Two articles published in the journal “The Monthly Notices of the Royal Astronomical Society” describe different aspects of a research that offers an explanation to the Great Eruption, an event in which the brightness of the Eta Carinae system increased between 1820 and 1843. A team of astronomers used various telescopes to gather new information about what’s happened in that area, concluding that originally there were three stars whose interaction ended up leading to the destruction of one of them.
About 7,500 light years away from the Earth, the Eta Carinae system was for some decades the second brightest star in the sky after reaching its peak of brightness in 1843. For that reason, in the 19th century it became famous but the instruments of that period didn’t enable astronomers to examine the Great Eruption in enough detail to explain its cause. After almost two centuries, the new generation instruments could finally allow to reconstruct the events observed at that time in what was called a forensic astronomy work.
A team of astronomers led by Nathan Smith of the University of Arizona and Armin Rest of the Space Telescope Science Institute carried out that task by exploiting a sort of light echo of those events. In simple words, a part of the light emitted by the Great Eruption rebounded on interstellar dust reaching the Earth only recently because it arrived on an indirect route, which is longer.
Various light echoes of the Great Eruption have been detected since 2003 by the 8-meter Gemini South telescope, Cerro Tololo Inter-American Observatory 4-meter Blanco telescope, and the Magellan Telescope at Las Campanas Observatory. Spectroscopic analyzes allowed to examine the echoes and to reconstruct the expansion of the materials that followed that event.
In particular, the spectroscopy obtained at the Gemini Observatory allowed to estimate the velocity of the gas expansion in that explosion between 10,000 and 20,000 kilometers per second. These are the typical speeds of supernovae, where there’s a destruction of the star, but it’s the first time that they’re detected in an event in which a star survives. The new data add a mystery to the Great Eruption mystery but eventually they may have contributed to its solution.
According to the researchers, the explanation for the Great Eruption and what was observed in the Eta Carinae system is that originally there were three stars and that two merged into a very massive one. The top image (NASA, ESA, and A. Feild (STScI)) illustrates the possible scenario that led to that explosion in the 6 panels.
- Eta Carinae initially was a triple-star system. Two hefty stars (A and B) in the system are orbiting closely and a third companion C is orbiting much farther away.
- When the most massive of the close binary stars (A) nears the end of its life, it begins to expand and dumps most of its material onto its slightly smaller sibling (B).
- The sibling (B) bulks up to about 100 solar masses and becomes extremely bright. The donor star (A) has been stripped of its hydrogen layers, exposing its hot helium core. The mass transfer alters the gravitational balance of the system, and the helium-core star moves farther away from its monster sibling.
- The helium-core star then interacts gravitationally with the outermost star (C), pulling it into the fray. The two stars trade places, and the outermost star gets kicked inward.
- Star C, moving inward, interacts with the extremely massive sibling, creating a disk of material around the giant star.
- Eventually, star C merges with the hefty star, producing an explosive event that forms bipolar lobes of material ejected from the monster sibling. Meanwhile, the surviving companion, A, settles into an elongated orbit around the merged pair. Every 5.5 years it passes through the giant star’s outer gaseous envelope, producing shock waves that are detected in X-rays.
Astronomers expect an actual supernova in the Eta Carinae system, meanwhile they try to study moments of the evolution of massive stars and interactions between different stars. A few weeks ago, an article published in the journal “Nature Astronomy” described a research on that system that concluded that the two stars are probably accelerating very high energy particles and that some will reach the Earth in the form of cosmic rays. In short, it’s a system that after almost two centuries of observations continues to be extremely interesting.