![hubble tuning fork hubble tuning fork](https://i.ytimg.com/vi/NgWGKw_E-G4/maxresdefault.jpg)
This dynamic explanation for spiral arm formation is supported by state-of-the art computer models of spiral galaxies. These may consist of collections of stars that are bound by each other’s gravity, and physically rotate together. New models however suggest that some arms at least could be real structures, not just ripples. Stars move in and out of these ripples as they pass around the galaxy. One of the most popular is the density wave model - the idea that the arms are not fixed structures, but caused by ripples in the density of material in the disk of the galaxy. There are several proposed mechanisms for how spiral arms form in galaxies. “The new results suggest otherwise, and that has a big impact on our understanding of how galaxies develop their structure.” “We always thought that the bulge size and winding of the spiral arms were connected,” Professor Masters said. The new work is based on a sample 15 times larger from the Galaxy Zoo project, where members of the public assess images of galaxies taken by telescopes around the world, identifying key features to help scientists to follow up and analyze in more detail. Hubble was limited by the technology of the time, and could only observe the brightest nearby galaxies. “This non-detection was a big surprise, because this correlation is discussed in basically all astronomy textbooks - it forms the basis of the spiral sequence described by Hubble,” said Haverford College’s Professor Karen Masters, Galaxy Zoo project scientist and lead author of the study. This suggests that most spirals are not static density waves after all. Now though, in contradiction to Hubble’s model, the new work finds no significant correlation between the sizes of the galaxy bulges and how tightly wound the spirals are. His key observation was that galaxies with larger bulges tended to have more tightly wound spiral arms, lending vital support to the density wave model of spiral arm formation. Hubble’s model is still used widely in astronomy textbooks. Image credit: Karen Masters, Sloan Digital Sky Survey.
![hubble tuning fork hubble tuning fork](http://pages.astronomy.ua.edu/white/101ay/images/galaxy/n4242_SBd.gif)
The Hubble Tuning Fork illustrated with images of nearby galaxies from the Sloan Digital Sky Survey.