The Caldwell Objects

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I F WE COULD PHOTOGRAPH HUMAN IMAGInation,
the result might look something like the
Eta (η) Carinae Nebula — an elaborate tapestry of
stellar energy fused into luminous folds of gases
and strung together with threads of dust. To the
naked eye the complex appears as a silver
pendant of nebulous matter in the sparkling
collar of the Carina Milky Way. It is the most
spectacular example of an emission nebula in the
heavens, and it resides in the richest and most
dramatic region of the Milky Way, a stormy
swath of suns between the Southern Cross and
the False Cross. Wide-field photographs show Eta
Carinae as a symphony of visual splendor; the
heart sees it as the Taj Mahal of galactic wonder.
Some books credit Abbe Nicolas Louis de
Lacaille with discovering the nebula. This claim
seems ludicrous. Today, the nebula is apparent at
a glance. So why wouldn't southern stargaz-ers
have known of it throughout time? For a clue,
let's look at Lacaille's "discovery" descriptions.
Looking at the nebula through a ½-inch 8x
telescope — very roughly the equivalent of a
small pair of binoculars — he noted "[t]wo small
stars surrounded by nebulosity." On another
night he described it as a" [l]arge group of a great
number of small stars, little compressed, and
filling out the space of a kind of a semi-circle of
15 to 20 minutes in diameter; with a slight
nebulosity widespread in space" (emphasis mine).
Would anyone today looking at this region of
space in binoculars call the nebulosity slight?
Equally remarkably, Edmond Halley is said to
have observed Eta Carinae, the bright star
embedded in the nebula, from St. Helena in 1677,
nearly 80 years before Lacaille. Yet he failed to
mention the presence of any nebulosity. Not until
James Dunlop observed the object in the 1820s do
we get a sense that a remarkable nebula can be
found here:
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[Eta (η)] Roboris Caroli*... is a bright star
of the 3rd magnitude, surrounded by a
multitude of small stars, and pretty strong
nebulosity; very similar in its nature to
that in Orion, but not so bright. . . . I can
count twelve or fourteen extremely
minute stars surrounding η in the space of
about 1'; several of them appear close to
the disk: there is a pretty bright small star
about the 10th magnitude north following
[northeast of] the η, and distant about 1'.
The nebulosity is pretty strongly marked;
that on the south side is very unequal in
brightness, and the different portions of
the nebulosity are completely detached. . .
. There is much nebulosity in this place,
and very much extensive nebulosity
throughout the Robur Caroli, which is
also very rich in small stars.
Is it possible that the nebula has been
gradually brightening over time? Today its
brightest parts are so intense they can be
glimpsed even in astronomical twilight, and the
Eta Carinae Nebula is as much a part of the
naked-eye fabric of the Milky Way as, say, the
Double Cluster in Perseus. So how could its
discovery belong to anyone in particular? Before
answering that question, let's see what modern
science has to say about the nebula and its source
of radiant energy.
The Eta Carinae Nebula is a remarkable ΗII
region, the largest known in our galaxy. It
measures some 260 light-years across — nearly 7
times the size of the Orion Nebula. The nebula
appears severely disjointed, with fantastic
blossoms of glowing gas clinging to a twisted
* Robur Carolinum (Charles's Oak) is a now-defunct constellation that
Halley designated in 1679. Eta Carinae was briefly known as Eta
Roboris Caroli. Eta Carinae also was known, in the 19th century, as Eta
Argus, when Carina was considered merely a part (the Keel) of Argo
Navis (the Ship Argo), another now-defunct constellation.
Deep-Sky Companions: The Caldwell Objects