NGC 7635. The Bubble Nebula

Click on the image for a larger version

© Josep M. Drudis-2016, Mount Lemmon Sky Center, AZ, USA.

NGC 7635 is an emission nebula located in the constellation Cassiopeia. This nebula emits, mainly, in H alpha and Oxygen III. The central bubble is one out of the three shells of emitting gas that surround the hot, O6.5 class, star SAO 20575 (also known as BD+60°2522). Amazingly, this star, which originated the bubble and fuels its emission with its enormous radiation output, is not located at the center of the bubble, due to its movement. It is the star located half way between the center and its lower left border. This star is about 4 million years old (just one thousandth of our Sun’s age) and is expected to explode as supernova in about 15 million years (give or take a couple million…). By coincidence, this nebula lies superimposed to a region where another supernova exploded many thousands of years ago. Its gas wisps were faint enough to remain undiscovered until 2005. These remains are out of this image (just a few arc minutes left of the left border).

This image was taken in connection with a Beta Testing trial at the Schulman Telescope (32 inch, atop Mt. Lemmon, AZ) in HaRGB. Due to inclement weather, the Blue images could not be taken during the trial and instead, the Green images were blended with narrowband OIII images (taken with a 17 inch telescope located in New Mexico) and mapped to Blue. This move can be justified because the blue tint of the nebula is much more due to OIII emission than to broadband Blue. Halpha blended with OIII acted as Luminance,  Ha blended with Red was mapped to Red, Green was mapped as is and Green blended with OIII was mapped to Blue.

Additional Information


Name(s): NGC 7635. Caldwell 11. The Bubble Nebula

Type: Emission Nebula

RA:  23h 20m 48s

Dec: +61º 10’ 58”

Constellation: Cassiopeia

Size (arcmin): 15×8

Magnitude: 10

Distance: 7,000 light years


Date: 2016-10-26 and 27

Location: Mount Lemmon, AZ, USA

Size (arcmin): 18×15

Telescope: Schulman 32” f/6.8   [OIII frames were taken with a CDK17″ f/4.5 located in Mayhill, New Mexico]

Camera: SBIG STX16803 (4096x4096pix)

Guiding: Astrodon MonsterMOAG off-axis guider

Total exposure: 8.75 hours (Ha: 3.5 hs; R: 1h; G: 1.75 h; OIII: 2.5 hs)

Processing: CCDStack, Photoshop CC 2017

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