Crescent Nebula (NGC 6888) in the Constellation Cygnus

« PreviousBack to Astrophotography Gallery» Next

The Crescent Nebula (NGC 6888) is an emission nebula lying along the "spine" of the constellation Cygnus the Swan among clouds of hydrogen gas. The nebula itself is an interaction of stellar wind from the hot, massive Wolf-Rayet star (WR 136 / HD192163 / HIP 99546) with gases from its own outer envelope, which was shed during an earlier phase of the star's evolution. Wolf-Rayet (WR) stars represent a late evolutionary phase of very massive stars that occurs after they expand into Red Supergiants (RSG), but before they explode in a type Ia or Ic supernova. The stars are extremely hot (surface temperature > 20,000 K) and display a unique spectral signature, with very strong, broad emission lines of different combinations of helium, nitrogen, carbon and oxygen; the specific combination of lines defines subcategories of WN, WC and WO stars. "Wolf-Rayet stars are thought to be the remains of stars initially more massive than ~30 solar masses that have ejected all or most of their outer envelopes through winds or interactions with companion stars, exposing the hot helium core" (Reference 1). WR 136 is expected to explode as a supernova within a few hundred thousand years.

According to Reference 2, the following summary outlines the evolutionary lifetime of a star of approximately 35 solar masses: 1) During its Main Sequence (MS) lifetime, stellar winds sweep out a thin shell or bubble in the interstellar medium. 2) After hydrogen in the core is depleted, the star expands into a Red Supergiant, while its core contracts and begins fusing helium. The star begins shedding its outer envelope via high stellar winds, filling the previously created bubble with its own outer atmosphere. 3) The star enters the Wolf-Rayet phase, with strong, fast Wolf-Rayet (W-R) winds that sweep up the star's previously ejected material, creating shock fronts within the existing bubble. The hot W-R star, which is a strong ultraviolet (UV) emitter due to its high temperature, excites the ejected material creating an emission nebula, often in a ring-formation. This glowing shell of ejected gases forms the Crescent Nebula, NGC 6888, shown here. Following the Wolf-Rayet phase, the star explodes as a supernova.

 

Crescent Nebula (NGC 6888)

Optics:8-inch RC telescope
Camera:QHY268M cooled CMOS camera
Exposure info:7 hrs Ha / 13 hrs O3 (10 min subs) / 1 hr each RGB for star color
Filters used:Astronomik H-alpha, OIII, RGB
date:July/August 2021
processing:Pixinsight-->Photoshop-->Lightroom

 

Below are the two narrowband channels, H-alpha and OIII, with most of the stars removed (via Pixinsight starnet); the images are inverted to better show the faint features of the nebula. What is fascinating to me is how different the topography of the W-R bubble appears in the two different wavelengths. According to Reference 2, "the elliptical nebular shell is enclosed by a complete, approximately spherical [OIII] bubble." That OIII bubble is quite clear in the lower left image.

Reference 2 writes in its conclusion: "NGC 6888 is a bubble formed by the mass loss of the precursor to the central star WR 136. The internal pressure of the shocked W-R stellar wind has swept up the ejecta from the previous RSG (Red Supergiant) phase. This material collides with a thin RSG shell formed at the MS bubble interface, fragmenting into the clumps seen today....The internal pressure of the stellar wind drives a shock into the material external to the visible shell. The shock is visible in [OIII] and H-alpha as a skin enveloping the clump network."

OIII filter, inverted, stars removedHalpha filter, inverted, stars removed

Plotting WR 136 / HD 192163 / HIP 99546 on a H-R diagram using data available in the literature shows the star clearly falls in the region of Supergiants.

 

An earlier version of the Crescent Nebula region (NGC 6888) presented as a 2-panel mosaic showing surrounding clouds of nebulosity.

DISCLAIMER: Please note that I am not an astrophysicst, but simply an amateur astronomer and astrophotographer. I am trying to learn some astrophysics and add some science to my hobby of astrophotography. My explanations above are intended as a brief introduction to what I have been learning, and to attempt to give some context to my efforts.

REFERENCES

1. Guidry, Mike. "Stars and Stellar Processes." Cambridge University Press, 2019.

2. Moore, Hester and Scowen, "Hubble Space Telescope Observations of the Wolf-Rayet Nebula NGC 6888." The Astronomical Journal, 119:2991-3002, 2000 June.