Nature vs. Nurture It is not well understood why we have so many galaxy types. It is a major area of current research.
Angular Momentum at collapse (nature)
Random motions in the collapsing cloud (nature)
Star Formation rate (nature? nurture?)
Galaxy Collisions (nurture)
Galaxy Mergers (nurture)
ALL these effects are occurring. This makes it difficult
to determine the importance of any one effect.. It is the combination
of all these effects (and perhaps others not yet identified) which give way to the
amazing array of galaxy properties found in the universe.
Evidence: On average Spirals spin faster than Ellipticals.
Problems: Ellipticals have a large range in rotation speeds.
Evidence: Models with high random motion preferentially create Ellipticals.
Problems: Remains to be proven observationally.
Evidence: An intense burst of star formation used up all Elliptical gas.
Problems: What triggers such large-scale star formation?
Evidence: Ellipticals are more common in areas of galaxy collisions (clusters).
Problems: Then why are Ellipticals found in complete isolation?
Evidence: Enormous Ellipticals at the centers of Galaxy Clusters. It is happening
NOW in our galaxy!