Astronomers know a thing or two about galaxies, including how many there are in the universe (at least 100 billion) and the different shapes they take (spherical, elliptical and spiral). But as to the particulars of the ginormous star-studded arms of the Milky Way and other galaxies of the spiral sort, astronomers have been largely in the dark.
With the help of a stupendous computer simulation that tracks 100 million separate "stellar particles," researchers at the University of Wisconsin in Madison and the Harvard-Smithsonian Center for Astrophysics showed that the galaxies’ arms reflect the action of forces from gas clouds within the galaxies.
Gravitation from the clouds perturbs the orbits of stars within the galaxies, kick-starting the formation of swirling "filamentary" structures that turn into the arms, study co-author Dr. Lars Hernquist, professor of astrophysics at the center, told The Huffington Post. He likened the swirling structures to the tendrils of cream that appear when it's stirred into a cup of coffee.
And once the arms appear, they don't go away.
"We show for the first time that stellar spiral arms are not transient features, as claimed for several decades,” study co-author Dr. Elena D’Onghia, professor of astronomy at the university, said in a written statement. “They are self-perpetuating, persistent and surprisingly long lived.”
Just how long do the arms retain their shape? According to Dr. Hernquist, evidence suggests they "stick around for the life of the galaxy."
A paper describing the research was published online in Astrophysical Journal.