During the 1840s, an apparently unremarkable star began to brighten. Over the course of roughly a decade, it became one of the brightest stars visible from Earth. Often, brightening like that means a supernova has destroyed the star, but η Carinae (or Eta Carinae) was still there when it was all over, and it underwent a number of smaller events over the ensuing century and a half.

Modern astronomy hardware has revealed that the resemblance to a supernova goes deeper than these early observations. Imaging of the complex nebula that surrounds η Carinae has revealed that a giant star had ejected roughly 10 times the Sun's mass worth of material into its surroundings during what's now known as the Great Eruption. Imaging also revealed that the system is a binary, containing a second enormous star in an eccentric orbit around the first.

We can't go back in time to observe the Great Eruption with modern instruments. But a team of researchers has been tracking its progress using echoes of light reflected off some dust that was more than 100 light years away from the star. The echoes reveal some material moving at a phenomenal speed—roughly 20,000 kilometers a second. That, combined with other unusual features of the system, led them to propose that there used to be three stars in η Carinae, and the outburst was the result of two of them merging.

A troublesome history

Coming up with a model that explains η Carinae is difficult in part because it's a system of such extremes. The smaller star is anything but small, having a mass that's likely to be in the neighborhood of 50 times that of the Sun. Its five-year orbit takes it in close proximity to its companion, a monster that could be more than 100 solar masses. Both of them are embedded in the Homunculus Nebula, the product of earlier eruptions that holds about 15 solar masses of material in its two main lobes, which roughly align with the stars' orbit.

Needless to say, we don't have detailed physical models of extreme circumstances like this. Complicating matters further, we've been observing it for well over a century, meaning we have a lot of data that must be incorporated. "Any model for η Car must face a daunting gauntlet of observational constraints," the authors of one of the new η Carinae papers note, before going on to explain one of the consequences: "To any proposed simple theory applied to η Car, one can usually respond with 'Yes, but what about... [insert obscure observational detail here]?'"

The paper focuses on two observational details, one from their own study and one from elsewhere. The one from elsewhere notes something odd about the system. The smaller of the two stars appears to have already gotten rid of all its hydrogen and is burning nearly pure helium. This would make it rather old as far as massive stars go and much older than the star it orbits, which appears to still have copious amounts of hydrogen.

The second one came courtesy of what are called light echoes. When light from an event strikes an object like a dust cloud, most of it either passes through or is absorbed. But a small fraction is reflected back, much like an echo involves reflected sound. Over the past few decades, the team behind the new paper has been identifying light echoes in the area of η Carinae and observing them whenever they could obtain telescope time. By matching these echoes with observations taken of the η Carinae system over a century ago, the researchers have been able to make a reasonable inference about the origin of the echoes. In their new study, they describe a source of echoes that seems to be replaying the Great Eruption.

The need for speed

These obviously don't have the resolution to allow us to "see" what the stars were doing at the time the Great Eruption took place—the echoes come off a cloud of dust, not a smooth, reflective mirror. Still, they provide some indications of what elements were present and how they were behaving. One key feature they capture is the motion of the materials expelled during the Great Eruption, which cause red- or blue-shifting of the light, depending on whether it was moving away from or toward the cloud of dust at the time. The degree of shifting is proportional to the material's speed.

And the speed ends up being unusually high. While the observations first showed material that was moving at speeds that might be expected if it was pushed out by an energetic star, it was gradually replaced by fast-moving material that was moving at up to 20,000 kilometers per second. Speeds like that have only been observed in material ejected by black holes or a supernova. Which, again, is inconsistent with the fact that the giant star at the center of the η Carinae system is still there.

The authors explain this change of speed by suggesting that there was material surrounding η Carinae due to earlier eruptions by its stars. This initially obscured the Great Eruption, but the fast-moving material eventually caught up with it and broke through its surface. At that point, the full extent of the Great Eruption's speed becomes visible in the light echoes.

This neatly explains the observations but doesn't explain how that speed came about in the first place.

Here, the researchers are at a bit of a loss. All the obvious explanations for strange behavior come up short, and, they admit, "At this point, we must resign ourselves to the fact that something fairly complicated has happened to η Carinae."

Possible explanations

One possible explanation is that the older-looking star formed elsewhere, was ejected from its home by gravitational interactions, and later ended up being captured by η Carinae. The researchers don't like that idea, because it involves two sets of rare events happening in the relatively short lifetime of a massive star.

The alternative is that the star formed where it is and only looks old. If the largest star present stripped off much of its orbital partner's hydrogen, you'd end up with a relatively low-mass, helium-rich star like the one that's present today. There's a chance that, during earlier eruptions, the larger star expanded enough so that the smaller one plunged through its outer layers, allowing hydrogen to be directly stripped off.

All of that would explain the apparently different ages of the two stars but not why the Great Eruption took place. One option is what's called a Pulsational Pair-Instability Eruption. These have been proposed based on exploration of stellar physics, but they haven't been observed. They could occur in stars with a very specific mass, which would cause the partial collapse of their cores once certain fusion reactions start. The energy released during this collapse causes the star to "pulse," triggering a new round of fusion reactions before a partial collapse restarts the process.

This fits η Carinae's behavior, as there seems to have been eruption events both before and after the Great Eruption. The problem is that the pulsational instabilities are expected to occur rapidly—a matter of years, rather than decades separating each event—and quickly lead to the catastrophic explosion of the star. Again, we feel compelled to remind you that η Carinae is still there. So either our models need significant refining, or this is a poor explanation for this star.

The number after two

There is an idea that does get this research team excited, however: η Carinae started as a system of three massive stars, and one of the three was eaten. In this model, the smaller, old-looking star started out in a close orbit with the central star and a third star was orbiting at a safe distance. But, as the central star stripped off material from its companion (making it look older than it is), the reduced mass caused the lighter star to orbit at ever-greater distances. Once far enough away, it would have started interacting with the third star.

At this point, the researchers have to propose that some strange things happened: the third star, through these interactions, moved inwards, eventually passed the old-looking one, and sent it spinning into its present, eccentric orbit. Not content to stop there, the researchers speculate that the third star eventually slammed into the central one. Prior to that collision, the three stars would have exchanged materials, accounting for some of the earlier eruptions of material. The Great Eruption, by contrast, involved the final fusion of the two stars.

Does this actually work? That's not clear; people haven't done much modeling of systems involving multiple massive stars. We also don't have a good idea of what would happen once two massive stars merge. Would it produce something like the Great Eruption? We simply don't know. So that means there are two aspects of the idea that we haven't modeled the physics on, which doesn't make it easy to evaluate in comparison to alternative scenarios that are unlikely but based on better-understood physics.

While we're waiting for astrophysicists to get a better grip on the interactions with massive stars, we can at least appreciate that η Carinae is both our most enigmatic laboratory for the study of massive stars and our closest, most-easily studied one. Plus, it's not unreasonable to think that the system might experience its final eruption in our lifetimes, which adds to its appeal.

Monthly Notices of the Royal Astronomical Society, 2017. DOI: 10.1093/mnras/sty1479, 10.1093/mnras/sty1500 (About DOIs).