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.
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....