H-alpha view of the Iris Nebula within molecular clouds

The Iris Nebula is a well known reflection nebula illuminated by the Herbig Be star HD 200775, a young star of spectral type B2Ve, which is still embedded in a gas and dusk envelope. The star, and thus the nebula, is located at a distance of about 1200 light-years. However, it is not known very well that this nebula is also an Hα emitter.

The Iris Nebula lies in the constellation Cepheus. The part of the nebula that is directly illuminated/ionized by HD 200775 is also known as Caldwell 4, LBN 487 or VdB 139. This nebula seems to lie within or in front of a larger molecular cloud.

Iris Nebula in H-alpha and continuum — Click on the image to load it at full resolution in a JavaScript viewer. Use the button to toggle Hα.
In both images, NIR (near infrared) is mapped to red, red is mapped to green and blue is mapped to blue. In the version with Hα, that emission line is added to the red channel. Stars are partially subtracted to improve the visibility of the nebulae.
By toggling between the images (click on the button), it can be seen that the HII regions (reddish) strongly correlate with the blue part of the reflection nebula, which is directly illuminated by the central star (which is no surprise). Furthermore, several tiny Hα-emitting star formation regions become visible within and around the dark nebulae west (right) of the Iris Nebula.
The red bow near the upper left corner of the image is caused by a reflection of the dying red giant T Cep, which is about 100 times brighter in NIR than in visible light (-0.496 mag in J band vs. 4.644 mag in G band).
In the Javascript viewer, the nebulae can be labelled using the '3' key or via the menu.

Iris Nebula in continuum — Click on the image to load it at full resolution in a JavaScript viewer. Use the button to toggle Hα.
In both images, NIR (near infrared) is mapped to red, red is mapped to green and blue is mapped to blue. In the version with Hα, that emission line is added to the red channel. Stars are partially subtracted to improve the visibility of the nebulae.
By toggling between the images (click on the button), it can be seen that the HII regions (reddish) strongly correlate with the blue part of the reflection nebula, which is directly illuminated by the central star (which is no surprise). Furthermore, several tiny Hα-emitting star formation regions become visible within and around the dark nebulae west (right) of the Iris Nebula.
The red bow near the upper left corner of the image is caused by a reflection of the dying red giant T Cep, which is about 100 times brighter in NIR than in visible light (-0.496 mag in J band vs. 4.644 mag in G band).
In the Javascript viewer, the nebulae can be labelled using the '3' key or via the menu.

Iris Nebula in H-alpha (without continuum). The central star HD 200775 is visible in this image because it is an emission line star.

Image data

FOV (full view in the JavaScript viewer):

1.35° × 0.89°

Position (J2000):

RA: 21h02m; DEC: 68°4′

Date:

2024

Location:

Pulsnitz, Germany

Instrument:

400mm Newton at f=1520mm

Camera Sensor:4

Sony IMX455

Orientation:

North is up (exactly in the image center)

Scale:

1 arcsec/pixel (at full resolution)

Total exposure times:

H-alpha (3.5nm):	16.9 h
Near infrared (SDSS I'):	7.2 h
Red (SDSS R' + 400-650 nm band-pass):	4.0 h
Blue (SDSS B'):	5.1 h

Image processing

All image processing steps are deterministic and none of the algorithms use machine learning (often referred to as “AI”), which tends to generate plausible looking fake details. The software used can be downloaded here.

The image processing steps were:

Bias correction, dark current subtraction, flatfield correction, noise estimation
Alignment and brightness calibration using stars from reference image
Stacking with outlier rejection, background estimation and optimal weighting based on noise estimation
Star subtraction where star positions and intensities are extracted from continuum images
Denoising and deconvolution of both components (stars and residual)
Dynamic range compression using non-linear high-pass filter
Color composition and tonal curve correction

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