Look into the night sky and you’ll glimpse the stars from hundreds of billions of galaxies. Some galaxies are swirling blue disks like our own Milky Way, others are red spheres or misshapen, clumpy messes or something in between. Why the different configurations? It turns out that a galaxy’s shape tells us something about the events in that galaxy’s ultra-long life.
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. The Milky Way and our nearest galaxy neighbor Andromeda fall into this category.
In theory, disk galaxies initially form from clouds of hydrogen. Gravity draws the gas particles together. As the hydrogen atoms draw closer, the cloud begins to rotate and their collective mass increases, which causes their gravitational force to also go up. Eventually, the 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 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.