LIGHT ECHOES FROM η CARINAE'S GREAT ERUPTION: SPECTROPHOTOMETRIC EVOLUTION AND THE RAPID FORMATION OF NITROGEN-RICH MOLECULES**This Letter includes data gathered with the 6.5 m Magellan telescope at Las Campanas Observatory, Chile.

We present follow-up optical imaging and spectroscopy of one of the light echoes of η Carinae's nineteenth century Great Eruption discovered by Rest et al. By obtaining images and spectra at the same light echo position between 2011 and 2014, we follow the evolution of the Great Eruption on a 3 yr timescale. We find remarkable changes in the photometric and spectroscopic evolution of the echo light. The i-band light curve shows a decline of …