If there's no friction in space, why do the arms of spiral galaxies sweep backwards?
鈥� The short answer is: differential rotation. Stars in a spiral galaxy, which includes a flat disc with a central bulge, orbit about its centre. Those further from the centre have longer orbits and therefore longer orbital periods, even though their velocities are similar to or greater than those of stars closer in. This results in more distant stars lagging behind.
But this raises further questions. Why do most galaxies have spiral arms in the first place? And why, given the much greater age of galaxies (often at least 10 billion years) compared with the typical orbital periods of their stars (a few hundred million years), have the spiral arms not wound up completely and disappeared? The answers to these questions are only partly understood.
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It seems certain that spiral arms aren't fixed structures, but rather result from compression waves that travel through the galaxy's disc.
Stars and gas clouds pile up within a wave to create a spiral arm 鈥� and the compression of gas clouds triggers the formation of more stars 鈥� but they can move in and out of the arm as they orbit. The arm is hence a pattern representing the pressure wave, whose rotation is independent of that of the stars.
An analysis of Hubble Space Telescope images suggests that galaxies initially had an irregular structure that could take a few billion years to settle down. Galaxies destined to be spirals gradually developed a disc appearance with a central bulge, but then required some kind of gravitational force to drive the pressure waves needed for spiral arm formation. Possibilities are the presence of companion galaxies or a bar-shaped concentration of mass extending from the central bulge. About two-thirds of spiral galaxies, including our Milky Way, contain such bars.
Once a pressure wave has formed, the gravitational force of all the stars and gas in the disc helps limit the spiral arm's tendency to wind up. A wave may also be able to reflect off the central bulge and set up a more stable standing wave. But it remains unclear how long spiral arms can last. In some galaxies, they may survive billions of years, while other galaxies might go through cycles of displaying spiral arms that wind up, but later produce new arms triggered by new pressure waves.
Richard Swifte, Darmstadt, Germany
鈥� As a matter of fact, there is friction of sorts in space, both from electric and magnetic fields, and from traces of matter. It might be minute by our usual standards, but on galactic scales it is quite important.
Such friction, however, has little to do with the shape of galactic arms. Any cloud of matter giving rise to a galaxy almost inevitably has a net rotation, and includes a lot of objects travelling in various directions.
鈥淭here is friction of sorts in space, from electric and magnetic fields and from traces of matter鈥�
Any object on an outward trajectory in its orbit within an arm tends to lag, and the further out it goes, the slower its angular velocity, lagging still further. Conversely, objects falling inwards tend to gain on objects further out. They orbit faster when further in because they are under more intense gravitational attraction. So inward, outward and constant orbits all increase the sweep of 鈥渢railing arms鈥�.
Jon Richfield, Somerset West, South Africa
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