Look at an image of the Milky Way galaxy, and you can't help but notice its exquisite spiral arms. For nearly 100 years, astronomers have tried to understand how the Milky Way and other spiral galaxies formed these dramatic patterns--and now they think they finally have the answer.
Current thinking about how spiral galaxies form traces back to an idea nearly 2 millennia old, to 2nd-century Egyptian mathematician Ptolemy. Trying to describe how the five planets in the night sky followed seemingly irregular paths, Ptolemy hit upon an idea he called epicycles. Basically, the theory posits that, as an object orbits, it performs little loops that make it appear to wobble. The idea was revived in the 1920s to explain similar irregular movements among the stars in the Milky Way and later still to describe the formation of galactic spiral arms. But there have been little data to support it.
Independent mathematician Charles Francis of Hastings, U.K., had been studying a cosmological problem that required him to analyze the motions of stars. So he compiled data on more than 20,000 stars from existing sky surveys whose position in our galaxy and orbital velocity had been accurately measured. Among the data, he noticed something that startled him: Nothing supported the epicycle hypothesis. So Francis and independent astronomer Erik Anderson of Ashland, Oregon, developed a simulation based on the velocity distribution of stars that they found in the sky-survey data. As the two report online today in the Proceedings of the Royal Society A, when they ran the simulation, it quickly--within a hypothetical 300 million years--formed spiral arms similar to those in the Milky Way. The stars simply followed the laws of gravitation, as set down by Isaac Newton over 300 years ago. The idea was so sufficiently simple, Francis says, "that I had rejected it before because I thought that if it were right, it would already be known. In fact, it worked straightaway, giving a perfect fit with the data."
As Francis describes it, the Milky Way and other spiral galaxies operate like a giant, spiral-grooved funnel into which billions of marbles are pouring. The grooves represent the gravitational field of the galaxy's spiral arms. Each marble finds a groove at the top of the funnel and begins to follow it down a never-ending slope, picking up momentum as it goes. Eventually, the marble gains enough momentum to jump free of its groove. It crosses to the next-highest one then falls back to a higher point in its original groove. (Watch the process here.)
Astronomer Rainer Klement of the Max Planck Institute for Astronomy in Heidelberg, Germany, says the paper "comes up with an elegant way of explaining the velocity distribution we observe in the solar neighborhood." He says new space missions scheduled in the coming years should map the position and motions of more stars "with unprecedented precision," and he predicts that the new data will support the paper's conclusions.