Thanks to the work of famous astronomer Edwin Hubble we know that just about any galaxy in the universe will have one of 4 different shapes, spiral, elliptical, lenticular, and irregular.
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.
Not every merger results in an elliptical galaxy. The Milky Way is actually quite old and large, but maintains its disk shape. It's been adding to its mass by simply drawing in dwarf galaxies, which are much smaller than our home galaxy, and collecting free gas from the universe. However, Andromeda, our disk-shaped sister galaxy, is actually headed straight for the Milky Way, Bassett told Live Science. So billions of years from now, the two spiraling galaxies could merge and each of the duo's starry disks will offset the other's rotation, creating a more random elliptical galaxy.
Not every merger results in an elliptical galaxy. The Milky Way is actually quite old and large, but maintains its disk shape. It's been adding to its mass by simply drawing in dwarf galaxies, which are much smaller than our home galaxy, and collecting free gas from the universe. However, Andromeda, our disk-shaped sister galaxy, is actually headed straight for the Milky Way, Bassett told Live Science. So billions of years from now, the two spiraling galaxies could merge and each of the duo's starry disks will offset the other's rotation, creating a more random elliptical galaxy.
For example, the younger age of disk galaxies is corroborated by their blue color. Blue stars are generally larger, and they burn faster and hotter (blue light has a higher frequency and is thus more energetic than red light). Meanwhile, elliptical galaxies are filled with older stars — called red dwarfs — that aren't burning quite as hot or fast.
Still, despite all we have learned about the massive celestial structures around us, there's still so much we don't know, Hummels said.
"Galaxy formation and evolution is one of the biggest open questions in the field of astronomy and astrophysics," Hummels said.
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