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up to 30 percent of stars had compositions indicating that they were formed in parts of the galaxy far from their current positions.
When the team looked at the pattern of element abundances in detail, they found that much of the data could be explained by a model in which stars migrate radially, moving closer or farther from the galactic center with time.
Astronomers decades ago proposed an evolutionary progression among normal galaxies, starting with the near-spherical ellipticals that gradually became squashed ellipticals, eventually changing into closed spirals, followed by open spirals, and finally culminating in irregular galaxies. Figure 2.19 schematically illustrates this evolutionary scheme, whose central idea is that galaxies originate with a more or less spherical shape and, as they grow older, their rotation tends to flatten them, first producing some ellipticity and then some spiral arms, prior to their breaking up as aged irregular galaxies.
On the other hand, given that the elliptical galaxies are so clearly old, then perhaps the evolutionary sequence runs in the opposite sense. Maybe irregulars are young and, having formed first, gradually evolve into ellipticals. It’s easy to imagine loose spiral galaxies wrapping up into tighter spirals and eventually becoming elliptical galaxies.