In 1837, Eta Car suddenly leapt from relative obscurity to become the second brightest star in the sky. Davidson estimates that it lost 10 to 20 percent of its mass before its brightness peaked in 1843. Originally more than 100 times the mass of our own star — typical of all such impostors — it expelled the equivalent of 15 to 20 Suns before fading in the late 1850s.
The event had seemed like a normal supernova, with a blinding grand finale of self-immolation. But in 1892 it faintly reappeared as a 6th- magnitude star, only to fade again a few years later. Some astronomers predicted it would return to its peak brightness by 1900. In fact, it is taking much longer: Nearly 200 years after its initial outburst, Eta Car has recovered only to about magnitude 4, with erratic periods of minor brightening.
Still, astronomers have closely monitored it for signs of impending eruptions. One potential telltale is Eta Car’s spectral signature, which shifts every 5.5 years when its much smaller companion star swings by. As the stars’ stellar winds collide during these flybys, the system emits less radiation at ultraviolet wavelengths. However, during the last several close passes, this has not happened. Davidson thinks that Eta Car’s outflow has become “almost a normal stellar wind,” and that may presage a major eruption.
“In the next 20 or 30 years, it’s going to change its appearance,” predicts Davidson. The star is already brighter than the Homunculus Nebula surrounding it, and he thinks it could brighten by another magnitude. “When it does, it will ionize the Homunculus Nebula, making it look like a planetary nebula, but 100 times brighter and with a central star,” or, as he also puts it, “A planetary nebula on steroids.”
Giant superflares from a dwarf
Stars like Eta Car are not the only celebrity lookalikes in our galaxy. Supernova imposters are a category of recurrent novae, systems that repeatedly surge on and off. Most consist of a white dwarf gulping gas from its companion until it overindulges, triggering periodic surface explosions up to a million times the white dwarf’s normal luminosity.