Iris Nebula

Lore & Background

The Iris Nebula was discovered in 1794 by Sir William Herschel and later added to the New General Catalogue (NGC). It is located near the Mira-type variable star T Cephei and the bright magnitude +3.23 variable star Beta Cephei (Alfirk). The designation NGC 7023 refers to the open cluster within the larger reflection nebula designated LBN 487. The nebula has a complex structure of clouds and filamentary structures, with highly inhomogeneous distribution containing several dense irradiated structures. Several photodissociation regions (PDRs) and dissociation fronts have been observed bordering the cavity as bridge illuminated ridges towards the northwest, southwest and eastern directions, driven by soft radiation fields with effective temperatures of 18,000 Kelvin. Outflows from the binary stars near the center have carved out a biconical cavity and triggered the formation of a few photodissociation regions (PDR 1 and PDR 2). Every filament of H2 has a corresponding extended red emission (ERE) filament, which is generally 10% wider. The ERE carriers are probably PAHs, produced through a two-step process involving photons of 10.5 eV and abundant optical and near-UV photons.

Reader's Guide

The Iris Nebula holds significance as the site of the first firm detection of neutral fullerenes (C60) in interstellar space, discovered using the Infrared Spectrograph on the Spitzer Space Telescope (Sellgren et al. 2007). This detection, along with prominent emission lines at 7.04 ±0.05 μm, made it one of the largest molecules detected in space. The fullerenes are located more towards the central star and in the central cavity, unlike the polycyclic aromatic hydrocarbons (PAHs) in the nebula. The discovery challenged the belief that fullerenes formed only in hot dense envelopes of evolved stars, revealing they can form efficiently in tenuous and cold environments illuminated by strong ultraviolet fields. Laboratory and theoretical studies suggest a pathway involving PAHs being converted into graphene under UV radiation, with photochemical modeling of circumovalene (C60H20) by Berné et al. (2015) predicting formation through full dehydrogenation, folding into a floppy closed cage, and shrinking via loss of C2 units until reaching the symmetric C60 molecule. The nebula also contains several young stellar objects (YSOs), with four candidate YSOs in the northwestern region, making it the most concentrated amount of YSOs in the Cepheus Flare. Observations support gravitational instabilities from pre-existing condensations compressed by hot gas as the likely formation scenario for these YSOs.

Did You Know?

• The Iris Nebula is illuminated solely by a binary system of intermediate-mass Herbig Ae/Be stars, including a +7.4 magnitude B2Ve-type star and a B5-type star.
• The nebula contains polycyclic aromatic hydrocarbons (PAHs), large organic molecules with carbons arranged in honeycomb-like shapes.
• The magnetic field strength of the starless core in the nebula is 179 ±50 μG.