Six galaxies have been caught in a dramatic transformation as the black holes at their centres suddenly developed voracious appetites.

A quasar is an extremely bright galaxy with a gluttonous black hole at its core. The black hole’s consumption of gas and dust emits large amounts of light, producing the quasar’s dazzling luminosity. Quasars fall into one of two categories depending on the spectrum of the light that they emit. Strangely, some quasars can switch between the two types, a mysterious process known as ‘changing their look’.

By comparing recent observations with archival data, Sara Frederick at the University of Maryland in College Park and her colleagues found six galaxies that brightened into quasars over less than a decade as their relatively dormant black holes suddenly became active. One galaxy needed less than three months to light up. During this process, the galaxies also changed their look.

Follow-up observations suggest that these are part of a new category of galaxies capable of suddenly consuming vast amounts of material, the authors write. Further study could help reveal why some galaxies can change their look.