Wednesday, 10 January 2018

The Horsehead

Few objects stimulate the imagination as much as the famous Horsehead Nebula (B33 in technical terms). Its suggestive shape has amazed people ever since its discovery in 1888 and it's become one of the most photographed objects in the sky. Unfortunately, to us, humble visual observers, this extraordinary patch of dust is a daunting target. Yes, I've read all of the reports that say that it was "easy to see in a C8" and I've even seen sketches, observed through a 6" without filters (!), that show it in all of its glory (and almost in 3D-Technicolor too). But are they trustworthy? No, unfortunately not. 

Sometimes, we visual astronomers behave like fishermen who claim to have caught a five-foot sardine. It's not just about showing off, but we also want to convince ourselves that we've really seen a very difficult object, even though it was on the border of visible, or sometimes beyond. Who can blame us? Often we spend many hours trying to find it, peering through that tiny little hole of our telescope's eyepiece until our other eyelid's sore of keeping it shut, our whole body's shivering because of the biting cold, our limbs numb, our noses dripping, our foreheads frosted, our brains begging us to go back indoors and preferably to sleep. Yet, we persevere because we want to find that particular object. It's like a trophy we desperately want to hang on the wall. 

In the case of our beloved Horsehead, which is so incredibly faint that it was hardly visible through my 18" binoscope, things tend to get a little out of hand. A H-Beta filter helps a lot to bring it out and makes this nebula accessible to smaller scopes under dark skies, but don't expect miracles. Given that a pair of these filters'd cost me €400 and that they're only useful on a handful of objects, I preferred to try and find it without. And with success, although it remained extremely faint as I've tried to reflect in my sketch. The background nebulosity, scarcely illuminated by the embedded newborn stars was hardly apparent as a somewhat lighter half of the field of view, compared to the darker left half. Something that struck me much more was the almost total absence of little background stars in that left half. Obviously, the light of background stars is completely blocked by the enormous cloud of dark dust that cuts through the field of view and of which the Horsehead is just a bulge sticking out. Fortunately for us, this peculiar bulge drifts in front of the delicate bright nebula behind it (IC434) and therefore becomes "easily" visible. Well, let's say that I've seen it. This whole area, some 1,500 light-years distant, is but a part of the gigantic Orion Molecular Cloud, a vast region in space where a lot of star formation takes place. Also the Orion Nebula is just a part of this complex. 

Much more evident in the same field of view, is a reflection nebula called NGC2023 (bottom-left on my sketch). Being over 4 light-years across, it's actually one of the largest reflection nebulae in our sky, brightly illuminated by the young and extremely hot giant star that lies within it.  

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