inside-out growth

The term inside-out growth refers to a pattern of star formation within spiral galaxies, that it initially occurs near the center, then spreads outward. This pattern was suggested by models and supported by observations of the ages, metallicity, and alpha enhancement of stars at different radii within the Milky Way and observable spiral galaxies. The star formation history of spiral galaxies remains a topic of research interest, e.g., to what degree inside-out growth describes it and what other factors (such as radial mixing) produce the observed stellar characteristics. The term outside-in growth naturally refers to the converse, which observations suggest is less common.