Every galaxy has a unique story to tell. The history of each one is evident in its shape. Scientists divide galaxies up into a handful of appearance-based categories. The Milky Way is what’s known as a spiral galaxy, meaning that it looks like a broad, flattened disc with a slight bulge protruding outward at its center.
That arrangement is the product of rotation speed, time and gravity. To learn more, we talked to astrophysicist Raja GuhaThakurta, Ph.D., a professor at the University of California Santa Cruz and an authority on the study of how galaxies evolve. It’s a field that invites a lot of debate.
“The physics of how these things form is not completely known or settled,” GuhaThakurta says. Nonetheless, it’s widely thought that most spiral galaxies begin their lives as spinning clouds of gas and dust. The speed at which they rotate matters a great deal. According to GuhaThakurta, massive, rapidly rotating clouds are more likely to become spiral galaxies.
Gravity attempts to pull these spinning, amorphous bodies into flattened planes. Over time, the clouds contract because of gravity and loss of energy due to friction. And due to a principle called the conservation of angular momentum, when a spinning object contracts, it rotates more rapidly.
You can see this in action at your local skating rink. Experienced ice skaters know to increase their twirl speed by drawing their arms inward.
Galaxy NGC 4302n (left) and galaxy NGC 4298 (right) are examples of spiral galaxies like our Milky Way. They appear different in the image taken by Hubble Space Telescope on April 24, 1990 because they are angled at different positions on the sky.
NASA and The Hubble Heritage Team
So, much like a spinning blob of pizza dough, spiral galaxies form when shapeless gas/dust clouds flatten out at high speed. The same physical forces also affect the look of the pointed “arms” that can be seen around the rims of such galaxies.
“The types of spiral arms are almost certainly related to the rotation rate,” GuhaThakurta says. Rapidly rotating systems tend to have a ring of small, tight arms. In contrast, those that move more slowly have longer, loosely wrapped ones.
To understand why, GuhaThakurta recommends trying a little home experiment: “Imagine stirring your coffee. Put a dollop of cream somewhere other than the center. You’ll notice that the cream will form a spiral pattern,” he says. Then, stir the brew with a spoon. If you do so rapidly, the pattern’s arms will get smaller and tighter.
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