A Butterfly Divided by Dust

Overview

IC 1318 is a large complex of glowing gas that surrounds Sadr (Gamma Cygni), the star at the centre of Cygnus, the swan. The whole complex is more than 100 light-years across. This frame looks at its best-known section, the Butterfly: two bright lobes of hydrogen with a long band of dark dust between them.

Sadr looks like it belongs to the nebula, but it doesn't. The star is roughly 1,800 light-years away, while the nebula sits at about 4,900 light-years. We just happen to see them along the same line of sight. Published distances for the nebula vary, so treat that figure as approximate.

The Dust Lane

The dark band cutting across the frame is catalogued as LDN 889. It isn't empty space. It's a cloud of cold dust and gas around 20 light-years thick, dense enough to block the light of the glowing gas behind it. The two "wings" on either side, catalogued as IC 1318 B and C, look separate, but they're physically connected behind the dust.

Look closely at the edges of the dark lanes and you'll find small dark knots and fingers pointing into the bright gas. These are denser clumps of the original cloud. Radiation from nearby hot stars is slowly eroding them, and the densest parts hold out the longest.

What Makes It Glow

The nebula is energised by hot, massive stars in the region, including an O-type star hidden behind the dust. Their ultraviolet light strips electrons from the surrounding atoms. When the electrons recombine, the gas gives off light at very specific wavelengths:

• Hydrogen-alpha (656 nm) traces most of the bright structure, including the fine red filaments running through the right half of the frame.
• Doubly ionised oxygen (501 nm) needs more energetic light to form. It shows up as the blue glow in the upper right.
• Singly ionised sulphur (672 nm) is fainter and follows the edges of the denser material.

Imaging Approach

The nebula was shot through 3 nm narrowband filters, which isolate hydrogen, oxygen and sulphur and reject almost everything else. Star colour came from a separate set of short red, green and blue exposures.

Before combining the narrowband data, I used continuum subtraction. Each narrowband filter also picks up a small amount of broadband light from stars and dust, so I scaled the starless RGB data to match each channel and subtracted it. What's left is closer to pure emission, and that's why the dark lanes go so deep and the fine filaments stand out.

For colour, I used the Foraxx palette, chosen by comparing options in Seti Astro's Perfect Palette Picker. Foraxx is a "dynamic" palette. Instead of mapping each gas to a fixed colour, it changes the blend pixel by pixel depending on how much hydrogen and oxygen are present:

• Areas dominated by hydrogen render warm red.
• Areas dominated by oxygen render blue.
• Areas where the two overlap render cream and gold.

The result keeps the natural feel of hydrogen-rich nebulae while still showing where the chemistry changes.

I followed Philippe Bernhard's "SHO with RGB stars (linear mix)" workflow for PixInsight for most of the processing, swapping in Foraxx for the colour combination step. The RGB stars were processed separately and blended back in at the end.