Dazzling in its fierce, fiery brilliance, Eta Carinae is a stellar system composed of at least two stars sporting a combined luminosity that is greater than five million suns. The primary star inhabiting this system is also extremely unstable and probably doomed to go supernova sometime in the foreseeable future. However, this bright system of stars is a trickster. In fact, the Eta Carinae primary star should have died long ago, but didn’t. Almost 170 years ago, this glaring stellar behemoth experienced an enormous outburst that liberated almost as much energy as a typical supernova blast, that heralds the demise of a massive star. Yet that powerful eruption wasn’t enough to send that enormous star screaming into eternity, and astronomers have been hunting for clues to explain this enormous outburst ever since.
While there is no way to travel back in time to the mid-1800s, and observe first hand the giant eruption in all of its original glory, astronomers can now enjoy a fiery encore performance of that dramatic stellar blast. This rebroadcast comes courtesy of some wayward light streaming out from the explosion. Instead of traveling straight to our planet, some of the emitted light from the outburst rebounded–or “echoed”–off of interstellar dust, and is now first arriving at Earth. The term for this phenomenon is a light echo.
This “time machine”, that Mother Nature has generously provided, is based on the lucky fact that light travels at a finite speed through space–and instead of zipping straight to our planet, some the light streaming from the outburst apparently took a detour. The wayward light rebounded, and this light echo is behaving like a love letter written 170 years ago that is only now first arriving at its destination.
A Mysterious Star
Eta Carinae is situated approximately 7,500 light-years from Earth in the constellation Carina, which makes it a relatively nearby star. Nevertheless, there is no definite evidence showing that Eta Carinae was observed before the 17th century. However, the German navigator, Pieter Keyser (1540-1596) did describe seeing a fourth-magnitude star that well may have been Eta Carinae. The star that Pieter Keyser observed around 1595-1596, was at the correct position for it to have been this mysterious star. In turn, Keyser’s observations were copied onto the celestial globes of the Dutch-Flemish astronomer Petras Plancius (1552-1622) and the French engraver and cartographer Jodocus Hondius (1563-1612), as well as the 1603 Uranometricia of the German lawyer and astronomer Johann Bayer (1572-1625). The Dutch explorer Frederick de Houtman’s (1571-1627) independent star catalogue, also produced in 1603, does not include Eta Carinae among the other 4th magnitude stars located in that region.
The first known reliable observation was produced by the English astronomer Edmond Halley (1656-1742) in 1677, when he recorded the star dismissively as Sequens–meaning “following” relative to another star within a newly discovered constellation then dubbed Robur Carolinum. Halley’s Catalogus Stellarum Australium was published in 1679. The mysterious star was also recognized and named Eta Roboris Caroli, Eta Argus, or Eta Navis. Eta was observed in what was later to be known as the constellation Carina. However, it was not generally known as Eta Carinae until 1879, when the stars of Argo Navis were finally designated as the daughter constellations situated in the Uranometria Argentina of the American astronomer Benjamin Apthorp Gould (1824-1896).
Halley determined an approximate apparent magnitude for Eta Carinae of “4” at the time of his discovery, which was later calculated to be magnitude 3.3 on the scale used by astronomers today. Several possible earlier observations indicate that Eta Carinae did not grow much brighter than this for most of the 17th century.
However, Eta Carinae brightened in 1837 from a 4th magnitude star to become brighter than the star Rigel. This proved to be the beginning of what is called its Great Eruption. Indeed, Eta Carinae became the second-brightest star in the sky between March 11 and March 14 1843. After 1856, it began to fade well below what the unaided human eye is able to see. A smaller eruption followed soon thereafter, when Eta Carinae reached 6th magnitude in 1892, before growing dim again. Since 1940, it has brightened consistently, growing brighter than magnitude 4.5 by 2014.
The blue-white star Rigel (Beta Orionis) is usually the 7th brightest star suspended in the dark night sky above our planet, as well as the brightest star in the Orion Constellation. However, it is occasionally out-dazzled within its constellation by the variable star Betelgeuse.
The duo of main stars within the Eta Carinae system display an eccentric orbit with a period of 5.54 years. The primary star is a true “oddball”, similar to a luminous blue variable, that started out at a hefty 150-250 solar-masses. However, the primary has shed at least 30 solar-masses already, and it is expected to go supernova in the near future. The behemoth star has the distinction of being the only one of its gigantic searing-hot kind known to produce ultraviolet laser emission. The secondary star of the fiery duo is extremely luminous–and very hot. This star is probably of spectral class O, and it weighs-in at an impressive 30 to 80 times solar-mass. The system is heavily blanketed by the Homunculus Nebula, which is composed of material hurled out from the primary during the Great Eruption. It is a dazzling denizen of the Trumpfer 16 open stellar cluster situated within the considerably larger Carina Nebula.
At the beinning of the 19th century, the binary Eta Carinae was very dim and unremarkable in appearace. However, its looks began to change during the first decades of that century as it grew ominously brighter. In 1843, Eta Carinae had become the second-brightest star in Earth’s night sky, out-dazzled only by the brilliant “dog star” Sirius–in spite of the fact that Sirius is one thousand times closer to us.
In 1827, the English botanist and traveler W.J. Burchell noticed the bizarre light display performed by Eta Carinae when it became a 1st magnitude star. Because of this, Burchell became the first to suspect that it varied in brightness. The English astronomer John Herschel (1792-1871) made a detailed list of accurate measurements in the 1830s revealing that Eta Carinae consistently glared at approximately magnitude 1.4–that is, until November 1837. On the night of December 16, 1837, Herschel stared up at the sky in amazement because this was when Eta Carinae suddenly and dramatically brightened sufficiently to out-shine Rigel–thus heralding the Great Eruption.
Observations conducted in radio, infrared, and in space have greatly expanded astronomical studies of Eta Carinae across all wavelengths of the electromagnetic spectrum. These studies revealed continuing alterations in the spectral energy distribution.
Finally, in July 2018, Eta Carinae was reported to have the most powerful colliding wind shock in our Sun’s general neighborhood in the Milky Way Galaxy. Observations conducted with NASA’s NuSTAR satellite provided greater resolution data that earlier studies using the Fermi Gamma-ray Space Telescope. By using direct focusing observations of the non-thermal source in the extremely hard X-ray band that is spatially coincident with the mysterious star, they showed that the source of the non-thermal X-rays varies with the orbital phase of the binary star system. Furthermore, the photon index of the emission is similar to that derived through analysis of the gamma-ray spectrum (y-ray).
Only the most massive stars in the Universe give rise to stellar mass black holes after they have gone supernova. The unstable, dying behemoth star of the Eta Carinae system is certainly massive enough to collapse into a black hole when it has finally reached the end of that long stellar road. But, it is such an extremely massive star that it may very well perish in a special–and spectacular–type of supernova blast called a pair-instability supernova. Pair-instability supernovae leave nothing at all behind–not even a black hole.
Stellar Survivor
In August 2018, a team of astronomers published their new findings, based on “modern astronomical forensics,” of the light-echo streaming out from the Great Eruption. The scientists, using ground-based telescopes, uncovered a surprise. Their new measurements, obtained of the 1840s eruption, show material expanding with record-breaking speeds up to as much as 20 times faster than expected. These rapid velocities are more akin to the extremely fast-moving material, screeching away as the result of a supernova blast, than they are to the relatively lazy and gentle winds emanating out from massive stars before they perish.
Based on this newly acquired data, the astronomers now propose that Eta Carinae’s Great Eruption may have been triggered by a prolonged battle among a trio of brawling sibling stars. The stellar skirmish likely destroyed one star, while leaving the other two squabbling siblings in a binary system. This brawl between stellar kin may have resulted in a violent blast when Eta Carinae feasted on one of its two tussling companion stars–shooting more than 10 times the mass of our Sun into the space between stars.
The new results are reported in two papers authored by a team of astronomers led by Dr. Nathan Smith of the University of Arizona (Tucson) and Dr. Armin Rest of the Space Telescope Science Institute (STSI) in Baltimore, Maryland.
The light-echoes were discovered in visible-light images obtained since 2003 with moderate-sized telescopes located at the Cerro Tololo Inter-American Observatory in Chile. Using the larger Magellan telescopes of the Carnegie Institution for Sciences’ Las Campanas Observatory and the Gemini South Observatory, also located in Chile, the astronomers then went on to use spectroscopy to study the traveling light. This enabled the team to measure the ejecta’s speed of expansion, that they determined to be zipping along at more than 20 million miles per hour. This breathtaking speed is fast enough to travel from our planet to the Kuiper Belt in only a few days. The Kuiper Belt is a distant, frigid, and dimly-lit domain beyond the orbit of the outermost planet Neptune. It is the reservoir of a dancing multitude of frozen comet nuclei, as well as the home of icy dwarf planets, such as Pluto.
The new observations are important because they provide precious clues that help astronomers solve intriguing mysteries still lingering about the Great Eruption.
“We see these really high velocities in a star that seems to have had a powerful explosion, but somehow the star survived. The easiest way to do this is with a shock wave that exits the star and accelerates material to very high speeds,” Dr. Smith explained in an August 2, 2018 Hubblesite Press Release.
Massive behemoth stars, like the unstable inhabitant of the Eta Carinae system, commonly perform their final farewell performance to the Cosmos when their cores collapse–and the erstwhile star evolves into either a neutron star or stellar-mass black hole after its demise. Astronomers observe this scenario in supernova blasts where the dying star is completely wrecked. Therefore, the question is: How do you have a star explode with a shock-driven tantrum, that isn’t sufficient to completely blast the star to smithereens? Some violent occurrence must have hurled just the right amount of energy onto the unfortunate star, causing it to eject its outer gaseous layers into space. But, the energy released is still not great enough to completely destroy the tragic star.
The team of astronomers now propose that just such an event could be caused by a merger between a duo of kindred stars. However, it has proven to be extremely difficult to devise a scenario that could work and also match the data accumulated on Eta Carinae.
The new model suggests that the most straightforward way to explain such a wide range of observations, pertaining to the Great Eruption, is with a strange dance between a trio of stars, where all three of the dancers exchange mass.
If that indeed was the case 170 years ago, then the current remnant binary system must have begun as a triple stellar system. “The reason why we suggest that members of a crazy triple system interact with each other is because this is the best explanation for how the present-day companion quickly lost its outer layers before its more massive sibling,” Dr. Smith continued to explain in the Hubblesite Press Release.
According to the new model, two massive stars were orbiting closely in their bizarre dance, while a third was dancing by itself farther away. When the most massive of the close binary stars approached the end of its hydrogen-burning “life”, it started to swell in size, and eventually dumped most of its material onto its slightly smaller (but nonetheless enormous) stellar sibling.
As a result, the sibling star gained enough weight to become a hefty 100 times the mass of our Sun. It has also grown very bright. The doomed donor star, now a “mere” 30 times solar-mass, has been stripped of its layers of hydrogen gas–thus exposing its searing-hot helium core.
Hot helium-core stars represent an advanced stage of stellar evolution in the “lives” of massive stars. “From stellar evolution, there’s a pretty firm understanding that more massive stars live their lives more quickly, and less massive stars have longer lifetimes. So the hot companion star seems to be further along in its evolution, even though it is now a much less massive star than the one it is orbiting. That doesn’t make much sense without a transfer of mass,” Dr. Rest explained in the August 2, 2018 Hubblesite Press Release.
Of course, the transfer of mass changes both the dancers and their dance within the disrupted system. The star with the helium-core now waltzes away from its binary sibling. Indeed, the star dances so far away that it begins to perform a gravitational ballet with the outermost third star of the trio–that had been dancing alone, until now–thus kicking it inward. Alas, after making a few close brushes, the star merges with its massive dancing partner, resulting in an outflow of material.
In the initial stages of the stellar merger, the ejecta is dense and ballooning in size at a relatively sluggish rate, as the stellar duo perform a deadly spiral ever closer and closer and closer. Finally, an explosion is triggered when the duo of inner stars finally merge, blasting off material at an impressive speed that moves 100 times faster. This matter ultimately catches up with the more slowly moving ejecta and slams into it like a wayward automobile into a wall–heating it up and causing it to glare furiously. This glowing material accounts for the brilliant light observed during Eta Carinae’s Great Eruption.
While all this is still going on, the helium-core star develops an elliptical orbit that carries it through the unstable star’s outer gaseous layers every 5.5 years. This interaction shoots out X-ray emitting shock waves.
A greatly improved understanding of the physics involved in Eta Carinae’s Great Eruption may help shed new light on the rather complicated dance performed between sibling stars in other binary and multiple stellar systems. This understanding is critical because it will solve some of the bewitching mysteries concerning the evolution and death of massive stars.
The astronomers published their findings in papers titled Exceptionally Fast Ejecta Seen in Light Echoes of Eta Carinae’s Great Eruption and Light Echoes From the Plateau in Eta Carinae’s Great Eruption Reveal a Two-Stage Shock-Powered Event. The two papers appear online in the August 2, 2018 edition of The Monthly Notices of the Royal Astronomical Society (UK)