels notes, “The formation and evolution of galaxies is one of the biggest open questions in astronomy and astrophysics.”