How spiral galaxies get their arms

The majority of galaxies aren't spiral shaped. Most of the non-spiral ones are comprised of stars, planets, gas, and other cosmic dust clustered together, like a boring bowl of trail mix with all the M&Ms picked out. That stasis is partly why astrophysicists have long been enamored by the perplexing beauty of spiral galaxies, such as our own Milky Way. Spiral galaxies are a clear minority in the universe (just 15 percent of all galaxies spin), and they tend to raise more questions than they answer.

For instance, are a galaxy's arms transient, merely a phase in its ongoing evolution? Or is the swirling, mesmerizing spin-cycle reflective of something stable, and quite possibly self-sustaining?

The answer, according to new research, may be the latter. Researchers from the University of Wisconsin and the Harvard-Smithsonian for Astrophysics have built an advanced new breed of simulation software to model just how disk galaxies form in the first place.

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According to Space.com, the program allowed researchers to "follow up to 100 million hypothetical stellar particles being tugged at by gravity and other astrophysical forces." This allowed astrophysicist Elena D'Onghia and her team to create a video animation of a spiral galaxy coming into its own:

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Using the model, the team found that star-creating molecular clouds, which are sometimes called stellar nurseries, tug and prod at a flat galaxy to initiate spin. Researchers affectionately refer to these molecular clouds as "peturbers."

"Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed," D'Onghia told Space.com. "It proves that once the arms are generated through these clouds, they can exist on their own through gravity." In other words, unlike a pinwheel that needs a constant stream of air to keep spinning, once a spiral galaxy gets going, it probably doesn't stop.