The Horsehead Nebula is one of a handful of select deep-sky objects that down the decades have acquired legendary, almost mystic status within the deep-sky community. It is arguably the most famous example of a dark nebula (catalogued as Barnard 33), which are cold, dense clouds of obscuring gas and dust that blot out the light from stars and other objects that lie beyond, from our perspective.
The Horsehead’s location, amid mighty Orion’s famously bright, well-known star-fields and brighter nebulae, is not hard to pin-point. However, appearing visually as just a small (five-arcminute long) notch silhouetted against the ionised gas of IC 434, a red emission nebula, it’s notoriously difficult to find and observe. Images reveal the Horsehead is truly an appropriate moniker and its been famous as such for many decades. Wide-field images of the Horsehead’s extended region provide a vista that has few equals in the entire sky.
Yet for such a famous and iconic object, it was unknown to the early great visual observers of the eighteenth and nineteenth century. It was first recorded in 1888 on a photographic plate exposed at Harvard University by William Pickering, but it fell to Williamina Fleming to first describe it on examining the plate. The most famous official designation assigned to the Horsehead is Barnard 33, after the prolific and extraordinary Edward Emerson Barnard, the great American observer whose 1927 catalogue of dark nebulae is perhaps his greatest legacy.
The Horsehead is an active site of low-mass star formation; images from professional observatories and the Hubble Space Telescope show a small but remarkably contrasting yellow nebula surrounding the star V615 Orionis and small red spots at the base of the Horsehead that are hidden protostars, with red streaks highlighting Herbig-Haro objects, which are jets of material emanating from protostars
How to observe it
Every observer will be familiar with the ‘Belt of Orion’, three bright stars, Mintaka (delta [δ] Orionis, magnitude +2.2), Alnilam (epsilon [ε] Ori, +1.7) and Alnitak (zeta [ζ] Ori, +1.7), which are orientated north-west to south-east and lie midway between Betelgeuse to the north and Rigel further south.
Home in on Alnitak (the fantastic Flame Nebula [NGC 2024 and Sh2-277] lies just to the north-east) and look for HIP 26756, a magnitude +7.5 star lying around 22 arcminutes to the south. This star is the most northerly of an isosceles triangle-shaped trio of stars that also includes magnitude +7.6 HIP 26694, which lies 13 arcminutes to the south-west and magnitude +7.9 HIP 26752, 25 arcminutes south. The Horsehead is located roughly in the centre of the triangle, about eight arcminutes south of HIP 26756.
Whether you are successful in catching a glimpse of your equine quarry will greatly depend on sky conditions. From a dark-sky location, wait for a moonless and very transparent night and get fully dark-adapted. How large a telescope you’ll need is open to debate; observers do report seeing it with instruments as small as 100–150mm (four to six inches) in aperture, or even large binoculars.
Once you’re certain you are looking in the right place, insert a hydrogen-beta narrowband filter, or a UHC filter, preferably with a 300mm (12-inch) telescope. (Have a go with a lesser aperture if you can spot the Flame Nebula, an easier target). Take your time and try to detect the dim glow of the aforementioned IC 434, the 90 x 14 arcminute-sized tapered finger of nebulosity running south from Alnitak. You may need to keep HIP 26756 out of your field of view and try using averted vision. If you can detect a faint notch, or hole, then you can pat yourself on the back!