My first Magazine Sky & Telescope - 01.2019

JANUARY 2019 OBSERVING Going Deep t THE HORSE’S HEAD SKETCHED The author’s drawing of the Horsehead Nebula and its surroundings combined many ields of view to produce this composite sketch. A little more than 4.5 hours of sketching time over several nights using a 28-inch f/4 Newtonian went into this drawing, which is based on views using magniications of 131× to 253× and a 1990s- era Lumicon hydrogen-beta ilter. Note how the top edges of the Horsehead’s proile have the greatest contrast. One of the more spectacular objects in this 3°-wide ield of view, and cer- tainly the brightest, is the double star Alnitak, Zeta Orionis, the easternmost star of Orion’s Belt — note that its double nature is not at all evident in the previous photos. The A compo- nent is the brightest O star in the sky. NE 60 JA N UA RY 2 019 • SK Y & TELESCOPE it unambiguously, but it’s been seen by other observers. Tracing the full extent of IC 434 is a challenge because it blends ever so gently into the sky background in all directions. It takes careful examination of the ﬁeld of view to determine where it disappears, and I’ve been surprised how far I’ve been able to trace it. Subtle east-west striations in the area west of the Horsehead can also be detected. Spectacular Surroundings Just northeast of the Horsehead Nebula is NGC 2023, a compact reﬂection nebula surrounding the 7.8-magnitude star HD 37903. Discovered by William Herschel in 1785, on the best nights you The Horsehead and Its Nebulae Object Type of Nebula Size RA Dec. Dark 6′ × 4′ 05 h 40.9 m –02° 28′ Emission 60′ × 10′ 05 h 41.0 m –02° 24′ NGC 2023 Emission + Reﬂection 10′ × 10′ 05 h 41.6 m –02° 16′ NGC 2024 Emission 30′ × 30′ 05 h 41.9 m –01° 51′ IC 431 Reﬂection 5′ × 3′ 05 h 40.3 m –01° 27′ IC 432 Reﬂection 8′ × 4′ 05 h 40.9 m –01° 29′ IC 435 Reﬂection 4′ × 3′ 05 h 43.0 m –02° 19′ B33 IC 434 Angular sizes and separations are from recent catalogs. Visually, an object’s size is often smaller than the cataloged value and varies according to the aperture and magniication of the viewing instrument. Right ascension and declination are for equinox 2000.0. We see IC 434 because of the O-type star Sigma (σ) Orionis. Its intense ultraviolet light causes IC 434 to glow by a process called photoionization, which produces its characteristic red glow seen in color photos. We see the Horsehead as a dark nebula, but on its opposite side it’s also photoionized by Sigma. We see a tiny bit of that glow around the top edge of the Horsehead’s silhouette, which enhances its con- trast. Seeing this thin bright edge is a challenge, but it is possible. On the best nights you might be able to see the faintly glowing equine outline of the horse’s mane, head, and brow, as well as the top of the muzzle. It’s an excit- ing observation and demonstrates that the Horsehead is an extension of IC 434 that’s silhouetted against itself. More difﬁcult to see is the bottom (eastern) edge of the Horsehead’s muz- zle. It’s much less distinct than the top, and whether you can see any contrast between IC 434 and the bottom of the muzzle will determine if you see a dark nebula shaped like a horse’s head in proﬁle or something more like Fleming’s “semicircular indentation.” Even more challenging is a very slightly lighter area in the neck that extends east-west and highlights the Horsehead’s mane. Detecting this area requires a pristine sky and superior observing skills. I have yet to identify may be able to see a fairly sharp edge on NGC 2023’s northwestern edge, which highlights how far off HD 37903 lies from the true center of the nebula. If NGC 2023 is an obvious glow without a nebula ﬁlter, you have a good chance of seeing the Horsehead. If you can’t see NGC 2023, then you’ll need to wait for a darker and/or a more transparent night. By far the most obvious bright neb- ula in the area is NGC 2024, the Flame Nebula. Also discovered by Herschel, it’s just east of brilliant Alnitak, or Zeta (ζ) Orionis, the easternmost star of Orion’s Belt. The Flame can be washed out by the glare of Alnitak so place the star just outside the ﬁeld of view. The Flame is an H II star-forming region, energized by nearby Alnitak. Streaks of dark nebu- losity divide it into three distinct parts that create its ﬂamelike shape.

My first Magazine Sky & Telescope - 01.2019 | Page 62