Why Are Most Galaxies Spiral-Shaped?

The structure of spiral galaxies is more complex than we thought. Here’s how the laws of physics contribute to the shaping of galaxies.

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What Process Creates And Maintains The Beautiful Spiral Arms Around Spiral Galaxies? I’ve Been Told That Density Waves Are Responsible — So Where Do The Density Waves Come From?
http://www.scientificamerican.com/article/what-process-creates-and/
“”The basic physics of why galaxies have spirals is known, but the details remain controversial, sometimes intensely so. Spirals exist only among flattened or ‘disk’ galaxies. These galaxies are differentially rotating–that is, the time to complete a full rotation increases with distance from the center. Differential rotation causes any disturbance in the disk to wind up into a spiral form. The trouble with this simple explanation is that the differential rotation would cause spiral features to wind up too quickly, so galaxies would not look like spirals for any appreciable length of time.”

How Do Spiral Galaxies Keep Their Shape?
http://earthsky.org/space/how-do-spiral-galaxies-keep-their-shape
“Astronomers sometimes call this the “wind-up problem” of the spiral arms. It has been observed that the inner parts of galaxies rotate faster than the outer parts. If so, are the inner parts of the spiral arms moving faster than the other parts? If that is happening, then the galaxy would have no choice but to wind up so much that the spiral structure would be thinned out and ultimately destroyed.
But we can see galaxies billions of light-years away – as well as those relatively nearby – that have maintained their spiral structure. How?”

Astrophysicists Discover Mechanism For Spiral-Arm Formation In Disk Galaxies
http://phys.org/news/2016-08-astrophysicists-mechanism-spiral-arm-formation-disk.html
“Astrophysicists at the University of Arkansas have discovered a mechanism for the formation of the spiral arms in disk galaxies. The finding was published in the Astrophysical Journal Letters, the journal of the American Astronomical Society. The discovery provides a better understanding for the formation of spiral arms in a kind of disk galaxy known as a spiral galaxy, said Hamed Pour-Imani, a physics doctoral student at the U of A and lead author of the study.”

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