Galaxies are star formation factories. Or at least, some of them are. Katherine Whitaker, a Yale graduate student, led a team of astronomers in surveying 40,000 galaxies. They found that while most of the galaxies were actively making new stars, a subset appeared to have completely shut down.
When viewed with the right filters, galaxies in the process of making stars appear bluer, whereas more passive galaxies are shifted toward the red spectrum. The astronomers examined galaxies that were up to 12 billion light years away (and consequently extremely young) as well as much closer ones. In most cases, it was easy to categorize each galaxy as being ‘on’ or ‘off’. As Pieter van Dokkum of Yale states:
We don’t see many galaxies in the in-between state. This discovery shows how quickly galaxies go from one state to the other, from actively forming stars to shutting off.
Consistent with current thinking, the astronomers believe that galaxies start out actively pumping out stars but then become passive later on. It’s not clear whether this cycle can be repeated, with sleepy galaxies coming back on line.