Sky and Telescope - October 2016 - 27

KEITH COOPER

LITTLE MAN The Hubble Space Telescope imaged the
famous Homunculus ("Little Man" in Latin) numerous
times over the past two decades, revealing the massive
nebula of gas ejected in the 19th-century explosion.

ESA / HUBBLE & NASA

L U M I N O U S , B LU E & VA R I A B L E

Eta Carinae, which outshines all else in the Carina Nebula
(left), is the brightest - and most famous - example of
a class of stars known as luminous blue variables. These
brilliant, massive, and aging stars change unpredictably:
they may vary day to day or week to week by 0.1 magnitude,
but on rare occasions they erupt, unleashing tremendous
amounts of mass. Luminous blue variables probably represent a short-lived stage in the evolution of massive stars.

classed as a luminous blue variable (LBV) - an aging
star prone to violent outbursts. Its companion, unseen
amidst the primary's glare, contains roughly 30 times
the mass of the Sun, but without direct observations,
astronomers don't know much more about it. The secondary could be an O-type main-sequence star or perhaps an evolved Wolf-Rayet star. The two companions
orbit each other every 5.5 years in an oval-shaped orbit.
Fewer than 20 LBVs are known in our galaxy, and a
signiﬁcant fraction display evidence of past eruptions.
Astronomers witnessed one such outburst during the
17th century when the LBV P Cygni ejected about 0.1
solar mass of material.
Although such an outburst could potentially explain
Eta Carinae's ﬂare around 1890, no such outburst has
approached the scale of the Great Eruption, when Eta
Carinae threw between 10 and 40 solar masses' worth
of gas and dust into the Homunculus. This twin-lobed
nebula now stretches 0.6 light-year long. The star's luminosity increased to 10 million times that of our Sun during the eruption, generating almost as much radiation
over the course of two decades as a typical supernova.
Yet, unlike in a supernova, Eta Carinae survived.
Numerous scenarios have tried to explain why. One
popular idea proposed that, if the star were spinning
fast enough to squash itself into an oblate shape, then
a regular LBV-style outburst would concentrate at the

ESO / IDA / DANISH 1.5-M / R. GENDLER, J-E. OVALDSEN, C. THÖNE & C. FERON

When John Herschel looked up
at the sky late one summer evening
almost two centuries ago, he knew it
didn't look right.
The great British astronomer had been living with
his family on the Feldhausen estate near Cape Town,
South Africa, cataloging the clusters, double stars, and
nebulae of the Southern Hemisphere. As he stepped
outdoors into the mild evening air on December 16,
1837, he scanned the sky and noticed a star in the constellation Argo Navis that seemed wrong. Herschel had
frequently observed this star, then known as Eta Argus,
at magnitude 1.4 throughout his time in South Africa.
This evening it had brightened dramatically, outshining even Orion's magnitude-0.13 star, Rigel.
Unbeknownst to Herschel, the star was undergoing
an incredibly violent process - now called the Great
Eruption - and today astronomers are still trying to ﬁgure out what happened. Herschel returned to England
the following year. Had he remained in South Africa, he
would have seen Eta Argus grow even more dazzling. By
1843, it had become second only to brilliant Sirius.
Eta Argus blazed for more than a decade but began
fading dramatically in 1858, dimming to 8th magnitude. The star experienced a brief resurgence between
1887 and 1894 in what is commonly referred to as the
Lesser Eruption, when it just about reached naked-eye
visibility. Today, it lingers at magnitude 4.5 - not too
shabby for a star some 7,500 light-years away.
In modern nomenclature, the star is called Eta Carinae. Its constellation, the ancient and unwieldy Argo
Navis (S&T: March 2015, p. 36) has long since been
broken into three more manageable pieces, with Carina
being the mythical ship's keel. Modern instruments
show that Eta Carinae is actually a binary system, with
a primary star about 100 times the Sun's mass that's

Sk yandTelescope.com October 2016

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Sky and Telescope - October 2016

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