"Even among Luminous Blue Variable [stars], η Car is unusual and its parameters are extreme."

That bit of science-speak roughly translates to "Even among the largest, most energetic stars, Eta Carinae has done things we can't explain, but find incredibly impressive." The top item in η Carinae's (η is the Greek letter eta) list of extreme behaviors involves producing a decades-long outburst that caused it to become the second-brightest star in the sky. This outburst released as much energy as a supernova and ejected many times the mass of the Sun. Yet somehow η Carinae remained intact.

Now, researchers have used a series of Hubble images to produce a timeline of the debris left behind by this enigmatic outburst. The new data reveals that this was just the latest in a series of eruptions, and we still can't explain why they happen.

η Carinae is actually a binary star system. η Carinae B, the smaller of the two stars, is still enormous, with up to 60 times the mass of the Sun. η Carinae A, however, is at least 90 solar masses and possibly much larger. Both are expected to be hundreds, if not thousands of times brighter than the Sun. The two share an extreme 5.5 year orbit: their distance varies between 1.6 Astronomical Units (an AU is the average distance between the Earth and Sun) to as much as 30 AU.

Saying much more about the two stars is tough, because they're embedded in a dense, lobed cloud of gas and debris known as the Homunculus Nebula. That material was put in place by what's called the Great Eruption. While the stars aren't currently visible to the naked eye, this wasn't always the case. Astronomers noted a brightening that started in the 1820s and continued until the Great Eruption in the 1840s, at which point η Carinae became the second brightest star in the sky. A second, smaller eruption occurred later that century, and the system has continued to vary in brightness since.

The cause of that brightening has been determined to be a staggering eruption of matter from the surface of η Carinae A. The star is estimated to have ejected between 10 and 15 solar masses of material into space, sending it off with a kinetic energy equivalent to 1050 ergs (a bit over 2 x 1027 megatons). That's nearly the amount of energy produced in a core-collapse supernova, which normally tears its star apart. Somehow, η Carinae A is still there.

So what could possibly cause an event like that? A rare class of large star, called Wolf-Rayet stars, also experience sporadic ejections of matter. But these tend to only eject about 10 percent of the Sun's mass—not several hundred percent.

A number of potential explanations have been proposed, all of them as extreme as the event itself. These include the idea that η Carinae started out as a three-star system, and the merger of two of the stars caused the Great Eruption. There's also the idea that, prior to its mass loss, η Carinae A was physically larger, allowing η Carinae B to collide with the outer layers of the stars at its orbit's closest approach.

These different models have consequences for the behavior of both the stars and the matter released during the Great Eruption. So three astronomers (Megan Kiminki, Megan Reiter, and Nathan Smith) joined forces to look more carefully at the Homunculus Nebula to see if the debris could help discriminate among these ideas.

The three astronomers were aided by the impressive longevity of the Hubble Space Telescope. More than a decade ago, they performed some observations of η Carinae using the Hubble. Recently, they got observation time specifically when the Hubble would be oriented in roughly the same way as it was back then, allowing them to minimize perspective differences between the images. They also trolled through the Hubble archives to find other images that included η Carinae. The astronomers adjusted these for the difference in location and pointing angle.

All told, they were able to track the material liberated in the Great Eruption for a span of 21 years using 11 different images. This allowed them to find the relative motion of 792 individual clumps of matter within the nebula. Once adjusted for perspective, every single one of these objects appears to be moving directly away from the star that ejected it. And, at least within the time window examined, there was no trend in the velocity, suggesting the material may still be moving at roughly the same speed as when it was first ejected. (It's moving pretty quickly, too, at speeds exceeding 1,450 km/second.)

The astronomers tested this conclusion by modeling where the material would have been 50 years ago and then comparing that to an archival photo of the star system. It all checked out.

So, if you know how fast the material is moving and where it is now, you can calculate when it left its point of origin, η Carinae A. Large portions of the material can be traced back to the early 1800s, suggesting that the great eruption actually occurred somewhat earlier than the brightening. This can be accounted for if the erupted material caused the brightening by slamming into other dust and gas in the neighborhood of the stars.

But some major features date back to the 13th century. And, oddly, all the material dating from this period is off on one side of the star, suggesting that the earlier eruption was radically asymmetrical. There are also hints of a smaller eruption in the 1500s that was equally one-sided.

This is pretty likely to rule out a merger as the cause of the Great Eruption. After all, it's not clear that a single merger event could produce eruptions for centuries afterward. In their forthcoming paper, Dr. Reiter and her colleagues state, "Non-repeating mechanisms such as the merger of a close binary in a triple system would require additional complexities to explain the observations." Whatever is causing these events also has to be capable of causing asymmetric eruptions in order to account for the distribution of matter in the nebula.

As for what the mechanism might be, the three astronomers have no suggestions. The last line of their paper simply sets out the hurdle that theorists must clear: "Models for this still-enigmatic star must therefore explain the recurrence of these major mass-loss events, along with their several-hundred-year timescale and their various asymmetries." For now, η Carinae remains a mystery and a challenge.

The arXiv. Abstract number: 1609.00362 (About the arXiv). To be published in Monthly Notices of the Royal Astronomical Society.