Ancient 'Great Eruption' Echoes Reach Astronomers

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When the binary star system Eta Carinae experienced a spectacular outburst in 1837 — dubbed the “Great Eruption” — there were no cameras or other sophisticated scientific instruments around to record the event for posterity.

But now, 170 years later, remnants of light from the Great Eruption are finally reaching Earth, providing new insight into how massive stars behave when they are on the brink of exploding.

Astrophysicists at the University of California, Santa Barbara and the Las Cumbres Observatory Global Telescope Network, announced the detection of this “light echo” in a Feb. 16 letter to the journal Nature.

UCSB Postdoc Federica Bianco, who compared the light echo to eyewitness reports from the 1800s, phrased the phenomenon best: “You are at the stadium, watching the game, and your team scores. But you do not have modern instruments, detectors and spectrographs to study it,” she said in a press release.

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“Now we are getting a replay — an up-close detailed view of our cosmic eruption,” she continued. “And just like with the replay, we get to see the outburst from a different point of view, as the light that we see now was originally traveling in a different direction than the light seen in the 1840s.”

Eta Carinae is a rare, massive binary star, and the dominant partner in this cosmic coupling belongs to the class of luminous blue variable stars. When it erupted 170 years ago, it became one of the brightest stars in the sky for a time. So why are we suddenly seeing light from that event again?

The astrophysicists explain that originally, the light traveled away from Earth, and then bounced off dust clouds, which rerouted it to Earth — just like an echo. The longer path means we are only now seeing that echo.

There might not be photographs, but there are a few historical eyewitness accounts on record to help astrophysicists determine that what they are seeing really is a “light echo” from Eta Carinae’s 19th century outburst.

In the 1830s, astronomer Sir John Herschel noticed an especially bright star in the southern sky while conducting a survey from Cape Town, South Africa. He dutifully sketched the region where the star appeared, particularly noting a dark ring in the upper part of the Carina Nebula that resembled a keyhole (see image, left).

Within a few years, however, that star had so faded in brightness that Herschel’s telltale keyhole was barely visible.

Today we know that Herschel’s bright star was Eta Carinae, experiencing a sudden burst of brightness thanks to a “supernova imposter” event, in which the star system shed a whopping 20 solar masses worth of outer shell.

You can still see the remnant of this stellar explosion in the Homunculus Nebula. It’s called that because Argentine astronomer Ernest Gaviola, who first observed it in 1950, thought it looked like a human figure, with a head, legs and folded arms.

There is even evidence that Australian aborigines may have spotted the Great Eruption around the same time, according to a paper that appeared last year in the Journal for Astronomical History and Heritage. Co-author Duane Hamacher maintains that the Boorong people of northwestern Victoria were aware of celestial objects and cast them as characters in their oral stories of the Dreaming — including the eruption of Eta Carinae.

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The evidence can be found in a paper by William Edward Stanbridge, a 19th century Australian astronomer who did a bit of star-gazing with two men of the Boorong clan, who recited those stories to him while pointing out the relevant stars overhead. Stanbridge dutifully recorded this information, matching the Boorong stars with their Western counterparts using a star atlas.

Alpha Centauri, for instance, was Berm-berm-gle, while Antares was Djuit, and Canopus was known to them as War (pronounced “Wahh”, meaning “Crow”). But when they pointed out Collowgullouric War (“Wife of Crow”), Stanbridge was stumped. He couldn’t identify the star on his chart. So he simply wrote, “Large red star in Robur Carol, marked 966. All the small stars around her are her children.”

In 1996, astronomer John Morieson came across Stanbridge’s work and re-analyzed it. You can see the constellation Carina to the left in the image below. (Eta Carinae is the brightest dot in the lower right corner.) On the right, Morieson “connected the dots” into something resembling a bird in flight — what he believes the Boorong would have pictured as the “wife of Crow.”

Morieson never published his thesis, but Hamacher and his collaborator, David Frew, came across it as they were rifling through historical records to build their case.

Hamacher figured folks would be skeptical of their claim that the Boorong story was a direct reference to Eta Carinae’s Great Eruption. That’s why he proffered an explanation on the Aboriginal Astronomy blog last year as to their reasoning process:

During the early 1840s, when Eta went through its great outburst and the time that Stanbridge was learning firsthand about Boorong astronomy, it was one of the brightest stars in the night sky (“large star”), it had a reddish color, was located in the now-defunct constellation of Robur Carol…. Star charts from the period refereed to the Carinae Nebula (surrounding Eta Carina) as ’966′.  Eta Carinae itself was designated ’968′, but was labeled as a 4th magnitude star.  This was probably a simple transcription error by Stanbridge, who did not recognize the bright star where his star charts claimed was a fairly mundane 4th magnitude star.  Finally, this region of the sky is rich in 4th and 5th magnitude stars, which would have been the “small” stars representing the children of the female Crow.

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Armin Rest of the Space Telescope Science Institute in Baltimore led the latest study, and spotted the echoes of Eta Carinae’s Great Eruption while comparing visible light images he’d taken of the star in 2010 and 2011.

That’s when he noted light “that seemed to dart through and illuminate a canyon of dust surrounding the doomed star system.” He concluded that it couldn’t just be material moving through space, which would only show up over decades of observation, not a single year.

It had to a light echo, giving the illusion of moving through time because each flash was reaching Earth at a different time after bouncing off dusty clouds. Comparison with historical records showed it was a remnant from Eta Carinae’s Great Eruption.

The color image at left shows the Carina Nebula, a star-forming region located 7,500 light-years from Earth. (Eta Carinae is near the top.)

The three black-and-white images pictured here show light from the eruption illuminating dust clouds as it moves through them. The images were taken over an eight-year span by the U.S. National Optical Astronomy Observatory’s Blanco 4-meter telescope at the CTIO.

This new study has already found some intriguing anomalies in Eta Carinae’s behavior compared to its fellow luminous blue variables, using spectroscopy, which gives a detailed “fingerprint” of stars telling astronomers the temperature and speed of ejected material.

For instance, the temperature of its outflow is much cooler that usual, around 8500 degrees Fahrenheit (5,000 Kelvin). Rest and his colleagues are revisiting their models for such stars to determine how this behavior might have occurred. They will continue to monitor Eta Carinae, and expect to see more brightening in six months or so, matching a similar period outburst in 1844.

“It’s as if nature has left behind a surveillance tape of the event, which we are now just beginning to watch,” said Rest. “We can trace it year by year to see how the outburst changed.