NGC7023 (Iris Nebula) taken last August 26, 2018 from Al Sadeem Astronomy Observatory using GSO RC 8-inch telescope and ZWO1600MC-Cool CMOS camera; processed in Pixinsight (Image Credit: Aldrin B. Gabuya/Al Sadeem Astronomy)

Caldwell 4 (more commonly known as Iris Nebula) is a bright reflection nebula situated about 1300 light-years away from Earth in the constellation of Cepheus. It was discovered by the English astronomer William Herschel on October 18, 1794. The nebula consist of a bright young star V380/SAO 19158, (app. Mag. +7.1) at the center illuminating a huge remnant cloud of stellar dust through reflection, where its light bounces off the surrounding nebula. This young star was thought to have formed about 5500 years ago. It spans about 30 light-years across. This nebula is part of a star cluster designated as NGC7023.

Researchers from Leiden University in the Netherlands found traces of polycyclic aromatic hydrocarbons (PAHs) existing and developing within IRIS Nebula using infrared images taken from SOFIA, Spitzer, Hubble and Canada-France-Hawaii telescopes. PAHs are flat molecules consisting of carbon and hydrogen atoms arranged in a honeycomb pattern which make up about 10% of the entire carbon content in the universe. Their analyses show that the intense ultraviolet radiation from the central star hit these PAHs, energizing the carbon and hydrogen atoms and fuse them together, creating larger, more complex compounds. This mechanism could give us better view on how life emerges in the universe, according to scientists.

The nebula is located about 6 degrees northwest of Alderamin or near variable stars T Cephei and Alphirk (see sky diagram below). It is best seen in northern temperate locations (24°-66°N latitude) during October, November and December when it is seen high in the sky. At magnitude +6.8, even at extremely dark, moonless, and clear skies, it appears as fairly large diffuse cloud making it difficult to spot with just binoculars or small telescopes. Its nebulosity will be mostly revealed through long exposure photography using medium (8-inch) or larger aperture telescopes.

Composite image of Iris Nebula from SOFIA (red & green) and Spitzer (blue) showing different populations of PAH molecules (Image Credit: SciTechDaily/NASA).
Artist’s Illustration of PAHs (honeycomb carbon-hydrogen structures) presence within Iris Nebula (Image Credit:


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NGC 7023 – Iris Nebula – Reflection Nebula (n.d.). Retrieved from