My first Magazine Sky & Telescope - 01.2019 | Page 63

Sigma Orionis Although it shows well without a nebula fi lter, I prefer the view using a DGM Optics Narrow Pass Band fi lter because it gives a nice balance of nebu- lar detail and stars. The Flame hides a star cluster made up of some 800 newly formed stars and protostars, ages 0.2 to 1.5 million years old, with the newest clustering toward its center. Discovered by near-infrared observations, the vast majority of these stars seem to have circumstellar discs. We can’t see this cluster in visual wavelengths, but it’s fascinating to know it’s in there. The Flame is approximately the same distance away as IC 434, the Horse- head, M42, and all the other nebulae in this region — about 1,400 light- years — because they’re all part of the Orion Molecular Cloud. We can think of them as being the brightest parts of the same nebula. The main difference is that their illuminating stars energize them to different degrees and from dif- ferent angles. IC 431, IC 432, and IC 435 are smaller, fainter versions of NGC 2023 — stars surrounded by faint reflection nebulosity — and bracket the Flame Nebula to its southeast and north- west. They’re fun to track down if you can tear yourself away from the more famous and obvious sights in the area. The multiple star Sigma Orionis that’s energizing IC 434 shines at mag- nitude 3.8 and is surrounded by a loose cluster of low mass and brown dwarf stars. It’s also part of the Orion OB 1 b stellar association, which includes all three of the Orion’s Belt stars. u THE HORSE’S HEAD UP CLOSE This European Southern Observatory image of the Horsehead Nebula was obtained with the 8.2-meter Kueyen telescope, one of the four Very Large Telescope units at Cerro Paranal in the Atacama Desert, Chile. Note the bright edge along the top of the horse’s mane, head, brow, and muzzle, all of which can be detected by visual observers. The more diffuse area along the bottom of the muzzle can also be seen but is much more dificult because of lower contrast. The most challenging area is the ever so slightly less dark area interior to the mane. The orientation is the same as in the other images. IC 434 B33 The Horsehead Nebula IC 431 Zeta Orionis (Alnitak) IC 434 IC 434 IC 432 NGC 2023 0.5 degree circle NGC 2024 (The Flame Nebula) IC 435 NE p THE HORSEHEAD AND COMPANIONS This labeled version of the author’s sketch helps set the scale of the Horsehead with its surroundings. The ½° circle centered on the Horsehead is about the same apparent size as the full Moon, and the Horsehead itself is nearly the same appar- ent size as the Dumbbell Nebula. The hydrogen-beta i lter tends to produce its highest contrast views when used with an eyepiece that gives a 4-mm to 7-mm exit pupil, so try to use eyepieces that fall in this range. Of course, by far the most obvious and spectacular object in this area is Alnitak. At magnitude 1.8, it’s actually a close visual double star, with its A and B components shining at magnitude 1.9 and 3.7, respectively, and separated by about 3″. They’re a beautiful sight when the seeing is steady enough to split them cleanly. A third, unseen star was discovered spectroscopically. The A component is the brightest O star in the sky, with the other two stars belonging to spectral type B. All this starry brilliance should be placed outside the field of view when trying to observe the Horsehead in a wide-field instrument. Even so, it’s pos- sible to see both at the same time if you keep the Horsehead Nebula in the sweet spot of your averted vision, then move Alnitak into the field of view. This corner of Orion would be irresistible even without the Horsehead Nebula, but there’s no doubt this most famous of the dark nebulae is the main attraction. Its shape really can look like the profile of a horse’s head, and even though I sometimes think of the chess piece, there’s no more aptly named object in the sky. ¢ Contributing Editor HOWARD BANICH and Chuck Dethloff, co- founder of the Oregon Star Party and a great friend of the author’s since 1991, still get excited about a great view of the Horsehead Nebula. Howard can be reached at hban- [email protected]. sk yandtele scope.com • JA N UA RY 2 019 61