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NGC 3372
Eta Car Nebula, Gum 33, RCW 53
RA 10:45:08
Dec -59°
52.0'
Bright nebula |
The giant Eta Carinae nebula is some 300 light
years across, twenty times the size of the Orion Nebula. Although it is 9000
light years away, it stretches across two degrees of sky. Burnham summarizes the
object as "remarkable, bright, extremely large, irregular, with dark lanes,
diameter 80' x 85'. Keyhole nebula, contains nova-like Eta Carinae." The
nebula takes the form of three enormous fan-shaped areas with dark lanes
between. In the telescope, the main area containing Eta itself is brightest, and
the dark lanes set it off with very definite borders. The distinctively shaped
Keyhole Nebula is the most famous dark zone seen near Eta, and is more prominent
to the eye than in long-exposure photos.
It was discovered by Lacaille and included in
his 1755 catalogue as Class III Nos. 5 & 6. In his half-an-inch 8x telescope
he saw it as "many faint scattered stars in circle 15-20 minutes diameter
filled with nebulosity."
Dunlop 309: "(Eta Roboris Caroli, Bode)
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 Eta 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 N.f. the Eta,
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, as represented in the figure. There is much
nebulosity in this place, and very much extensive nebulosity throughout the
Robur Caroli, which is also very rich in small stars." Dunlop observed it
13 times.
h: In his great work "Results of
Astronomical Observations made during the years 1834,5,6,7,8, at the Cape of
Good Hope; being the completion of a telescopic survey of the whole surface of
the visible heavens, commenced in 1825, by Sir John F. W. Herschel, Bart., K.H.
... " published in 1847 he writes about "Eta Argus and the great
Nebula surrounding it. There is perhaps no other sidereal object which unites
more points of interest than this. Its situation is very remarkable, being in
the midst of one of those rich and brilliant masses, a succession of which
curiously contrasted with dark adjacent spaces (called by the old navigators
coal-sacks), constitute the milky way in that portion of its course which lies
between the Centaur and the main body of Argo. In all this region the stars of
the milky way are well separated, and, except within the limits of the nebula,
on a perfectly dark ground, and on an average, or larger magnitudes than in most
other regions." Herschel then gives the results of several star counts or "star
gages" conducted over an area of 47 square degrees, and concludes that: "the
amazing number of 147,500 stars must have passed under review. In the midst of
this vast stratum of stars occurs the bright star Eta Argus, an object in itself
of no ordinary interest on account of the singular changes in its lustre has
undergone within the period of authentic astronomy." He then relates a
short history of Eta's magnitude as seen by observers before him, starting with
Halley in 1677 and ending with Taylor's observation in 1833 which recorded it as
second magnitude. "When first observed by myself in 1834, it appeared as a
very large star of the second magnitude, or a very small one of the first, and
so it remained without apparent increase or change up to nearly the end of 1837,
in November of which year it was noticed of its usual brightness, or at least
without exciting any suspicion of a change ... It was on the 16th December 1837
that ... my astonishment was excited by the appearance of a new candidate for
distinction among the very brightest stars of the first magnitude, in a part of
the heavens with which being perfectly familiar, I was certain that no such
brilliant object had before been seen. After a momentary hesitation, the natural
consequence of a phenomenon so utterly unexpected, and referring to a map for
its configurations with the other conspicuous stars in the neighbourhood, I
became satisfied of its identity with my old acquaintance Eta Argus. Its light
was however nearly tripled. While yet low it equalled Rigel, and when it had
attained some altitude was decidedly greater. It was far superior to Achernar.
Fomalhaut and Alpha Gruis were at the time not quite so high, and Alpha Crucis
much lower, but all were fine and clear, and Eta Argus would not bear to be
lowered to their standard. It very decidedly surpassed Procyon, which was about
the same altitude, and was far superior to Aldebaran ... From this time its
light continued to increase. On the 28th December it was far superior to Rigel,
and could only be compared with Alpha Centauri which it equalled, having the
advantage of altitude, but fell somewhat short of it as the altitudes approached
equality. The maximum of brightness seems to have been obtained about the 2nd
January 1838 ... it was judged to be very nearly matched indeed with Alpha
Centauri ... After this its light began to fade ... On the 20th, it was 'visibly
diminished - now much less than Alpha Centauri, and not much greater than Rigel.
The change is palpable.'" He continues to discuss the course of the
outburst, and notes that "a strange field of speculation is opened by this
phenomenon." He ends the discussion with the observation that: "in the
beginning of 1838, the brightness of this star was so great as materially to
interfere with the observations of that part of the nebula surrounding it which
is situated in its immediate vicinity, and, in particular, almost to obliterate
that extremely curious oval or lacuna which forms to conspicious a feature in
the figure of the nebula annexed, and of which, had I not previously secured a
correct representation, I should then scarcely have been able to have done so to
my own satisfaction. The accurate representation of this nebula with its
included stars has proved a work of very great difficulty and labour, owing to
its great extent, its complicated convolutions, and the multitude of stars
scattered over it. To say that I have spent several months in the delineation of
he nebula, the micrometrical measurement of the co-ordinates of the skeleton
stars, the filling in, mapping down, and reading off of the skeletons when
prepared, the subsequent reduction and digestion into a catallogue, of the stars
so determined, and the execution, final revision, and correction of the drawing
and engraving, would, I am sure, be no exaggeration. Frequently, while working
at the telescope on these skeletons, a sensation of despair would arise of ever
being able to transfer to paper, with even tolerable correctness, their endless
details. However, by breaking it up into parts, and executing each part
separately, it has been accomplished ..." This account gives us a rare
glimpse of Herschel the man and sets an example for all deepsky observers.
Herschels magnificent sketch spans almost exactly a square degree of sky: "Of
this about four-sevenths are occupied by the nebulous branchings and their
included vacuities, and this portion only I have thought it requisite to
triangulate and catalogue. The number of stars within this area whose places
have been determined is 1,203 ... " In the introduction to his discussion
of the nebulosity, he comments: "It would manifestly be impossible by
verbal description to give any just idea of the capricious forms and irregular
gradations of light affected by the different branches and appendages of this
nebula. In this respect the figure must speak for itself." Of the
surrounding sky in which the nebula is situated, he writes: "Nor is it easy
for language to convey a full impression of the beauty and sublimity of the
spectacle it offers when viewed in a sweep, ushered in as it is by so glorious
and innumerable a procession of stars, to which it forms a sort of climax,
justifying expressions which, though I find them written in my journal in the
excitement of the moment, would be though extravagent if transferred to these
pages. In fact, it is impossible for any one with the least spark of
astronomical enthusiasm about him to pass soberly in review, with a powerful
telescope and in a fine night, that portion of the southern sky which is
comprised between 6h and 13h of RA, and from 146 to 149 NPD [-56 to -59
Declination], such are the variety and interest of the objects he will
encounter, and such the dazzling richness of the starry ground on which they are
represented to his gaze." A footnote reads: "The first three hours of
the zone thus marked out are remarkable for their fine double stars. Among the
nebulae which occur from 9h to 12h we have .. the beautiful planetary nebula
[NGC 2867], a perfect planet in appearance, with an attendant satellite; the
falcated nebula [NGC 3199]; Eta Argus with its nebula; the superb cluster [NGC
3532]; the blue [Herschel's italics] planetary nebula [NGC 3918], a most
exquisite and unique object ; and the beautiful cluster of various coloured
stars about Kappa Crucis [NGC 4755]." Hershel incorrectly deduces that the
nebulosity lies "at an immeasurable distance behind that stratum [Milky
Way]", since "this nebula does not show any appearance of
resolvability into stars ... [and] has therefore nothing in common with the
milky way, on the ground of which we see it projected." Of the appearance
of the nebulous complex, he writes: "There are, however, certain features
to which it is necessary to refer more particularly in illustration of our
figure ... The whole extent of the nebulosity to the south is somewhat greater
than can be included in the figure, but it grows so faint beyond the oval
vacuity in the upper part that I have not considered it necessary to trace it
beyond that limit. Nor am I quite sure that the south following portion of the
area of the figure in which no nebula is represented is in reality absolutely
free from it. The interior of the oval above mentioned is perfectly so. It is
also nearly devoid of stars, four minute ones ... only having been perceived in
it." Herschel was concerned with the possibility of change taking place in
the nebula, and wrote: "Great attention has been paid to exactness in the
situations of the minute stars ... which mark out the form of this oval with
respect to its borders. The two large stars .. on its south following side are
fairly immersed in the nebula, as are also [a star] on its south preceding, and
[two stars] at its northern extremity. Close to the great star A, is situated
that singular lemniscate-oval vacuity which forms so strange a feature of this
nebula. Its area is not entirely devoid of light. A thin nebulous veil seems as
if extended over its northern loop on the preceding side. Four stars .. are
placed precisely on its edge, and will serve as excellent detectors of change in
its form, should any occur..." The rest of his discourse relates to the
measurement and construction of the system of skeleton stars he employed to
accurately draw the nebula. The NGC records it as "remarkable object, Great
neb, Eta Argus."
Burnham writes that the star "Eta Carinae
is located in one of the most splendid regions of the southern Milky Way, the
great diffuse nebuloisty NGC 3372, often called the 'Keyhole Nebula', remarkable
both for its great size and the complexity of its structure ... dark lines
divide the nebulosity into several separate islands of glowing light; the
brightest of these contains an irregular dark elongated mass - the keyhole
itself - from which the nebula derives its name. In addition to this nebulosity,
which forms a brilliant setting for Eta Carinae, the star itself is surrounded
by a much smaller nebulous shell which is expanding at a rate of about four
arcseconds per century ... bright nebulous condensations in the shell were
detected visually by R.T. Inness in 1914, and were at first recorded as faint
'companion stars'. "
Sanford says "the whole area is a delight
to the visual observer, and each increase in telescopic aperture is rewarded by
more fascinating detail."
Hartung writes: "Eta Carinae with its
associated star clusters and the great diffuse gaseous nebula enveloping it form
one of the finest telescopic objects; on a clear dark night the region is
beautiful beyond description, even for small apertures. The nebula is
diversified by prominent dark lanes which indicate absorbing matter and it
extends in irregular luminous clouds far beyond a large field. The bright orange
Eta is surrounded by an orange red nebula about 3 arcsec wide, just visible with
a 4-inch, and the spectrum of the star shows numerous bands with the red
hydrogen-alpha shining like a tiny lamp at one end; this too can just be seen
with a four-inch."
Harrington calls it "among the most
amazing objects in the entire sky. Visible to the unaided eye, this huge glowing
cloud is exquisite even in the most modest optical equipment. Dark rifts divide
the nebula into several distinct regions, with the most prominent patch clearly
teardrop shape. Entombed within this cloud is Eta Carinae itself, a dazzlingly
orange star with an unusual past ... closely surrounding Eta Carinae is a tiny
reddish glow measuring only 3 arc seconds wide. This diminutive cloud may be
seen through moderate-size instruments at high magnifications. For a good
over-view of the entire network of nebulosity, use your lowest-power
widest-angle eyepiece. Even then, NGC 3372 will more than fill the field."
Simon Tsang writes that "the nebula is
segmented by prominent dark lanes and has a darker patch, the Keyhole, just west
of the star Eta. With its beautiful swirling outer extensions, this is without
doubt the most splendid emission nebula I've ever seen."
A wide-field photo, good for visual
comparison, may be found in Astronomy (1993) June, p 83.
Steve Coe, in an IAAC contribution, writes: "Eta
Carina is immediately obvious naked eye as an elongated bright spot in the
Southern Milky Way. In the 10X50s it is over one degree in size with averted
vision and is much brighter in the middle. There are 14 stars involved within
the nebulosity. There is an obvious dark lane that forms a "V" shaped
through the nebula with the brightest nebulosity and the star Eta Carina both to
the the north side of this dark lane. Averted vision makes the nebula grow
significantly in size. The field of view is stunning, a rich Milky Way field
with this large, bright nebula dead center is unique."
11x80: This nebula is
wonderful in 11x80 binoculars, which show a complex region of dark and bright
nebulosity, the bright nebulosity split by multi-legged dark nebula. Eta Carinae
itself is prominent and easily visible as a slightly orange star within the
textured nebulosity. The immediate impression of the nebula is of two brighter
lobes of nebulosity divided by a dark rift. This rift branches dimly into a
Y-shape east and south of Eta. The southern part of the nebulosity appears as an
elongated, bright mass with large, fainter winglike extensions west and east.
The northern half, containing Eta, appears mainly elongated to the north-west,
away from Eta. The whole four degree field of view is filled with many, many
stars. The background field is very rich with the unresolved haze of the rich
Milky Way background. I get the impression that these stars are quite shy; as
soon as you look away, they pop into view, but if you look at them, they vanish
again. Look for the two bright orange stars in the field; w Carinae to the south
and t2 Carinae to the north-west. [AS]
10-inch f/5 Newtonian: A
10-inch f/5 at 30x shows this as a very complicated area of contrasting
nebulosity and black sky; Eta itself appears slightly orange and is clearly
bloated. The whole one degree eyepiece is filled with delicate textured wisps of
this beautiful nebula. (suburban skies) [AS] |
