My first Magazine Sky & Telescope - 01.2019

Discovery Photographed in 1888 with an 8-inch astrograph at the Harvard College Observatory (HCO) in Cambridge, Massachusetts, the discovery plate, B2312, was examined by Williamina Fleming of the HCO, who published her description of the photo in 1890. In Volume 18 of the Annals of the Harvard College Observatory, she mentioned a “semicircular indentation” that we now call the Horsehead Nebula: A large nebulosity extending nearly south from ζ Orionis for about 60′. More intense and well marked on the following side, with a semicircular indentation 5′ in diameter 30′ of ζ. Although the discovery was initially credited to the HCO’s Director Edward Pickering in the 1895 edition of the Index Catalog (IC), in Volume 60 of its Annals in 1908 the HCO correctly listed Fleming as discoverer of the Horsehead Nebula. The nebula was cataloged as B33 by E. E. Barnard in 1919. The “large nebulosity,” now known as IC 434, was originally discovered by William Her- schel in 1786. Visual Detection IC 434 provides the slightly brighter background against which the Horse- head is silhouetted. Without this back- ground the Horsehead would be invis- ible to visual observers, so it’s IC 434 that’s crucial to detect — if you can’t see IC 434 you won’t see the Horsehead. An important tool for visually observing IC 434 is a hydrogen-beta nebula ﬁ lter. Sometimes referred to as the “Horsehead Filter,” it increases the contrast between IC 434, the back- ground sky, and the Horsehead. If you have one, use it. If you don’t, get one. It’s useful on more than just the Horsehead, of course, but its nickname is deserved because it can make this famously dif- ﬁ cult object a lot easier to see. Although I was able to detect the Horsehead using an Ultra-High Con- trast ﬁ lter with my 20-inch, borrowing Chuck’s hydrogen-beta ﬁ lter increased contrast immensely and produced a much more satisfying image. On the other hand, if you’re ever under a pristine dark sky with Orion near the meridian, try to see the Horsehead without a ﬁ lter and you may be pleas- antly surprised. On these rare nights I’ve thought it looked like a knight, the chess piece. If you’ve never seen the Horsehead visually, there are two confounding factors to overcome. The ﬁ rst is to get yourself under a truly dark and trans- parent sky. Without a quality dark sky you have almost no chance to see the Horsehead visually no matter how large or ﬁ ne a telescope you’re looking through. Even the hydrogen-beta ﬁ lter won’t help much. The second factor is expectation, and being aware of this is just as important as the observing conditions. We’ve all seen wide-ﬁ eld photographs of the Horsehead and its environs (like those in this article) showing the Horsehead as a rather small object in relation to its spectacular surroundings. Because of photos like these, there’s a good chance you’ll expect the Horsehead to appear smaller in your eyepiece than it actually appears, and to have more contrast with its background. If so, your eye might skip over its much larger apparent size and subtle contrast. 0° M78 δ IC 431 IC 432 2024 ζ ε –2° η σ IC 434 2023 IC 435 B33 –4° ORION M43 2 3 4 5 6 7 τ ι υ 5 h 40 m 1 M42 –6° –8° from even mildly light-polluted skies, frustrating far too many observers for far too long. But it’s surprisingly easy to see under truly dark and transpar- ent conditions — and with a smaller telescope than you may imagine. Rigel 5 h 30 m 5 h 20 m β NE p THE HORSE’S HEAD APPEARS A cropped scan of plate B2312, the discovery image of the Horsehead Nebula (at center) from the Harvard College Observatory, has also had the colors reversed for better contrast. Taken with the 8-inch Bache doublet refractor in 1888, this plate was examined by Williamina Fleming in 1890 when she discovered the Horsehead Nebula. The Horsehead is included in E. E. Barnard’s 1919 catalog of dark nebulae as B33. We now know the Horsehead is the closest nebula pillar sculpted by photoionization, and that protostars are forming within it. To calibrate your expectations, the Horsehead is approximately the same apparent size as M27, the Dumbbell Nebula, and has even less contrast than the Pillars of Creation in M16. A big telescope isn’t required to see the Horsehead. I’ve been able to see it quite well in scopes down to a 4.25- inch Newtonian from the unspoiled skies of Steens Mountain in southeast Oregon, illustrating that a top-notch sky is more important than the size of the telescope. Details In her plate notes, Williamina Flem- ing mentioned that IC 434 is “more intense and well marked on the follow- ing (east) side,” but visually it doesn’t have a sharp eastern edge, even though it does fade more gradually westward. The sharpest boundary is created by the Horsehead Nebula itself, which helps tremendously with its detectability. Even so, on most nights the Horse- head will appear as a dark, fuzzy-edged scoop into IC 434 that’s only slightly darker than its surroundings. IC 434 is quite faint even with the help of a hydrogen-beta ﬁ lter. sk yandtele scope.com • JA N UA RY 2 019 59

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