NGC6888, also called the Crescent Nebula, is an emission nebula around a Wolf-Rayet Star, WR136.  It’s located in the Milky Way, approximately 5000 light years away, and it has an apparent size of 18 x 12 arcminutes, making it 26 x 17.5 light years across.  It’s estimated to be 30,000 years old.

Although it may seem similar since it is a nebula around a star, the Crescent Nebula is not a planetary nebula, and its ultimate ending will be very different from a planetary nebula.  Planetary nebulae occur when an intermediate mass star, 1 to 8 solar masses, expands into a red giant, sheds its outer layer, and shrinks to a white dwarf.  The high temperature and wind from the white dwarf ionizes the shed outer layer, making the beautiful nebulae.  The Crescent Nebula was made by a massive star, estimated to initially be <= 50 solar masses.  When it was a main sequence star, fusing hydrogen early in its life, its solar wind blew a bubble in the gasses surrounding it.  When it became a Red Super Giant, its slow solar wind filled the bubble with its outer layer, estimated to be 25 solar masses worth of material. And when the star collapsed into a super hot Wolf-Rayet star, now about 21 solar masses in size, its fast solar wind compressed the red super giant and Wolf-Rayet material into ionized filaments and clumps.  Eventually, the Wolf-Rayet star will fuse its matter creating heavier and heavier elements until it reaches iron, when it will implode and create a supernova.  

Visually, the nebula appears to have an outer Oiii “skin” and a “clumpy” Ha interior.  The Oiii skin is the boundary between the main sequence bubble and the Wolf Rayet shell, and the clumpy Ha interior is the red super giant material compressed by the Wolf Rayet shell.  A “blowout” in the Oiii skin can be seen in the lower right in the blue Oiii in this image.  The Interstellar Medium (ISM) – the cold low density gas between stars – may have been less dense in this direction, allowing the Oiii skin to blow out in this direction and not in other directions where the ISM is denser.   

I used data from my driveway in Friendswood, Texas with suburban Bortle 7 – 8 brightness skies (lots of light pollution) to make this image.  In order to capture the detail in this nebula and the outer Oiii shell, I needed a lot of data: 12.4 hours of Ha data and 15 hours of Oiii data, plus an hour of RGB data for the stars, taken over ten nights.

This is a narrowband image, mapping Oiii to blue and Ha to red.  My goal was to capture both the details in the Ha and the outer Oiii shell. 

Some people think this looks like a cosmic “brain.”  What do you think?  Isn’t our galaxy beautiful?

Camera geek info – Narrowband:

• Williams Optics Zenith Star 73 III APO telescope
• Williams Optics Flat 73A
• ZWO 2” Electronic Filter Wheel
• Antila HO and RGB filters
• ZWO ASI183MM-Pro-Mono camera
• ZWO ASIair Plus
• iOptron CEM40
• Friendswood, Texas Bortle 7-8 suburban skies

Frames:

• July 4, 2024
  • 104 60 second Gain 150 Ha lights
  • 30 1.0 second Gain 150 Ha flats
• August 2, 2024
  • 95 60 second Gain 150 Oiii lights
  • 30 0.5 second Gain 150 Oiii flats
• August 8, 2024
  • 215 60 second Gain 150 Oiii lights
  • 30 0.5 second Gain 150 Oiii flats
• September 7, 2024
  • 247 60 second Gain 150 Oiii lights
  • 30 0.5 second Gain 150 Oiii flats
• September 12, 2024
  • 237 60 second Gain 150 Ha lights
  • 30 1.0 second Gain 150 Ha flats
• September 13, 2024
  • 198 60 second Gain 150 Oiii lights
  • 30 0.5 second Gain 150 Oiii flats
• September 17, 2024
  • 133 60 second Gain 150 Ha lights
  • 30 1.0 second Gain 150 Ha flats
• September 19, 2024
  • 271 60 second Gain 150 Ha lights
  • 30 1.0 second Gain 150 Ha flats
• September 20, 2024
  • 143 60 second Gain 150 Oiii lights
  • 30 0.5 second Gain 150 Oiii flats
• September 21, 2024
  • 17 60 second Gain 150 red lights
  • 20 60 second Gain 150 green lights
  • 20 60 second Gain 150 blue lights
  • 30 0.05 second Gain 150 red flats
  • 30 0.02 second Gain 150 green flats
  • 30 0.02 second Gain 150 blue flats
• 30 Flat Darks from library
• 30 Darks from library

Processing geek info:

• PixInsight
• BlurXterminator
• NoiseXterminator
• StarXTerminator
• Generalized Hyperbolic Stretch
• NBColourMapper