Caldwell Catalog C4 — Hubble · Caldwell 4 · NASA & ESA

Iris Nebula

Nebula in Cepheus · 1,300 light-years

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Type	Nebula
Constellation	Cepheus
Right ascension	21h 01m 36s
Declination	+68° 10′
Object distance	1,300 light-years
Apparent size	10′ (arc-minutes)
Approx. true diameter	3.8 light-years across
Also known as	C4 · NGC 7023

Caldwell 4, popularly known as the Iris Nebula, is a nebula (NGC 7023) in Cepheus — a luminous cloud of interstellar gas and dust. It appears as number 4 in the Caldwell catalogue, Sir Patrick Moore's 1995 list of 109 bright deep-sky objects for amateur astronomers. It spans roughly 10 arc-minutes on the sky. It is one of the showpiece objects Sir Patrick Moore chose for his Caldwell list of bright targets the Messier catalogue left out.

Although it appears about 10′ wide on the sky, its real span is roughly 3.8 light-years — the difference is a measure of how far away it lies.

Observing Iris Nebula

Iris Nebula lies in the constellation Cepheus and is best placed in the evening sky around August. Its maximum altitude above the horizon depends on your latitude. It sits far enough north to be circumpolar from mid-northern latitudes, and never rises for most of the southern hemisphere. Note that the listed magnitude is spread across the whole object — diffuse galaxies and nebulae are often harder to see than the number alone suggests, so dark, moonless skies help a great deal.

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Sources

Position, magnitude and angular size from the OpenNGC catalogue (Mattia Verga).
Object identity cross-referenced with SIMBAD (CDS, Strasbourg).
Image credits and the original observation source are linked with each photograph.

Related objects

Orion Nebula — nebula
Ring Nebula — nebula
Eagle Nebula — nebula
Carina Nebula — nebula
Lagoon Nebula — nebula