Echoes from a long-gone explosion may help astronomers understand one of the most massive and mysterious stars in the sky. The star Eta Carinae weighs in at about 90 Suns and shines hotly inside a barbell-like nebula called the Homunculus, the vestiges of the star’s so-called Great Eruption. This event, which lasted from 1838 to 1858, boosted the star’s luminosity to make it temporarily the second-brightest star in the night sky (it's not even in the top 100 usually). Now, an international team of astronomers reports in February 16th’s Nature that they may have evidence that the eruption didn’t go down quite like many researchers think it did.
“The cause of the Great Eruption is without doubt one of the biggest puzzles in stellar astronomy,” says Michael Corcoran (NASA Goddard), who did not participate in the current study. What is known is that the star spouted material in the form of two big globes, losing in all about 10 solar masses during the eruption. Somehow, the star survived. This material has been expanding into space since then at speeds that are high, but not as high as those attained by supernova outflows.
Eta Car is a rare, unstable type of star known as a luminous blue variable. These stars’ hiccups are suspected to happen when an increase in the star’s radiation drives material off the star’s surface as an “opaque wind.” While this theory is the standard one, not everyone buys it: in 2008 Nathan Smith suggested that the eruption happened when Eta Car suffered an internal explosion.
Smith (now at the University of Arizona’s Steward Observatory) is a coauthor on the new paper, which also argues against the wind explanation. The astronomers used observations from Las Campanas Observatory and the Cerro Tololo Inter-American Observatory, both in Chile, as well as data from the Faulkes Telescope South in Australia to reveal light echoes from the Great Eruption. A light echo is just what it sounds like: a reflection of the light burst of a stellar explosion, bouncing off nearby interstellar clouds of gas and dust. Analysis of the light echo indicates that the eruption’s temperature was about 5,000 kelvins (8,500° F) — but that’s at least 2,000 kelvins too low for the dominant wind model. The authors conclude the opaque wind idea just doesn’t cut it.
The evidence is reasonable and sure to spark debate, Corcoran says, but it’s not certain that the wind theory doesn’t work at lower temperatures. “I think the jury’s still out on determining the nature of the Great Eruption,” he says.
Theodore Gull (NASA Goddard) agrees. He notes that the study is a snapshot of a long-term, ongoing study and that continuing the study over the next decade will provide enough data to carefully compare the echoes with visual observations from the 1800s. Such comparisons could reveal correlations between the two, such as matches in light variations, and insight into whether the initial event was a spike or a gradual change. “That will be the proof of the pudding.”