If you live in the northern hemisphere, I’m sure you’ve very much noticed the daylight hours have become much shorter – but have you noticed the return of the winter stars during the early morning hours? If you’re up before dawn the constellation of Orion sits high in the sky and with it brings promises of “Dark Knight Ahead”….
In this beautiful h-alpha image of B33 and NGC2024 taken by Gordon Haynes, we’re getting a preview of one of the most sought after dark nebulae in the heavens – the “Horsehead”. The long tongue of nebulosity which makes it visible is IC 434, first discovered photographically by Edward Pickering in 1889. But it wasn’t until January 25, 1900 that Isaac Roberts picked up the dark notch on a photo he’d made and E.E. Barnard visually recognized it around 1910.
The ever-vigilant, and visually astute Barnard made his first publication of the “dark knight” in Dark Regions in the Sky Suggesting an Obscuration of Light – Astrophysical Journal, Vol. 38, pages 496-501. In 1919, he officially cataloged it as B33 in On the Dark Markings of the Sky – with a Catalogue of 181 Such Objects where it remains to this day as an astronomical favorite. What makes this 1,600 light year distant dark globule of dust and non-luminous gas so important? Well, a recent study done using the h-alpha wavelength and the 2.34 m Vainu Bappu Telescope were done to test fractal structure. Ten sample readings of the box dimension of this image were taken using a fractal analysis software, giving an average value of 1.6965725. The sample dimensions were found to be different from the topological dimension of one. Importantly, the box dimension of B 33 was not found to be significantly different from that of the Julia set (box dimension 1.679594) with c = -0.745429 + 0.113008i. This provides compelling evidence to show that the structure of the Horsehead nebula is not only fractal, but also that its geometry can be described by the Julia function f(z) = z2 + c, where both z and c are complex numbers.
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While that’s cool, I wanted to go even deeper. I checked into SCUBA and this is what I found from the works of D. Ward-Thompson (et al):
“We present observations taken with SCUBA on the JCMT of the Horsehead Nebula in Orion (B33), at wavelengths of 450 and 850 mum. We see bright emission from that part of the cloud associated with the photon-dominated region (PDR) at the `top’ of the horse’s head, which we label B33-SMM1. We characterise the physical parameters of the extended dust responsible for this emission, and find that B33-SMM1 contains a more dense core than was previously suspected. We compare the SCUBA data with data from the Infrared Space Observatory (ISO) and find that the emission at 6.75-mum is offset towards the west, indicating that the mid-infrared emission is tracing the PDR while the submillimetre emission comes from the molecular cloud core behind the PDR. We calculate the virial balance of this core and find that it is not gravitationally bound but is being confined by the external pressure from the HII region IC434, and that it will either be destroyed by the ionising radiation, or else may undergo triggered star formation. Furthermore we find evidence for a lozenge-shaped clump in the `throat’ of the horse, which is not seen in emission at shorter wavelengths. We label this source B33-SMM2 and find that it is brighter at submillimetre wavelengths than B33-SMM1. SMM2 is seen in absorption in the 6.75-mum ISO data, from which we obtain an independent estimate of the column density in excellent agreement with that calculated from the submillimetre emission. We calculate the stability of this core against collapse and find that it is in approximate gravitational virial equilibrium. This is consistent with it being a pre-existing core in B33, possibly pre-stellar in nature, but that it may also eventually undergo collapse under the effects of the HII region.”
So it’s a chance thing… It just happens to look like a cosmic chess piece. But this is one chess piece that has the odds stacked in its favor for starbirth. This shapely cloud of H2 molecules may have a density within its internal clumps that could reach up to 105 H2 per cubic centimeters or more and have their own internal magnetic field which will provides support against their own gravity. Deep inside, the dust blocks out the stellar ultraviolet radiation, getting darker and colder – just like our northern hemisphere nights. Near the center, the carbon changes and the chemistry becomes exotic – stars begin to form in a process very similar to condensation. The pressure appears to be building inside B33…
And tomorrow’s “Dark Knight” will be lit by new stars.