The Naked-Eye Visibility of Nebula NGC 3372

[AJames]

In the times before the significant brightening of the star Eta Carina in the early to mid 19th Century no one seems to have actually described its naked-eye visibility - at least as we see it today. We could readily assume that the nebulosity should have been seen by many people and observers before this time, but no observational records exist.

Two distinct possibilities are;

1) Either the brightness of eta Car hid the nebulosity from view

2) That the nebulosity was different than it was today.

All observations, and even Nicolas LacaIlle, do not describe naked-eye visibility in this region. It was after about 1815, when eta Car had risen above 3rd or 4th magnitude, that the nebulosity possibly was extinguished by its light. By 1822, around the time when Sir Thomas Brisbane, James Dunlop and Charles Rumker had arrived in Sydney, Australia, and had moved inland to constructed the Paramatta Observatory, the star had risen to somewhere around 2nd magnitude.

No doubt, it was the brilliancy of the eta Car that made the nebulosity not as seen visually to the eye but only as a telescopic target.

Naked-eye visibility of the nebulosity only could occur once ? Car had dropped in brightness. One of the first to record fact that I have found was by Sir John Herschel own son - the Lieutenant J. Herschel observations on the 23rd November 1868 from Bangalore. He says (Lt. J. Herschel, J.W.F. Herschel; "The Great Nebula round Eta Argus", MNRAS, 29, 82 (1868)) that;

"The nebulae is easily seen with the naked-eye. the eye selects the object as naturally as it would choose the Pleiades at that altitude... Compared [the nebula] directly with Nebecula Major (preceding by six hours...) that the Nebula Eta Argus was intrinsically brighter"

What do others think of this proposittion??

[bojan]

The nature of the nebulosity suggests that its brightness was not changed in such a short time.
The reason for lack of records therefore must be the effect of "hiding" behind the much brighter eta.

[CosMos (Rich)]

"The Eta Carinae Nebula was discovered by Abbe Lacaille during his 2-year journey to the Cape of Good Hope in 1751-52. Lacaille made two catalog entries situated in the region covered by this object: Lac III.5 and Lac III.6, which the elder literature both identifies with NGC 3372. While the description of III.6: "Large 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", matches well with what Lacaille should have seen in his 0.5-inch refractor, he describes III.5 as: "Two small stars surrounded by nebulosity". Ronald Stoyan has found that close to Lacaille's position for this object, there is indeed a small cluster, Collinder 228, which would match Lacaille's description when observed with such a small telescope."

http://www.seds.org/messier/xtra/ngc/n3372.html

[glenc (Glen)]

Dunlop 309 = NGC 3372
(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. Figure 14 is a very correct representation of it; the circle A B is about 1° and 37' diameter, with the star eta in the centre. 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 l0th magnitude north following 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. 13 observations.
James Dunlop 1826