The Meerkat of Science: Eta Carinae
Space is filled with fascinating stars. There are diminutive red dwarves, there are stars like our own sun, that spin in orbit around each other. You have stars that are so large they would engulf Jupiter and even Saturn, if placed within our solar system. You have stars that shine brighter than a thousand sun-type companions. There are so many incredible, wondrous stars in the cosmos, that it is difficult to know which one to honour first, but I thought I’d take a look at a unique, turbulent binary system, the system of Eta Carinae.
The earliest firm record of Eta Carinae comes from Edmond Halley, who noted the star’s position in 1677, though it’s possible that references to a bright star in the southern hemisphere are in fact references to Eta Carinae. The star shot to fame within the astronomical world due to two huge eruptions, made the star easy to spot in the night sky. The Great Eruption, in the middle of the 19th Century, and the Lesser Eruption, in 1890, may not be the only occasions that Eta Carinae has produced powerful stellar explosions, though the observational history of these events is hard to pin down.
What isn’t hard to pin down is the erratic nature of Eta Carinae.
A True Giant
The primary star, Eta Carinae A, is believed to weigh in at 100 solar masses, placing it in the high end mass category. There are more massive stars out there, but not many, and certainly not many that can be observed in as much detail. The precise details of Eta Carinae A are proving difficult to determine, thanks to the huge plumes of ejected gas and material from the earlier eruptions, but scientists have pieced together enough to make some reasonable conclusions of the primary star’s mass, and they have also been able to determine that this is a star on the edge.
In previous posts, I mentioned that stars are a balancing act between the fusion at their cores, and the force of gravity. These two competing forces form an equilibrium, and in the end, gravity wins when the star runs out of fuel to sustain fusion. In the case of Eta Carinae A, the forces at work in the core are so powerful, they have brought the star to the brink of the Eddington Limit.
The Eddington Limit is the theoretical point at which the power within the star exceeds the pressure of the gravity bearing down. If this limit is breached, the star will lose material in powerful explosions. Such is the turbulent energy churning away within Eta Carinae A, that it is thought to have touched this limit, and at times, barely held itself together. The star is unstable, and there is evidence that the internal structure of the star has not fully recovered from its prior outbursts.
Complicating the evolution of the primary star is the presence of a secondary star, about which less is known. Eta Carinae B is believed to be quite massive in its own right, anywhere from 30 to 80 solar masses, and it has a strong, fast solar wind, which creates a channel in the wind of the primary as it orbits. This orbit is highly irregular, and thus the secondary star will be alternately stretching and compressing the primary. Needless to say, this has a serious effect on the evolution of the primary!
The Future
Eta Carinae A is going to explode as a spectacular supernova in the astronomically near future, and there is speculation that it could be so powerful, it deserves a category of its own, namely a hypernova. The explosion could rip through the previously ejected material, and light up the Carina Nebula, where the star resides. How this impacts Eta Carinae B is unclear, though the most likely outcome is for the secondary star to be roughly and quickly ejected from the system. It’s widely believed that Eta Carinae A has enough mass to collapse into a black hole, and thus could offer us a glimpse into the process of black hole formation.
When might all this happen? Well, it is hard to say. Direct observations of the stars are impossible at the present time. Eta Carinae A is known to be unstable, and prone to violent outbursts. Astronomers have seen other stars display similar eruptions, and those stars have blown themselves up a few years later. On the other hand, it’s been more than 150 years since the Great Eruption, and the star is still with us. Such massive stars are rare, and therefore there’s a lot we haven’t yet observed of their behaviour. This can make definitive predictions of their futures challenging. Nonetheless, what we can be reasonably certain of is that Eta Carinae A will provide an incredible picture when it finally does go supernova, and the possibility cannot be ruled out that this might be soon!
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