The Elephant's Trunk (IC 1396A)

I pointed my telescope towards the Cepheus constellation during the first week of September to capture IC 1396A, better known as the Elephant's Trunk Nebula. The dark, snaking column running through the center of the frame is a dense cord of cold molecular gas and dust about 25 light-years long, sitting inside the much larger IC 1396 emission nebula. IC 1396 itself is one of the biggest emission nebulae in the northern sky, roughly 3 degrees across, which works out to about 100 light-years of actual size once you factor in its distance of around 2,400 light-years from Earth. For scale, 3 degrees is about six full moons lined up side by side.

The whole region is lit up by HD 206267A, the massive, hot O-type primary star of the multiple system HD 206267, sitting near the center of IC 1396. Everywhere that starlight reaches, it ionizes the surrounding hydrogen gas and makes it glow. The trunk itself survives because it's currently dense enough to shield its own interior from that onslaught.¹

As discussed in my last post, the color in narrowband images like this one isn't strictly artistic license, but rather it's a mapping of which atoms are being excited and how. Two emission lines dominate: hydrogen-alpha (H-alpha) at 656.3 nanometers and doubly-ionized oxygen (OIII) around 500.7 nanometers.² Wavelengths outside of these emission lines are generally filtered by my Optolong L-eNhance narrowband filter so the remaining H-alpha and OIII can be mapped to a, SHO Hubble palette. The Optolong L-eNhance does not let Sulfur II through its filter, so I’m missing some of the spectrum of a true SHO palette. The Foraxx utility in PixInsight synthesizes this wavelength to create the familiar Hubble palette.

Where HD 206267A's ultraviolet radiation is actively stripping electrons off hydrogen atoms, you get a bright, rimmed edge. Along that ionization front, two different processes light it up: recaptured electrons cascade back down through hydrogen's energy levels to produce the H-alpha light, while free electrons collide with oxygen ions and kick them into an excited state that decays to produce the OIII light.³ Behind that front, in the trunk's shadowed core, the gas is neutral, cold, and dark, because the dense dust blocks the radiation from getting through.

That's why the trunk has such a striking two-part look: a glowing, sharply outlined "head" facing the ionizing star, and a long, dark, dusty tail trailing away from it. The shape itself is a visible record of which side is under bombardment.

While reading about the Elephant’s Trunk Nebula I learned the same radiation and stellar wind compressing and eroding the trunk from the outside is also causing new stars to form inside it through a mechanism called radiation-driven implosion (RDI). As the ionization front pushes into the cloud, it compresses pockets of gas ahead of it past the threshold needed for gravitational collapse, essentially squeezing new stars into existence as a byproduct of destroying their parent cloud. The trunk isn't a static shape. It's a structure actively being consumed from the outside while manufacturing its own generation of stars on the inside.⁴

Elephant's Trunk Nebula
by Justin Pickens

Total integration: 22h 17m
Integration per filter:
Multiband: 22h 17m (1337 × 60")

Equipment:

For more information, visit AstroBin:
https://app.astrobin.com/i/mz8vq2

¹ Teets, B. (2025, December 17). The Elephant's Trunk. Dyer Observatory, Vanderbilt University. https://dyer.vanderbilt.edu/2025/12/17/the-elephants-trunk/

² Optolong Optics Co., Ltd. (n.d.). L-eNhance filter for color CMOS or CCD color camera or DSLR. Retrieved September 17, 2026, from https://www.optolong.com/cms/document/detail/id/16.html

³ ESA/Hubble, "Painting with Oxygen and Hydrogen"; Byrne, Krumholz & Kim (2025), "The HyLight Model for Hydrogen Emission Lines in Simulated Nebulae," arxiv.org/abs/2509.20361.

⁴ Getman, K. V., Feigelson, E. D., Sicilia-Aguilar, A., Broos, P. S., Kuhn, M. A., & Garmire, G. P. (2012). The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an H II region. Monthly Notices of the Royal Astronomical Society, 426(4), 2917–2943. https://arxiv.org/abs/1208.1471