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NGc 7023 is also known as The Iris Nebula. It is a reflection nebula. The characteristic blue color of this type of nebulae appears when light from a star (in this case it is V380 Cep or HD 200775, a variable star, 7.36-13.40) is scattered off a neighboring dust cloud. The spectrum of the polarized reflected light is similar to that of the original star, only bluer. Many nebulae show this color (M45 and part of M20 are good examples of this). Logically, reflection nebulae are less dense than dark nebulae, allowing the light not only to be reflected, but also transmitted through the nebula. “Thicker” nebulae appear dark or “black”. This central star is located in an area with considerable amounts of gas and dust, that can be spotted as brown reflections in the area surrounding NGC 7023.
Additional Information
Object
Name(s): NGC 7023, Caldwell 4, Iris nebula
Type: Reflection Nebula
RA: 21h 01m 35.5s
Dec: +68º 10’ 00.1”
Constellation: Cepheus
Size (arcmin): 18×25
Magnitude: +7.1
Distance: 1,300 ly
Image
Date: 2014-08-20
Location: Mayhill, Mew Mexico, USA
Size (arcmin): 54×36
Telescope: Planewave CDK 20” f/4.5
Camera: FLI PL11002M (4008x2672pix)
Guiding: not recorded
Total exposure: 125 min (L: 30 min; RGB: 95 min)
Processing: CCDStack, Photoshop CC 2016 and PixInsight