With all of the rich targets available in the Milky Way this time of year, I thought to myself, why not image one I already did again? This time, I went a little extra on it.
NGC 6888, commonly called the Crescent Nebula, is the result from the stellar wind of a Wolf-Rayet star that lies 6,700 light years away. The nebula is an emission nebula that is energized by the stellar wind of this star, and it displays the common Hα and OIII emission lines at a deep red and a cyan, respectively. The complex, brain-like structure of the nebula is due to the stellar wind from the central star. The star is readily visible in the center of the nebula.
Recipe One
This recipe is basically the microwave hamburger from the gas station. It comes ready to go and just needs a tiny bit of culinary basics, which consists of pushing a few buttons. The ZWO SeeStar is a compact all-in-one electronically-assisted astronomy machine. You just plop it in your yard and control it from a phone or tablet. This image is 60 minutes using the built-in light pollution filter.
Serves 2.39 arcseconds per pixel.
Recipe Two
This recipe consists of the basic beginner amateur home astronomer ingredients. You will need
- 72mm doublet refractor
- Reducer/flattener bringing the focal length to 376mm
- ASI294MC camera, with 4.63μm pixels
- EQM-35 mount
- UV/IR filter
Combine ingredients using a dash of velcro wire ties and a splash of dew straps. Place into yard for 11 nights, or until 970 light frames (48.5 hours) have been completed. Garnish with 100 fresh dark frames, 100 flat darks, and 100 flat frames. Make sure dew does not form on sensor while cooking.
Serves 2.66 arcseconds per pixel.
Recipe Three
This is the expert class Le Cordon Bleu recipe. You will need
- 12in of aperture
- 1200mm of focal length
- ASI2600MC with 3.76μm pixels
- EQ8-R Pro mount
Ingredients must be very finely prepared and collimated. Pay special attention to accurate guiding. Place into permanent amateur dome observatory, with full robotic control. Set It and Forget Itâ„¢. After 11 nights, or 840 light frames have been captured (56 hours), garnish with 100 fresh dark frames, 100 bias frames, and 100 flat frames.
Serves 0.65 arcseconds per pixel.
Taste Test
The SeeStar image is the FITS stack as produced by the SeeStar, and the other two images were processed in PixInsight using manual calibration, registration, local normalization, and integration. All images were finished with neural network-based tools: BlurXterminator, StarXterminator and NoiseXterminator. Generalized Hyperbolic Stretch was used for the starless stretching, and arcsinh stretching was used for the stars. Limited local histogram equalization and color saturation was used.
Below is a montage of all three images, with the SeeStar and widefield images star aligned and scaled to match the native resolution of the deep field image. Click the picture to see the striking differences in resolution. The SeeStar image, left, used the built-in light pollution filter, which is a wide dual band filter. The widefield image, middle, used a cheap UV/IR cut filter. The deep field image, right, used no external filter, but the camera does include a UV/IR cut sensor window.
For the limited time I imaged this with the SeeStar, the results are pretty impressive. You can begin to make out the OIII shell.
The widefield scope result was kind of disappointing. The sensor seems less sensitive than the deepfield camera, but this may come down more to the widefield scope being f/5.2 and the deepfield scope being f/4. The detail is not strikingly different from the SeeStar, integration time considered. The field of view is nice for large targets, though, unlike this target.
The deepfield image turned out pretty impressive considering it is full broadband from Bortle 6 skies. I was unsure if I would get much detail in the OIII shell, but I am pleased with the results.
The field of view of my widefield scope is almost three times that of my deepfield scope, or nine times by area.
And the SeeStar S50 has a field of view about the same as my deepfield scope!
The Secret Ingredient
The key to the success of these recipes is WR 136, also known as HD 192163, the Wolf-Rayet star responsible for this breathtaking nebula. This star, readily visible in the center of the nebula, has an apparent magnitude of 7.5, placing it just at the naked eye limiting magnitude of experienced observers in the darkest skies. It is, however, pretty bright in terms of galactic stars, so into the spectrograph it went! I took about 20 minutes of exposure on this star in my ALPY 600 spectrograph, 500 exposures of around 2.6 seconds. I calibrated the frames with darks and halogen bulb flats, wavelength calibrated against the Fraunhofer lines in the daytime sky, and did a crude response correction using the star Vega and an A0V reference spectrum. Therefore, fluxes are not perfectly absolute in the following spectrum.
The first thing to notice, compared to the average main sequence star, is that this spectrum does not show absorption but emission. The hydrogen Balmer series is present, denoted in cyan labels. There are also numerous helium lines, denoted in pink labels, including the Pickering series of singly ionised helium as well as neutral helium. There is a carbon doublet in the middle of the chart. Finally, there are a few prominent nitrogen lines, denoted in yellow labels. The prominence of nitrogen over carbon gives this star a WN6 spectral type classification. The subclassification of 6 is derived from ratios of helium and nitrogen ion emissions. The temperature of this star is estimated to be 70,000K (125,000° F), so the Plank curve of blackbody radiation would peak deep into the ultraviolet, giving little noticeable continuum in the optical band like we see with main sequence stars like Vega or the Sun.
Wolf-Rayet stars are an evolutionary stage of O-type stars, which are the largest type of main sequence stars. These types of stars are thought to be quite rare, with only 20,000 of them estimated to exist in the Milky Way. Because these stars are so large, gravitational attraction causes their cores to be extremely hot, and this causes them to burn through their hydrogen fuel at an astounding rate. When the hydrogen runs out, the star begins to swell immensely, becoming a red supergiant and shedding the remaining hydrogen outer layer. This expelled mass becomes the visible nebula of the star. At this point, the bare helium star begins fusing helium and heavier elements. The star begins losing mass at an enormous rate due to strong convection mixing and radiation-driven stellar wind that causes particles to be able to exceed the escape velocity of the star. Eventually, the star fuses heavier and heavier elements, which take less and less time as the elements become heavier. When the core becomes iron, the star collapses, and, depending on mass, could become a blackhole or a supernova.
This star is 600,000 times more luminous than our little Sol, five times the radius and 21 times more massive. WR 136 shed off around five solar masses worth of matter 200,000 years ago when it exhausted the hydrogen fuel in its core, and it is still shedding matter at a rate of one solar mass every 50,000 years. This initial outburst is the visible shell that is the nebula, and the stellar wind, traveling at 3.8 million miles per hour, is what gives its complex structure. The intense ultraviolet radiation from the star is what illuminates the nebula. WR136 is estimated to be 4.7 million years old and is at the end of its life. It’s estimated that this star will go supernova in a few hundred thousand years. Live fast, die young.
