Scientists have modeled the complex history of Eta Carinae, an extraordinary star that has been the focus of human attention for decades.

Eta Carinae or Eta Carinae is a binary star system (η Car, η Carinae) that is surrounded by the expanding Homunculus nebula in the shape of an hourglass. It owes its birth to the most powerful explosion of a star in the middle of the 19th century. The modern name was given in 1950 by Enrico Gaviol (Ramón Enrique Gaviol). She has been visible to the naked eye for over ten years.

Two stars in the Eta Carinae system move around a common center of mass in elongated elliptical orbits (eccentricity 0.9) with a period of 5.54 Earth years. The main component of the system is a hypergiant, a bright blue variable (YHV), which originally had a mass of 150-250 solar masses, of which about 30 solar masses have already been lost. It is one of the largest and most unstable stars known, and its mass is close to its theoretical upper limit. It is expected to go supernova in the astronomically close future (several tens of millennia). Eta Carinae itself is extremely massive, more than 100 times the mass of the Sun.

Light from the components of the Eta Carinae system is strongly absorbed by the small bipolar nebula Homunculus measuring 12 × 18 arc seconds, which consists of matter from the central star ejected during the Great Flare. This Carina A loses its mass so quickly that its photosphere is not gravitationally bound to the star and is “blown away” by radiation into the surrounding space.

A recently published study focused on one hypothesis that the binary star system used to be triple, which eventually became unstable and caused the stars to merge. As more detailed observations of Eta Carinae are made, this scenario becomes more popular, but still lacks detailed theoretical studies.

Astronomers at Monash University performed the first comprehensive and detailed theoretical calculations for this scenario. They first ran dynamic three-body simulations to see how the triple system became unstable. Scientists started with a stable system in which one star orbits in a wide orbit around two other stars in close orbit. As the most massive star approaches the end of its life, it expands and begins to transfer matter to its companion. This makes the system unstable and results in the merging of two stars. The process has been going on for several thousand years. Scientists have found that before the final merger, stars can suddenly change places and meet each other at close distances, touching surfaces of each other.

In addition, the researchers performed additional N-body simulations to see how the star responds to these close collisions. By combining orbital dynamics and close collision simulations, they found that multiple collisions could reproduce the disordered structure outside the Homunculus Nebula.

Hydrodynamic modeling, in turn, showed how the stream from the merger of stars takes the shape of the hourglass that we see today. Scientists have proposed a new scenario that uses similar ideas about how the SN1987A triple ring nebula formed. When stars merge, a tremendous amount of energy is released within the star, causing the Great Eruption. But unlike supernovae, most of the energy and mass remains in the star. This energy seeps out slowly over the next century in the form of strong bipolar winds. The wind sweeps away the inner parts of the explosion ejection and forms a hollow shell. Our simulations show that with this scenario we can accurately reproduce the shape and size of the Homunculus Nebula.

The scientists’ combination of simulations successfully reproduced the main features of the surrounding Eta Carinae nebula and provided compelling support for the triple star system scenario. This not only provided insight into the origin of Eta Carinae, but also many other astronomical objects that can be created by merging in ternary systems. For example, it is believed that some of the massive black holes discovered by LIGO (GW190521) were created this way. Using information from Eta Carinae’s research, we can learn much more about the formation of exotic objects in the universe.