The young star Eta Carinae –one of the most massive in the Milky Way – survived a titanic eruption 170 years ago. Although located relatively far away from Earth (about seven thousand light-years away, as compared with the average distance of naked-eye stars of about a thousand light-years), it can be seen easily by people in the southern hemisphere because it is extraordinarily bright—about five million times more luminous than our Sun.
Astronomers have suggested one reason it is so bright is because it is very massive – perhaps as much as 200 times more massive than our Sun, making it one of the most massive stars known. Massive stars consume their hydrogen much more quickly than Sun-like stars, and are thus hotter, brighter, and die within a few million years.
Eta Carinae resides in a large molecular cloud (the Carina Nebula) surrounded by a double-lobed structure of gas and dust that probably resulted from prodigious mass ejections and intermittent winds from the star (or others nearby).
“Eta Carinae is a binary star surrounded by a complex nebulosity which is all expanding away from the central star,” wrote Monash University astronomer and binary-star expert, Ryosuke Hirai, in an email to The Daily Galaxy. “In the 1940s to 1950s the nebula was much smaller, and we could only make out a vague shape of the nebula with the telescopes at the time, which looked like a blob with a head and two legs sticking out of it. That’s how it started to be called the “Homunculus” nebula, which is a Latin word for a “little person”. Today we can see the surrounding nebula in much more detail and we know the Homunculus looks more like an hourglass, with lots of blobs and ejectiles surrounding it.”
The Great Eruption of 1837
Eta Carinae itself is known to be highly variable; John Herschel (the son of Astronomer Royal William Herschel) first called attention to this star and a particularly dramatic flaring event it underwent in 1837 dubbed “The Great Eruption.” Scientists have debated whether the whole region is dominated by active star formation and/or whether a supernova may have gone off nearby, all of which would contribute to the variability and complex structures. They have also suggested that the Great Eruption was due to the merger of a pair of binary stars, and that analogous events may power the extreme events seen in other galaxies.
“Many astrophysicists have been investigating how the Great Eruption was triggered and how it created the bipolar Homunculus nebula, but it is extremely difficult to explain all the observed details self-consistently,” notes Hirai in his email.
As radiation and shocks from stellar flares propagate outward through the interstellar medium they encounter wisps and cloudlets of material that then light up – “echoes” of the flaring events themselves.
In 2018, a team of Harvard CfA astronomers reported finding a new light echo from careful subtraction of images taken at different epochs. The light echos are reflections of luminous Eta Carinae off of dust grains in the surrounding nebula. Due to the extra distance the light echos travel, there is a corresponding time lag, and so the light echos reveal the nature of Eta Carinae at different times and from different viewpoints. The new light echo is somewhat brighter than others they had seen, and was distinct in its character: it fades away more slowly and shows different spectral characteristics.
Evidence of a Triple-star System?
The scientists analyzed images and spectra of the nebula taken at optical wavelengths with the CTIO Blanco and Magellan Baade and Clay telescopes, and in the infrared with the IRAC camera onboard Spitzer. The spectra reveal for the first time very high expansion speeds in the gas, up to fifty million miles an hour, and evidence of a two-phase eruption which the scientists can trace back to the Great Eruption. They interpret the results to argue for a unstable triple-star system that led to the merger, kicking out the original primary star. (They also detect evidence for prior eruption activity – as much as 600 years before the Great Eruption.)
The new scenario differs in several key ways from earlier suggestions, and can more easily explain a wider variety of observations. There is a record of observation of Eta Carinae dating back to John Herschel, with many detailed results over the past decades.
In a new 2021 paper, researchers solved the mystery surrounding the 19th Century eruption and ejection of the surrounding Homunculus nebula, based on recent observations, including light-echo spectra of the eruption that has been the center of a long-standing mystery. The paper suggests that it most likely resulted from a stellar merger in an unstable triple system. If in fact Eta Carinae’s eruption really was a triplet merger of this sort, these data offer new insights into how very high mass stars form and evolve in their environments.
The Last Word — Erupted Multiple Times Over the Past Millennium
Lead author, Ryosuke Hirai, summed up current findings about Eta Carinae in his email to The Daily Galaxy:
“We think that Eta Carinae used to be a triple system, and the Great Eruption was caused when two of the stars collided and merged with each other. As the two stars merge, the orbital energy is rapidly released and ejects a fraction of the star through an energetic explosion. However, this doesn’t immediately create the hourglass-like shape nebula but we expect it would be mostly spherical like in normal explosions.”
“The energy from the merger is not enough to blow up the whole star so most of the star is left with a huge amount of excess energy remaining in it.,” Hirai wrote. “The star should also be rapidly spinning because of the large angular momentum brought in from the orbit. We expect this excess energy to leak out in the form of strong stellar winds, and it should be stronger around the poles because of the rapid rotation. This bipolar wind sweeps up the inner parts of the ejected material from the eruption, shaping it into the Homunculus nebula we see today. Our hydrodynamical simulations show that this scenario can create a nebula very similar to the observed shape.”
“The blobs and ejectiles outside the Homunculus nebula,” Hirai notes, “have expansion velocities slower than the Homunculus, meaning that they should have been shot out centuries before the Great Eruption. This indicates that Eta Carinae should have erupted multiple times over the past millennium, which cannot be explained in a stellar merger scenario. We hypothesized that the complicated orbital dynamics of the triple system before the merger could have caused the stars to sometimes closely encounter each other and trigger multiple mini-eruptions. By carrying out N-body dynamical simulations, we found that this scenario can reproduce the distribution of the ejectiles outside the Homunculus nebula.”
The furious expansion of a huge, billowing pair of gas and dust clouds are captured in the NASA/ESA Hubble Space Telescope comparison image of Eta Carinae at top of the page. Even though Eta Carinae is more than 8,000 light-years away, structures only 15 billion km across (about the diameter of our solar system) can be distinguished in this sharp Hubble image. Dust lanes, tiny condensations, and strange radial streaks all appear with unprecedented clarity. (J. Hester/Arizona state University, NASA/ESA)
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Harvard-Smithsonian Center for Astrophysics