During the 1840s, an apparently unremarkable star began to brighten. Over the course of roughly a decade, it became one of the brightest stars visible from Earth. Often, brightening like that means a supernova has destroyed the star, but η Carinae (or Eta Carinae) was still there when it was all over, and it underwent a number of smaller events over the ensuing century and a half.
Modern astronomy hardware has revealed that the resemblance to a supernova goes deeper than these early observations. Imaging of the complex nebula that surrounds η Carinae has revealed that a giant star had ejected roughly 10 times the Sun’s mass worth of material into its surroundings during what’s now known as the Great Eruption. Imaging also revealed that the system is a binary, containing a second enormous star in an eccentric orbit around the first.
We can’t go back in time to observe the Great Eruption with modern instruments. But a team of researchers has been tracking its progress using echoes of light reflected off some dust that was more than 100 light years away from the star. The echoes reveal some material moving at a phenomenal speed—roughly 20,000 kilometers a second. That, combined with other unusual features of the system, led them to propose that there used to be three stars in η Carinae, and the outburst was the result of two of them merging.