Around 7,500 light-years distant in the constellation Carina the Keel, the star Eta Carinae, had a massive explosion that started in 1837. The enormous energy release, which was later dubbed the “Great Eruption,” briefly elevated Eta Car (as it is known informally) to the position of one of the brightest stars in the night sky. Eta Car is still around 5 million times more bright than the Sun, despite the fact that its light has significantly decreased since then.

The cosmic material released during Eta Car’s catastrophic catastrophe is still visible inside the broader Carina Nebula today. This cosmic debris has subsequently created an expanding, bipolar nebula surrounding the star system known as the Homunculus, or “little man.” In fact, scientists have hardly looked away from Eta Carinae since its explosion caught their attention in the first place.

According to David Espinoza, an astronomer at the National Autonomous University of Honduras, “there are still a lot of things we don’t know about Eta Car” despite 185 years of intensive study. It performs a number of odd things.

Michael Corcoran, a NASA Goddard Space Flight Center and Catholic University of America astronomer who has been researching Eta Car for the past 30 years and was Espinoza’s colleague and previous Ph.D. advisor, concurs. One of the great mysteries of stellar astronomy, according to Corcoran, is why this star burst and how it was able to survive such a powerful assault.

However, Corcoran’s area of expertise, X-ray astronomy, has provided some solutions. He says that this is so because “X-rays emanating from Eta Car are quite confined.” Other wavebands have harder-to-disentangle information because the radiation emanates from bigger areas surrounding the star.

Probing Eta Carinae with X-rays

Eta Car was an X-ray variable star, as Corcoran and his colleagues found in the early 1990s. Around the same time, Brazilian astronomer Augusto Damineli discovered that Eta Car is actually a binary system after studying a large collection of ground-based optical spectra. The presence of the smaller star in this system has been strongly supported by X-ray investigations, despite the fact that it has never been directly observed. According to current estimations, the companion star is around 30 Suns smaller than the bigger (primary) star, which has a mass of about 90 Suns.

Corcoran later oversaw a group that used NASA’s Rossi X-ray Timing Explorer to track the X-ray production from Eta Car in the late 1990s. These investigations revealed that Eta Car has a peak in its X-ray emissions every 5.5 years, during the conjunction of the two stars, each of which orbits the galactic center with a very eccentric orbit. The X-rays are at their brightest at such periods. The X-ray signal then almost completely fades after a few weeks, only to quickly reappear and restart the cycle.

The regularity of Eta Car’s X-ray fluctuation, according to Corcoran, is “amazing.”

Eta Carinae: A blast from the past

Now, Espinoza, Corcoran, and their colleagues present the first-ever estimate of the X-ray energy emitted during Eta Car’s Great Eruption of the 19th century in a new publication that was published on July 8 in The Astrophysical Journal. They assembled more than 300 observations from NICER, a NASA X-ray observatory on the International Space Station, to accomplish this.

The quickest, lightest material is expelled first during the preceding blast wave, and then the slower, denser stuff is released afterwards, according to Espinoza. In other words, the material that spreads the farthest from the star is shown by these low-energy X-rays as the eruption’s vanguard.

Espinoza examined the data and found that Eta Car’s emission of low-energy X-rays has decreased steadily and essentially linearly since the initial NICER observations were made in 2017. He and Corcoran were able to demonstrate that the X-ray power alone generated by the explosion was approximately 10 million times more than the total radiant output of the Sun across all wavelengths by extending this behavior back in time, all the way to the 1840s.

The amount of energy released during the Great Eruption in optical wavelengths and the kinetic energy of the slower-moving material that makes up the Homunculus Nebula are equivalent to this, which was far more than anticipated.

The approach the researchers took for their latest investigation is also not novel. In fact, it is quite similar to the method used by cosmologists and astrophysicists to turn back time on the universe’s growth and conclude that a Big Bang must have occurred.

What caused the Great Eruption of Eta Carinae?

In the future, Corcoran and Espinoza want to use information from additional X-ray telescopes, including as NASA’s Chandra X-ray Observatory and ESA’s XXM-Newton X-ray observatory, to improve their estimate of Eta Car’s 19th-century detonation. They also eagerly look forward to measuring the motions of both the X-ray emitting gas and the stars themselves using the upcoming XRISM satellite, a joint Japanese/U.S. X-ray telescope scheduled for launch in April 2023.

The puzzle of what caused the star to explode 185 years ago cannot be solved by more accurate estimates of Eta Car’s energy output, nor can they tell us if or when such an outburst may happen again. They can, however, undoubtedly aid astronomers in putting some restrictions on the up to this point suggested eruption models.

For instance, Ryosuke Hirai of Monash University has investigated the hypothesis that the triple star system Eta Car, in which two of the three stars eventually merged due to orbital instabilities, may have genuinely existed. Hirai’s theory postulates that this merging caused the dramatic flare-up that was noticed by renowned astronomer John Herschel and others in December 1837. According to Corcoran, “if it actually was the outcome of the merging of two stars, that process could be modeled in detail, and the X-ray data might play a key part in helping us rule in, or rule out, such stellar merger models.”

The intriguing life of Eta Carinae

Astrophysicists have a rare chance thanks to Eta Car to learn more about the most massive and powerful stars in the Milky Way. Eta Car is the most massive and brightest star within 10,000 light-years of Earth, after all.

Espinoza observes that “the majority of the stars in the sky are not alone.” Astronomers frequently make light of the fact that three out of every two stars in the sky are binary stars like Eta Car. Giant stars like these disperse the heavy elements necessary for the formation of rocky planets and life across the cosmos by stellar winds, eruptive events, and supernova explosions.

Corcoran adds that these enormous star systems may also be the precursors of the binary black holes whose collisions are picked up by gravitational wave detectors like LIGO. The formation and evolution of some of the greatest merging black holes discovered by LIGO, those with masses between 60 and 70 solar masses, are still a mystery. However, according to Corcoran, “they probably come from star systems like Eta Car.” “Nature is providing us a detailed look at an ancestor of those types of systems.”

When he initially set his sights on Eta Car in 1992, Corcoran had no idea that it would hold his interest for the following 30 years, but it has.

It’s not a black hole, but it’s kind of like one because once you fall into it, it’s difficult to escape, he claims.

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