Sky and Telescope - October 2016 - 28

SCIENCE PHOTO LIBRARY

The Case of Eta Car

UNDER SOUTHERN SKIES John Herschel erected a 20-foot
telescope on the Feldhausen estate near Cape Town, South
Africa, during the 1830s.

star's poles and become extra-powerful. Yet even in the
most extreme case, it was hard to see how this mechanism could rival the energy and mass expelled by Eta
Carinae. So that idea, among others, fell by the wayside.
Today astronomers deliberate between two main
alternatives, a pre-supernova explosion or a stellar
merger - and the debate is far from settled.

The False Supernovae
During the late 1990s, as various teams undertook
systematic searches for supernovae, they began turning
up so-called supernova imposters - stars that appear
to explode, sometimes even more than once, yet somehow survive. Several of the imposters eventually did
explode for real, most notably the supernovae known as
SN 2006jc and SN 2009ip. In fact, had we witnessed Eta
Carinae's Great Eruption in another galaxy, it may well
have looked like a supernova imposter.
"But we still don't have a theoretical model that
explains them," admits Nathan Smith (University of Ari-

zona). Smith has been chasing supernova imposters for
almost a decade. Although their origin remains a mystery, he and others have found a characteristic feature to
distinguish their quarry: narrow emission lines.
When a massive star's outer layers erupt in an imposter explosion, the outﬂow of hot, dense hydrogen gas
emits light at speciﬁc wavelengths, known as emission
lines. Because the gas is moving relatively slowly, the
emission lines are narrow - unlike the broad lines characteristic of a true supernova. If the same star really does
go supernova later, its shockwave will ram into the back
of the older material, reheating it and again releasing
narrow emission lines. In fact, the obscuring gas may
be so dense that it hides the supernova's broad lines,
making it diﬃcult to diﬀerentiate imposters from the
real thing.
Imposter events tend to occur late in a star's life,
sometimes just years before a true supernova, so it's
likely that the fault lies with the aging process. As a stellar core begins to run out of hydrogen for nuclear reactions, it must instead use the elemental ashes of prior
fusion for fuel - this is how stars forge increasingly
heavier elements in their centers. Within these heavyfusion regions, Smith explains, bubbling and swirling
motions due to convection and turbulence could mix the
gas in just the right way to create violent instabilities -
in other words, the potential for explosions.
Yet if imposter explosions precede the real thing, why
is Eta Carinae still shining happily 173 years after the
peak of its Great Eruption? "There's some missing ingredient that we don't understand," says Smith.

Crashing Stars
Then again, perhaps it's not just a missing ingredient
but an entirely diﬀerent recipe. Simon Portegies Zwart
(Leiden University, The Netherlands) and Edwin van den
Heuvel (University of Amsterdam, also in The Netherlands) recently proposed a game-changing alternative:

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OVER THE CENTURIES Eta Carinae ﬂashed its way to celebrity in 1837 before plunging back into obscurity two decades later.
More recently, the system has edged its way back into naked-eye view. E. FERNÁNDEZ-LAJÚS ET AL., ASTRONOMY & ASTROPHYSICS 2009 / HTTP://ETACAR.FCAGLP.UNLP.EDU.AR

October 2016 sky & telescope

HTTP://ETACAR.FCAGLP.UNLP.EDU.AR

Sky and Telescope - October 2016

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