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The super secret sky

February 2012: As history shows us, spotting the nebulosity within the Pleiades is no easy task.
stephen_james_o_meara_new
On the next clear moonless night, when you’re far away from city lights, try searching out the veils of nebulosity adorning the Seven Sisters (the Pleiades) like bridal attire. What you see will depend on a variety of factors: the size of your telescope, the clarity of the night, the darkness of the sky, the magnification you use, your field of view, and your vision.
 
Telescopic observers learned these facts over a period of decades in the middle to late 19th century as some of the most reputable astronomers of the time debated the existence of the brightest veil: NGC 1435, otherwise known as Tempel’s Nebula and the Merope Nebula (because of its proximity to the star Merope). The story behind the astronomical quest to spy that and other nebulosity in the Pleiades (M45) is much too complex to review here in detail. Instead, I will highlight some of the more interesting asides — ones that help illustrate how difficult it can be to verify visual claims. These historical fragments should also help you put your own observing experience into perspective.

The “mystic” veil
While in Venice, Italy, German-born Wilhelm Tempel turned his 4-inch Steinheil comet-seeker toward the Pleiades on October 19, 1859. He hadn’t looked at the group in 6 months, he said, and consequently found a “large bright nebula,” about 30' in length, just south of Merope. Except for an uncertain observation by French astronomer Edme-Sébastien Jeaurat in 1786, Tempel’s was the first well-documented sighting of nebulosity in the Seven Sisters.
NGC-1435
NGC 1435, also known as Tempel's Nebula (and the Merope Nebula), was notoriously hard for historical observers to confirm, and some astronomers doubted its very existence for decades. Anthony Ayiomamitis
In a letter to Christian A. F. Peters, editor of the Astronomische Nachrichten, dated December 23, 1860, Tempel said that at first he mistook it to be a “fine large comet, but convinced myself on the following night, the 20th, that its position remained unchanged.” He went on to observe the nebula repeatedly through a variety of instruments, including the 8-inch and 11-inch refractors at the Royal Florentine Observatory in Arcetri, Italy (now simply known as Arcetri Observatory). Through these telescopes, he was “greatly surprised to find it so plain, large, and (in one direction at least) so sharply defined.”

Not so fast
Skepticism arose, however, when several renowned observers using large refractors and reflectors could not see the nebula. German astronomer Heinrich d’Arrest was among Tempel’s biggest critics. After failing to detect the veil through his 11-inch refractor in Copenhagen, d’Arrest jousted, “I have hitherto been able positively to see nothing. … I therefore, even yet, am of opinion that this nebula is variable, otherwise the original announcement of the discovery … must be looked upon as being greatly exaggerated.”

Others, including fellow German Eduard Schönfeld, disagreed. Observing September 20, 1862, with a 6.5-inch refractor in Mannheim, Germany, he said the nebula “instantly stuck out in the local telescope.” Later, he added that it had shone “very clearly” and was “immediately conspicuous, without accurate knowledge of the position.”
IC-349
Ghostly nebla IC 349 is another of the Pleiades' secret veils, lying just 36" southeast of Merope. E. E. Barnard discovered the object in 1890, and it too possesses a contentious history. NASA/Hubble Heritage Team (STScI/AURA)
In response to mounting swipes, d’Arrest admitted, “After a long effort, I actually set eyes on Tempel’s Nebula.” But, in support of his own argument that Tempel’s claim was exaggerated, he added the following verbal parry: “This is the faintest object which I remember ever having seen in the refractor.” Adding fuel to the fire was the fact that even those observers who rallied support for Tempel could not agree on the nebula’s exact form, casting more doubt on the veracity of the claims.

In his “Great Nebula in the Pleiades,” in an 1888 issue of Knowledge, noted English astronomer Arthur Cowper Ranyard opined, “The observations are worth examining, as they throw some light on the differences which are always likely to exist when observations are pushed into the border-land of vision, where by reason of the extreme faintness or minuteness of the objects examined, the eye begins to fail, and the imagination begins to play a larger and larger part in filling up the gaps where the senses of the eye-straining observer fail him.”

Because we now know that the Merope Nebula does not vary in brightness, the reason for the diverse opinions among these respected observers is clear: Large, dim nebulae can escape notice (due to lack of contrast) in a long-focal-length telescope at high power, which gives a limited field of view.
Wilhelm-Tempel
Wilhelm Tempel, a 19th-century German astronomer, spotted NGC 1435 just south of Merope in 1859. A year later, he called it "plain, large, and ... sharply defined." Still, other astronomers doubted the nebula's existence for decades to come. Robert Bunsen
In fact, British observer Reverend Thomas W. Webb spied the Merope Nebula through a 2-inch telescope in 1863 while finding it “invisible” through an 11-inch. Even d’Arrest finally admitted, “Here are nebulae, invisible or barely seen in great telescopes which can easily be seen in their finders.”

The nebula grows
As observers began scrutinizing the Merope Nebula, new details emerged. In 1864, German-born French artist and astronomer Hermann Goldschmidt thought the entire cluster was surrounded by nebulous matter, extending over an area of 5°. Others agreed in general that Tempel’s nebula was larger than listed.
 
Italian astronomer Giovanni Schiaparelli, for instance, wrote to Tempel, asserting, “Not only does [the Merope Nebula] reach almost up to Electra, but it encompasses that star and Celaeno as well. … It is singular that so many persons should have examined the Pleiades without paying attention to this great nebula, which nevertheless is so evident an object with a clear sky.”

Clarity began to take shape in 1886 with photographs by French brothers Paul and Prosper Henry taken through the 13-inch refractor at Paris Observatory. They showed for the first time not only the Merope Nebula (as four nearly parallel streaks) but also wisps of nebulosity extending from the stars Maia and Electra.
Pleiades-with-labels
The Pleiades (M45) may be familiar to most skywatchers, but their hidden veils of nebulosity are somewhat harder to make out. NASA/ESA/AURA/Caltech
A photo finish
Inspired by the photographic evidence, new visual studies ensued. One observer, Austrian astronomer Rudolf Spitaler, used the 27-inch Grubb refractor in Vienna to see some astounding detail: “At least the whole Pleiades region west and north of Alcyone is covered by an extended nebulosity, of which all previously perceived, apparently isolated nebulae are merely bright knots of light.”
 
This observation, in turn, was vindicated in October 1886 when Welsh engineer and amateur astronomer Isaac Roberts made a 3-hour exposure with the 20-inch Grubb reflector. It showed not only Alcyone, Maia, Electra, and Merope “surrounded by nebulae, but that the nebulosity extends in streamers and fleecy masses, till it seems to almost fill the spaces between the stars, and to extend far beyond them.”
 
The only visual detail that went unnoticed in the photographs was IC 349, which eagle-eyed Edward Emerson Barnard spied in 1890 through the 36-inch refractor at Lick Observatory in San Jose, California. He called it the “new Merope nebula” because the 30''-wide cometary glow (the brightest part of the nebula) lies only 36'' southeast of Merope and thus hides in the star’s glare. Alas, even this discovery has seen challenges. But that’s another story.
 
Now it’s your turn. Let me know what you see at someara@interpac.net.
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