For a short spell in the mid 1800s, η Carinae, a hypergiant star in the Southern Hemisphere’s Carina constellation, had the look of what astronomers now call a supernova: It ejected some 15 solar masses of gas and dust, released 1043 joules of energy, and briefly became the second-brightest star in the sky. But unlike a star undergoing a supernova, η Carinae survived the flare-up. The event, known as the Great Eruption, has long puzzled astronomers, and now a study by Megan Kiminki, Nathan Smith (both at the University of Arizona), and Megan Reiter (University of Michigan) suggests there’s more to η Carinae’s story.
By analyzing 20 years of Hubble images, Kiminki and her coworkers were able to estimate the velocities of hundreds of objects in the expanding cloud of ejecta that surrounds the star. Extrapolating the trajectories back in time, they could infer the moment, give or take a few decades, when each object was hurled from the star. The results indicate that the ejecta couldn’t have originated from a single eruption; there had to be two others—one in the 1500s and another in the 1200s. In the image, the colors of the velocity arrows indicate the time of an object’s ejection (red, 1800s; yellow–green, 1500s; blue–violet, 1200s). The star itself is obscured by an oblong cloud known as the Homunculus Nebula. The apparent 300-year cycle of outbursts rules out at least one popular explanation for the Great Eruption—that it was a one-time event triggered when η Carinae swallowed a smaller companion star—and places new constraints on others. (M. M. Kiminki, M. Reiter, N. Smith, Mon. Not. R. Astron. Soc., in press.)
