What's puzzling astronomers most is that the galaxy at the center of the Phoenix Cluster appears to be in an active "starburst" period—birthing more than 740 stars a year, based on data from NASA's Chandra X-ray Observatory, theNational Science Foundation's South Pole Telescope, and eight other telescopes on Earth and in orbit.

Among central cluster galaxies, "the previous record-holder forms stars at a rate of around 150 stars per year, so this is absolutely crushing that record," McDonald said.