NEBULAE
The Edge of the Pelican
— IC 5070, close-up —
Processed by Nicolas Rolland, Paris. 9 hours gathered by Roboscope at Apollo 11 Observatory, Fregenal de la Sierra, Spain.
This is the boundary itself, at a third of an arcsecond per pixel. The wide view shows where the shape comes from; this one shows what it is made of.
Where the front meets the dust
The dark lane that draws the pelican's outline is not a clean edge. Along it, the ionisation front driven by the hot star hidden behind the cloud is working into the cold material — and where the material is dense enough to resist, it survives as a ridge or a column while everything around it is converted. The result is a boundary made of protrusions, each one a piece of cloud that was harder to remove than its neighbours.
Not Deneb
The source has been credited to Deneb for a long time, on the grounds that it lies in the same direction and is one of the most luminous stars in the sky. It is also at about 2,600 light-years, against 1,800 for the nebula. The star actually doing the work was identified in 2005: an O-type star embedded in the dust between IC 5070 and the North America Nebula, catalogued 2MASS J20555125+4352246, invisible at optical wavelengths.
Three lines, three depths
At this scale the choice of narrowband filters stops being a matter of colour. Where an ionisation front works into a cloud, the three lines used here do not come from the same place: OIII is emitted closest to the source, where the radiation is hardest, H-alpha behind it, and SII in the cooler layer furthest in, where the gas is only partly ionised. The front is therefore stratified, and at a third of an arcsecond per pixel that stratification is wide enough to resolve. The colour gradient across each ridge is not decoration — it is a reading of how deep the radiation has got.
Nine hours at a third of an arcsecond
A Planewave CDK17 of 432 millimetres at f/6.8, with a CMOS sensor sampling at 0.314 arcseconds per pixel, from Fregenal de la Sierra in August 2024 — 540 frames over nine hours, in SII, H-alpha and OIII. That sampling is four to five times finer than the wide-field view of the same object, and it is what the subject is for: the ridges along the front are a few arcseconds across, and at a coarser scale they merge into a smooth edge. Nine hours is short for narrowband, and it shows in the faint outer material rather than in the ridge — the front is bright, and it is the part of this nebula that nine hours will carry. Gathered by Roboscope at the Apollo 11 Observatory.
Hubble palette (SHO): SII in the red channel, H-alpha in green, OIII in blue. The colours are a mapping of three emission lines, not a record of what the eye would see.
