Most galaxies in the observable universe, including our own, have that distinctive spiral or pinwheel shape. This design is one of the cosmos' loveliest compositions, and it echoes an odd predilection for spirals that can be found throughout creation.
The odd part is that no one really knows why these galaxies are shaped as they are. Furthermore, from what we can tell, that spiraling effect is essentially an optical illusion. Trace Dominguez investigates this cosmic mystery in today's DNews report.
Most galaxies spin around a central point, usually a supermassive black hole. Look at the typical galaxy shape, and it's easy to imagine that each of those spiral arms is a solid cluster of stars, revolving eternally around that cosmic axis. But if that were true, then the stars at the end of a spiral arm would have to be moving much faster than the stars near the center. Otherwise the galaxy would not hold its shape.
Everything we know about physics tells us that the opposite should be true: Objects move faster when they're closer to the point in space that they orbit. Otherwise, they lose the eternal battle with gravity and get sucked in. For those keeping score at home, this is Kepler's Third Law.
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The only conclusion is that those spiral arms aren't really discrete clusters of stars at all -- we're not really looking at what we think we're looking at. Astronomers have come up with two possible explanations for the optical illusion.
The first proposes a kind of cosmic traffic jam effect. We're not seeing individual stars when we look at a spiral arm, but rather an endless sequence of stars moving in an out of density waves. It's like looking at a traffic jam from a distance: The line of cars appears solid, but the vehicles are constantly moving and flowing.
The other explanation suggests that each star formations in a galaxy causes a kind of shock wave that kicks off still more star formation. The rotation of the galaxy stretches the bright new stars into the familiar spiral arms, then they burn out and die as the chain reaction propagates and the arms move on.
This is all radically simplified, of course. You'd need several doctorates and/or exotic alkaloids to really wrap your head around it all. Check out Trace's video for some helpful visual aids.
-- Glenn McDonald
Scientific American: What Process Creates And Maintains The Beautiful Spiral Arms Around Spiral Galaxies? I've Been Told That Density Waves Are Responsible -- So Where Do Density Waves Come From?
Earth Sky: How Do Spiral Galaxies Keep Their Shape?
Phys Org: Astrophysicists Discover Mechanism For Spiral-Arm In Disk Galaxies