A candidate for the most interesting known star has been shown to be even more complex than previously thought, with interactions with its companion shaping the enormous field of gas and dust it is throwing off.

Eta Carinae A is a titan among stars, so large it forced a rethink of the maximum brightness a star could reach. In the mid 19th Century it became the second brightest star in the sky for a period, an astonishing achievement since it is 20 times as far away as Canopus, the giant it temporarily displaced. Its fame was only stifled by the fact that it lies too far south to be seen from most of the northern hemisphere.

Soon however, something much more important was stifling Eta Carinae's luster. Having thrown off 10-40 times the mass of the sun as if it was no big thing, the clouds emitted started blocking Eta Carinae's light from reaching our eyes. It's still visible in small telescopes as an intensely red star in the middle of a magnificent star cluster, but most of the light is captured in the light year wide dust cloud known as the Homunculus Nebula.

^{Credit John Gleason & Steve Mandel​. Eta Carinae and the cluster of stars around it.}

Eta Carinae B is about 30 times the mass of the sun and at least 100,000 times the brightness – an enormous star by ordinary standards, but a third or less of the mass of the primary. The relationship between the two is hard to observe so closely hidden behind such an impenetrable wall. It is only in the last 20 years years that astronomers have become confident two stars exist inside the nebula, with an orbital period of 5.5 years.

By examining 92 swathes across the nebula (up from 5 previously) in wavelengths from the infrared to the ultraviolet a team lead by Wolfgang Steffen of the Universidad Nacional Autónoma de México created a model of the Homunculus. Their model is both more detailed than anything before, and avoids some past assumptions that may be inaccurate. Their work is published in the Monthly Notices of the Royal Astronomical Society, where they reveal the nebula is not as symmetric as previously thought, and has “circumpolar trenches” in each lobe.

The authors believe the lack of symmetry is a consequence of the influence of the companion star. Although they leave open the question of whether the companion helped trigger the explosion in the first place, they believe the evidence points to the explosion occurring when the two stars were at their closest point, which seems something of a coincidence. It is believed Eta Carinae B has a highly eccentric orbit, and the authors suggest, “This creates a large-scale (≳1600 au) time-averaged wind cavity on the apastron side of the system.”

^{Credit: NASA Goddard (inset: NASA, ESA, Hubble SM4 ERO Team). Previously undetected irregularities in the nebula indicate the influence of Eta Carinae's companion.}

The study of exceptional stars like Eta Carinae is more than a hobby of people who like the extremes. As the paper notes, “Massive stars have major impacts on their host galaxies via the input of ionizing photons, energy, and momentum into the interstellar medium.” Moreover, many astronomers suspect that the events of 180 years ago are preliminary to a supernova explosion, possibly very soon.