The Crescent Nebula (NGC 6888) and its Wolf-Rayet Star (WR 136)

NGC 6888
All Images Copyright Steve Cannistra

Please click here for a medium-size (35%).

Position Angle: 333 degrees
RA (WR 136): 12h 20m 07s (J2000)
Dec (WR 136): +38degrees 21' 17" (J2000)

WR 136 is a Wolf-Rayet (WR) star that lies deep in the heart of the Crescent Nebula (NGC 6888), shown here as a narrowband image comprised of data from Ha and OIII (please read the section "Photographic Details" below for information regarding how this image and spectrum were acquired).  WR stars begin life as massive O-type stars, which burn hydrogen to helium in their core so rapidly that they last only about 1 million years (in contrast, our Sun is a less massive G-type star that burns hydrogen more slowly and has an expected lifetime of 10 billion years).  Once the hydrogen in their core is depleted, O-type stars leave the main sequence of the Hertzsprung-Russell diagram and enter into the Red Giant phase, during which they expand in size and eventually burn helium in their core.  WR stars are known for their high temperatures, in the range of 20,000-210,000 degrees K, as well as intense stellar winds that cause loss of their outer hydrogen shell, leaving behind the helium core.  Helium in the core fuses into heavier elements such as nitrogen, carbon, and oxygen, which are excited by intense radiation within the core to produce the characteristic emission lines of WR 136As shown in the spectrum above, these major emission lines include He I, He II, NIII, N IV (and to a lesser extent C IV).  Also note the absence of major hydrogen absorption lines (which would usually appear as dark lines in the spectrum characteristic of the hydrogen Balmer series).  This is due to the fact that an outer hydrogen layer no longer exists in most WR stars, and therefore cannot absorb hydrogen-specific wavelengths emitted by deeper regions of the star (such as H beta and H alpha).  The intense stellar winds emitted by WR stars rapidly expand outward at a speed of 3,000 km/s, losing the equivalent of our sun's mass every 50,000 years.  This stellar wind eventually collides with slower moving gas that was shed thousands of years earlier by the dying star, resulting in a shock wave that releases radiation in the form of Ha (656.3 nm) and OIII (500 nm) emissions (among others).  This is what creates the image of the Crescent Nebula shown above.  The hydrogen and oxygen signals were captured in this image by using narrowband filters that isolate their respective wavelengths (hydrogen signal is mapped to the red channel, and oxygen signal is mapped to blue and green channels).  It is thought that once WR stars exhaust their helium fuel within the core, they will eventually end their life in a Type Ib or Ic supernova explosion.

Photographic Details:
This is a narrowband image comprised of Ha signal (Red channel) and OIII signal (Green and Blue channels), representing a total of 6 hours of exposure.  The spectrum was acquired using a Star Analyzer 100 attached to a Samyang/Rokinon 135mm lens and a Canon T7i full spectrum modified DSLR camera (the actual first order spectrum is shown in the inset, just below the label "Angstroms").  Information regarding the Star Anlyzer 100 and the RSpec software used to analyze this spectrum can be found in many online sources- it is a 1.25" transmission diffraction grating, which permits slitless spectroscopy that can be suprisingly effective at producing low resolution but nonetheless informative spectral data.

- Dates of image acquistion:  Ha and OIII data- July 11 and 12, 2022.  Spectroscopic data- August 3, 2022.

- Main imaging scope:  Celestron 9.25" standard SCT (native f10) with 0.63 focal reducer (spaced to achieve f6.1  = 1444mm effective focal length), on the Takahashi NJP Mount.

- Spectrum optics: Samyang/Rokinon 135mm lens with attached Star Analyzer 100 transmission diffraction grating.

- Autoguider:  None.

- Camera 1
(for NGC 6888): ZWO ASI1600 (Gain 200, offset 40, Read noise 1.55 e rms), -20C, 3.8 micron pixel size, image scale 0.54"/pixel given above set up; Baader Ha and OIII narrowband filters; ZWO 7-position filter wheel for 2" filters.

- Camera 2 (for WR136 spectrum): Canon T7i (800D), self-modified for full spectrum sensitivity by removing 1) the IR cut filter (low pass filter #2, LPF2) as well as 2) the UV/IR filter (LPF1).

- Exposure (NGC 6888):  Total exposure 6 hours
(3 hours Ha, 3 hours OIII, 60sec subs, unguided dither in CCD Commander software every 2 subs, ASI1600 Gain 200, offset 40, Read noise 1.55 e rms).

- Exposure (WR136 spectrum):  Total exposure 20 minutes (60 sec subexposures x 20), ISO 800.  The subs were combined in MaximDL using 1 star alignment, which works well since the spectra of all stars in an image like this will be oriented in exactly the same way.

- Post-processing:  NGC 6888- Calibrated, aligned, and stacked in Maxim, deconvolution in CCDStack, followed by DDP in ImagesPlus (IP).  The final HOO image was processed in Photoshop CS.

- Post-processing:  WR 136 spectrum- Raw subs from the Canon T7i (taken at position angle 250 degrees to minimize overlap between spectrum and star field) were converted to TIFF files in Canon Digital Photo Professional (DPP), then combined in MaximDL (1 star alignment on WR136), with subsequent processing in RSpec software.

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