The so-called Wolf-Rayet stars are a rare set of stars characterised by highly unusual spectra, marked by prominent broad emission lines of ionised helium, nitrogen or carbon. Their surface temperatures range from 20.000 K to over 200.000 K, far exceeding those of most other types of stars.
In 1867, from the Paris Observatory using the 40 cm Foucault telescope, French astronomers Charles Joseph Étienne Wolf and Georges Antoine Pons Rayet visually observed (prior to the widespread use of photographic plates) the spectra of three eighth-magnitude stars in the constellation Cygnus. To their surprise, the spectra displayed very broad emission lines rather than the far more common absorption lines or bands. These stars would later be designated Wolf–Rayet (WR) stars.
The stars they identified were HD 191765, HD 192103 and HD 192641, now known respectively as WR 134, WR 135 and WR 137. In the first image on the right. WR 134 and WR 135 can be seen, lying less than one degree apart, The third, WR 137, lies outside the current field of view but can be found in my earlier RGB image of the region.
WR 134 is located approximately 6.000 light-years from Earth and is surrounded by a faint nebulous bubble, shaped by the star’s intense radiation and powerful stellar wind (*). Though only about five times the radius of the Sun, its temperature exceeds 63.000 K, and its luminosity is roughly 400.000 times greater than that of our Sun.WR 134 is also a variable star, classified as an Algol-type eclipsing binary, with the designation V1769 Cygni.
(*) Today seems that it is still unclear which of the two stars, WR 134 or WR 135, is primarily responsible for creating the shell (For example, have a look at this).
My image is presented in the Ha-OIII-OIII palette (with RGB stars "taken" from this image) and accumulates 33 hours of exposure time. As usual, I first calibrated the image with SpectrophotometricColorCalibration, but then modified the balance of the G and B channels to get a more pleasing (subjective) view. Since the OIII component of the nebula is so weak, I gave a little more time to the exposure with this filter (18 h OIII vs. 15 h Ha), but it is now clear to me that it was not enough.New Narrowband Version, updated June 2025
As mentioned previously, I first imaged this area in 2023. However, I was not fully satisfied with the result after processing, and this season I decided to revisit the field in an effort to improve the image.
To achieve this goal, I began by collecting additional data this time using an SII filter, adding 16 hours of exposure and bringing the total integration time to 44 hours.
In addition, I opted for a new approach to colour mapping. Starting from a base SOO palette, I applied a method introduced by Adam Block, incorporating hydrogen-alpha data colourised in yellow. This results in what could be described as an S Ha – O Ha – O palette (with RGB stars).
For the processing, I followed a more careful workflow, with particular attention to enhancing the detail in the faint nebulous bubble surrounding WR 134, while also aiming to reduce the visual impact of the stars. The final result can be seen in the two images on the right: one showing a cropped detail of the region, and the other displaying the full wide-field version.
Click on the images for a full resolution version, or go to the Gallery section for complete exposure details.Image processing: Pixinsight.
Observatory automation and remote operation with Talon6.
In 1867, from the Paris Observatory using the 40 cm Foucault telescope, French astronomers Charles Joseph Étienne Wolf and Georges Antoine Pons Rayet visually observed (prior to the widespread use of photographic plates) the spectra of three eighth-magnitude stars in the constellation Cygnus. To their surprise, the spectra displayed very broad emission lines rather than the far more common absorption lines or bands. These stars would later be designated Wolf–Rayet (WR) stars.
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