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Eta Carinae (Eta Car) is one of the most massive binary stars in the MilkyWay. It became the second-brightest star in the sky during its mid-19th century"Great Eruption," but then faded from view (with only naked-eye estimates ofbrightness). Its eruption is unique among known astronomical transients in thatit exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3kpc away, spatially resolved studies of the nebula have constrained the ejectedmass and velocity, indicating that in its 19th century eruption, Eta Carejected more than 10 M_solar in an event that had 10% of the energy of atypical core-collapse supernova without destroying the star. Here we report thediscovery of light echoes of Eta Carinae which appear to be from the 1838-1858Great Eruption. Spectra of these light echoes show only absorption lines, whichare blueshifted by -210 km/s, in good agreement with predicted expansionspeeds. The light-echo spectra correlate best with those of G2-G5 supergiantspectra, which have effective temperatures of ~5000 K. In contrast to the classof extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption,the effective temperature of its outburst is significantly cooler than allowedby standard opaque wind models. This indicates that other physical mechanismslike an energetic blast wave may have triggered and influenced the eruption.

Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption