For instance, "cool objects emit redder light than hot objects," Rest explained. "Look at an ember — if it is relatively cool, it is dark red, and the hotter it gets, the bluer, whiter it gets. So overall the color of the spectra gives you already a good idea how hot or cold the star is."
Now, using the Magellan and du Pont telescopes at Las Campanas Observatories in Chile, Rest and his colleagues have discovered spectra from the Great Eruption, by looking for light from the explosion that bounced or echoed off interstellar dust tens of light-years from the star.
"The missing piece, the big gap, has been that there were no spectra of the Great Eruption," Rest said. "Now we have it, and it gives us the chance to better understand how this eruption happened and what caused it."
Surprisingly, their observations suggest the Great Eruption is different from so-called "supernova impostors," events that resemble the explosive supernova deaths of stars but are thought to be eruptions from bright blue variables. For example, the Great Eruption was only about 8,540 degrees Fahrenheit (4,725 degrees Celsius), much cooler than allowed by the stellar winds used to explain supernova impostors.
"This star's Giant Eruption has been considered a prototype for all supernova imposters in external galaxies," said study co-author Jose Prieto, now at Princeton University. "But this research indicates that it is actually a rather unique event."