Sky & Telescope - March 2020 - 27

studies to take a telescope to the South Atlantic island of Saint Helena and remeasure
the far-southern stars with much greater
accuracy. He reclaimed the stars that
Plancius had added to Argo and turned
them into the constellation Robur
Carolinum (Charles's Oak) to ﬂatter
Charles II, then king of England. The
two different visions - Greater Argo
versus Old Argo plus Robur Carolinum
- coexisted and competed for the better
part of a century.
With that as background, let's visit the
Southern Hemisphere and take a look at the stars
contested between Plancius and Halley.

PL ANCIUS GLOBE: NATIONAL MARITIME MUSEUM, GREENWICH,
LONDON, CAIRD COLLECTION; NGC 3293: ESO / G. BECCA RI

Greater Argo Navis
The pot of gold at the end of the Carina Milky Way is the
incomparable Eta Carinae Nebula, NGC 3372, which is a
large, prominent bright patch to the unaided eye under any
but the brightest skies. It appears much bigger and brighter
than the Orion Nebula, despite being six times more distant, in the next Milky Way spiral arm in from our own. The
nebula becomes overwhelmingly complex and beautiful with
even the slightest optical aid, crisscrossed with dark dust
lanes and spangled with bright concentrations. Every increase
in aperture and magniﬁcation reveals something new; you
could spend a lifetime observing this object and see something new each time you look.
By amazing good luck, the Eta Carinae Nebula lies
directly behind one of the densest concentrations of nakedeye star clusters in our own arm of the Milky Way, and those
clusters are embedded in an extremely rich star ﬁeld. The
overall effect of bright-on-bright-on-bright is unlike anything else in the sky. To my eye, this area outshines the star
clouds in Cygnus, Scutum, and Sagittarius by a large margin.
It's full of fascinating features, but for now we'll concentrate
on the four brightest clusters.
NGC 3532 rivals the Eta Carinae Nebula in brightness,
though not in size. It's a stunning, huge, rich collection
of bright stars through binoculars and small telescopes.
NGC 3293 is a tiny, almost stellar bright patch to the
unaided eye, and optical aid reveals it as an extremely rich
and compact group of bright and medium-bright stars, which
are responsible for its nickname, the Little Gem. NGC 3114
is intermediate in size and brightness between NGC 3532 and
NGC 3293. Through binoculars it's a sparse group of bright
stars that reminds me of a claw.
Well away from the Milky Way, the Southern Pleiades, also
called IC 2602, consists of 2.8-magnitude Theta (θ) Carinae
surrounded by a group of 5th-magnitude stars that I can only
partially resolve without optical aid.
Tracing up the Milky Way far to the northwest, the False
Cross draws the eye irresistibly. It consists of Kappa and
Delta Velorum from the Old Keel plus Iota (ι) and Epsilon (ε)

t PLANCIUS GLOBE Petrus Plancius was a DutchFlemish astronomer and cartographer. He is renowned
for having produced the first complete celestial
globe, such as the one shown here.

Carinae. I have drawn dashed lines onto
the standard Sky & Telescope stick ﬁgure
to emphasize the asterism. It's strikingly
similar in shape and orientation to the
Southern Cross, though a tad bigger and
fainter. I see both groupings as kites rather
than crosses, since both lack a center star.
Continuing two of the kite's sides to the northwest, the parallelism between Gamma Velorum and
much fainter Chi (χ) Carinae reinforces the bond between
Old Argo and its southerly extension.
Greater Argo does indeed look like a ship, much more so
than Old Argo, since it's now a whole ship rather than just
a half. Not only has Old Argo been stretched far to the left,
it's also pivoted counterclockwise so that the stars of the Old
Keel now mark the bottom of Vela, the Sails.

Enter Lacaille
That's how things stood when Lacaille sailed for South Africa
in 1750. His main purpose was to determine the scale of the
solar system by measuring Mars's parallax in conjunction
with Jérôme Lalande back home in France. That's exactly
q LITTLE GEM NGC 3293 is a young open star cluster seemingly
superposed on wispy clouds of gas and dust lanes in this European
Southern Observatory image. The stars likely formed about 10 million
years ago, and the cluster lies at a distance of some 7,600 light-years.

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Sky & Telescope - March 2020

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