In the mid-nineteenth century, one of the most massive stars in our Milky Way galaxy suddenly got much brighter. For ten years, indeed, it was the second-brightest star in the sky.
It's been unclear exactly what happened to Eta Carinae - but the Carnegie Institution's Jose Prieto, now at Princeton University, says his 'light echo' technique has shown the eruption was very different from previously thought.
As a Luminous Blue Variable (LBV), Eta Carinae has periods of dimness followed by periods of brightness, caused by increased instability and loss of mass.
But the Great Eruption was an unprecedented event, in which the star lost several times the mass of the sun. Scientists have until now believed that it was caused by a stellar wind.
But the Space Telescope Science Institute team has now examined eight years' worth of images showing 'light echoes' of the Great Eruption bouncing off interstellar dust.
This dust is tens of light years from the star, meaningthat the light's reaching Earth now, rather than in the 1800s when it was spotted directly.
They then used the Magellan and du Pont telescopes at Las Campanas Observatories in Chile, to obtain spectra of the echoes of light. This gave them information about the chemical composition, temperature and velocity of the ejected material.
And, surprisingly, the observations show that the Great Eruption is different from other events thought to be eruptions from LBVs. For example, the Great Eruption was significantly cooler than allowed by simple stellar-wind models used to explain such so-called 'supernova impostors'.
"This star's Giant Eruption has been considered a prototype for all supernova imposters in external galaxies," says Prieto. "But this research indicates that it is actually a rather unique event."
In other words, scientists still don't know exactly what caused Eta Carinae to erupt and lose such a quantity of mass without being destroyed.