The Crescent Nebula (NGC 6888) and its Wolf-Rayet Star (WR
136)
Characteristics:
Position
Angle: 333 degrees
RA (WR 136): 12h 20m 07s (J2000)
Dec (WR 136): +38degrees 21' 17" (J2000)
Description:
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 136. As
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.
Please
note: Graphics on this website
may not be reproduced without author permission.
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