Each hemisphere of the Earth has that one giant star that just might explode as a supernova. Over the past year or so, we’ve talked about our northern hemisphere giant a lot: that’s Betelgeuse, the red shoulder star of Orion that became markedly fainter in late 2019. Now, the southern hemisphere supernova hopeful is in the news because it is threatening to drown out the light of its nebula.
This star, Eta Carinae, got its name from being one of the brighter stars in its constellation, but it was far from the brightest, at least until 1827 when it suddenly grew in brightness until in March 1843 it became the brightest star in the southern hemisphere and the second brightest star in the entire sky. It is believed that during this event, Eta Carinae expelled much of its atmosphere into what is now called the Homunculus Nebula.
Following all the excitement, Eta Carinae faded from view for decades before a slow rebrightening started in the early 1900s. Ever since Eta Carinae has been continuing to brighten, and today it is back to being a fairly bright star. Or should I say stars? While a single giant star is responsible for the behaviors we’re seeing, this star is likely aided and abetted by a companion star that both shapes the Homunculus Nebula and interacts with the giant star’s wind.
These two stars orbit every 5.5 years and the highly elliptical nature of this orbit means they dance together and apart over this time. Until recently, when the stars came together, mass transfer from the massive star to the companion led to the system shining in extreme ultraviolet. That is until recently, which seems to indicate that the star is no longer passing along its mass the way it used to.
During both the 2014 and 2020 close passages, no extreme ultraviolet light was seen, and this seems to indicate that the massive star has settled into a less sharing phase; instead of blasting out its material, it has more normal stellar winds and is hanging out getting brighter and brighter over time. If this new behavior persists, Eta Carinae will get bright enough that its light will make it difficult to image the surrounding nebula, but this will only happen if this behavior continues.
This work was presented by Kris Davidson at this week’s virtual American Astronomical Society meeting. Kris points out, with a distance of 7500 light-years, Eta Carinae is the only massive star of this type that is close enough to study in detail, and for all we know, this star has already gone supernova, and now we’re just waiting for the light from that explosion to reach us here at Earth.