NGC 7635 is a large emission nebula, visually located in the constellation Cassiopeia. Its defining feature is the large bubble at the center of this image, which is
a Wolf-Rayet bubble. The nebula is predominantly red in the true-color version, because (i) ionized hydrogen emits in the
red part of the spectrum; (ii) the dominant emissions captured in this photo are from hydrogen being ionized (stripped of its electron) by the
Wolf-Rayet star inside the nebula. The bubble in the center of the image is created by the high-velocity stellar winds from
the huge, hot central star pushing the gas away from the star.
The field is thought to be about 7,100 light years from Earth; at that distance, the bubble would be about 7 light years across. Visually, this field is about the width of a full moon
(although very dim by comparison).
As befits such an iconic object, I had imaged this region 22 years earlier, in the very early days of my imaging work. I was (and still am) quite pleased with the old image, which really was the first image
I took as I became more comfortable with the entire process. To see what a bit of acquired skill, much better location and much better equipment/software
can do, compare this current image to my original one here.
Also befitting such an iconic object, this has been imaged by Hubble (in the Hubble palette, like the third image in my stack above); to see what that marvelous instrument can do with it, click
here.
I have presented this object in four different formats; I like each one in its own way. This is the order in which they appear as you cycle through (by repeatedly clicking on the photo, waiting for
each to download; each is labeled in the lower left corner), starting with a reddish version:
(i) A true-color version (the top photo in the stack), with the color created by imaging through red, green and blue filters (with a significant amount of Ha and OIII data blended into various channels, in varying percentages; Ha emissions are in the red spectrum, and OIII emissions are blue-green, so I have blended Ha into the luminance layer and the red channel, and OIII into the green and blue channels).
(ii) A true-color version (the top photo in the stack), with the color created by imaging through red, green and blue filters only (no Ha or OIII data included). It is interesting to see how much the addition of the Ha and OIII increases the detail, but it's also interesting to note the more vibrant star colors that often appear in the RGB version.
(iii) A version in the Hubble palette (a lot of the Hubble photos, including and especially the famous "Pillars of Creation," are made with this set of filters, since it's a useful set for scientists to see what's actually happening), which shows SII emissions as red, Ha emissions as green, and OIII emissions as blue. Because Ha emissions so dominate this nebula, I have significantly de-emphasized them in this rendering. With that, the OIII emissions show as pure blue (would be a blue-green otherwise), and the SII emissions show as a ruddy brown/yellow (would really show up much otherwise). I like the clear blue when a nebula has significant oxygen emissions, as is the case here. This form of combining results in magenta-colored stars, which I have significantly desaturated.
(iv) A pure Ha version, showing only emissions from the ionized hydrogen atoms in the nebula.
Copyright 2023 Mark de Regt