In the 1830s and 40s, astronomers and even casual stargazers noticed that the star Eta Carinae was brightening, quickly becoming the second brightest star in the night sky. At a glance it seemed like the star had gone supernova, but it strangely survived the explosion. Thanks to the weirdness of space, modern astronomers have gotten the chance to travel back in time to witness the event, and discovered that Eta Carinae's outburst may have been the star violently cannibalizing a sibling.
Today, Eta Carinae still bears the scars from that explosion, in the form of a cloud of debris surrounding it that's known as the Homunculous Nebula (of which you can 3D print a model, if you're so inclined). We now also know that Eta Carinae is a binary system, made up of a large, unusual star and a smaller, hotter partner.
Although some clues were left behind, exactly what happened to the system 170-odd years ago remained a mystery. But space is a weird place, and sometimes it plays reruns of old events – in 2015, astronomers watched a past supernova play out again thanks to the light-bending effects of a massive galaxy cluster, through a phenomenon known as gravitational lensing.
In a similar vein, astronomers have had front-row seats to the explosion of Eta Carinae since about 2003. This time though, the replays aren't the result of gravitational lensing but "light echoes," where some of the light from the event bounced off interstellar dust and was reflected towards Earth, arriving 170 years or so later.
To find out what happened to Eta Carinae, a team of astronomers has now peered towards the system using telescopes at the Las Campanas Observatory and the Gemini South Observatory, both in Chile. Using spectroscopy, the researchers were able to measure how fast the material was thrown out of the star during the explosion, and clocked it at over 20 million mph (32 million km/h).
"We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived," says Nathan Smith, co-lead researcher on the project. "The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds."
Based on their data, the researchers suggest that the simplest explanation for the event, and the observed state of things now, is that the two stars of Eta Carinae have a long-lost sibling. In their proposed scenario, the system was once home to three stars, two of which were large and relatively close together, while the third orbited further away.
As part of the normal life cycle of such stars, the larger of the closely-knit pair began to swell up, and its buddy would have swallowed up that material and grew to about 100 solar masses. The donor star, meanwhile, would have shrunk to about 30 solar masses and remained as just a hot helium core – seen today as the secondary star in the binary system.
"From stellar evolution, there's a pretty firm understanding that more massive stars live their lives more quickly and less massive stars have longer lifetimes," says Armin Rest, co-lead researcher on the project. "So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn't make sense without a transfer of mass."
But there's more to the story. This mass transfer would have shaken up the gravitational balance of the system, sending the helium-core star drifting away from its more massive partner. That journey, in turn, disrupts the orbit of the third star, which would have been thrown back inwards, towards the big star. After a slow dance of doom, the two stars finally collide and throw off waves of material, causing a glow that was seen as a brightening in the mid-19th century.
The story could help researchers better understand the physics of unusual star systems, as well as the life and death of stars. An animation of the events can be seen in the video below.
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