No Winds of Change for Eta Carinae

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 Hubble Space Telescope captured the glory of the about-to-go-supernova Eta Carinae. When it blows, it may be as big as the largest stellar explosion seen so far.
Nathan Smith / Jon Morse / NASA
“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.”

5 thoughts on “No Winds of Change for Eta Carinae

  1. Kris Davidson

    Your news item "No Winds of Change for Eta Carinae" is wrong and in some respects it violates basic journalistic standards. Why doesn’t your item credit the lead author for the Nature paper, Armin Rest? Here are the principal facts of the case.
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    (1) The theoretical predicted temperature was 5500-6500 degrees K, not 7000. I was the author of the cited source.
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    (2) The spectral-classification method that the researchers used to deduce 5000 degrees K is fundamentally flawed. They used a mass-production spectral-classification technique that no careful expert would employ for such a special case.
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    (3) They based their 5000 degree K estimate on a set of stars that are all far less luminous that Eta Car’s great eruption. Hence they are not valid comparisons.
    (4) The Great Eruption was not a star’s atmosphere; instead it was a mass outflow. This fact totally invalidates their temperature estimate.
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    In summary, there is no basis for the news-item claim. And you have violated basic journalistic standards in two ways: You didn’t even mention Armin Rest, and you failed to consult real experts on the temperature and physics of the Great Eruption.
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  2. S&T Editors

    This article has been vetted and approved as scientifically accurate by several leading experts who study Eta Carinae. S&T stands by this article as accurate given the current state of knowledge of this enigmatic star. — The Editors of Sky & Telescope

  3. BMayfield

    I’m glad to see the editorial defense of Camille Carlisle’s article. Her report, as well as Kelly Beatty’s 2008 article are very interesting indeed. By all means check out the link, and don’t miss A.A.Szautner’s excellent post about the increasingly rapid run-thru of nucleosynthesis in massive stars. His comment, entitled "Imminent?" suggests that Eta Car, being 7.5k light years away, has in fact already gone supernova. The wave of photons may arrive at any time. So, when will it’s greatest eruption show up? Will we live to see it? I hope to, and in fact, I expect to.

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