HEIC: A Giant, Smouldering Star (Campbell's Star, HD 184738)

[bystander]

A Giant, Smouldering Star
ESA/HEIC Hubble Picture of the Week | 2013 Sep 16

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This new image, snapped by NASA/ESA Hubble Space Telescope, shows the star HD 184738, also known as Campbell’s hydrogen star. It is a Wolf-Rayet star — an evolutionary stage for stars with a mass of over 20 times that of our Sun, when they are rapidly blowing away material and losing mass. This type of star is named after two French astronomers, Charles Wolf and Georges Rayet, who first identified them in the mid-nineteenth century.

Stars like HD 184738 are short-lived, very massive, and extremely hot, with surface temperatures of up to 40 times higher than that of our Sun. They are also very luminous, though as they predominantly emit in the ultraviolet and X-ray parts of the spectrum, they may not appear to be exceptionally bright. The star’s distinctive fiery red colour is caused by its nitrogen content.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Jean-Christophe Lambry.

Credit: ESA/Hubble & NASA
Acknowledgement: Jean-Christophe Lambry

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[geckzilla]

Nice one. Coincidentally, I was looking at this in the HLA (it looks way different in the preview) picking out the data to process it myself when I realized it was the same object that I was just reading about not even an hour prior.

[Ann]

The star’s distinctive fiery red colour is caused by its nitrogen content.

Obviously I can only comment on the star's B-V and V-I indexes. I don't know anything about its B-R index, or its V-R index. It could be that the star is extremely bright in R, and it is very likely, of course, that the star or its surroundings produce lots and lots of Ha emission.

I do note, however, that the star's B-V index is -0.013, which suggests, although it doesn't prove, that the overall color of the star is blue. I also note that the V-I index of the star is 0.01. This, too, suggests that the star is blue rather than red in color.

I note, too, that the star was imaged through one infrared, two red, one green and one blue-green filter, which may possibly not be the best way to find out whether or not this star is fiery red in color.

Ann

[geckzilla]

There are narrowband filters all the way from f375n to f953n in the archive and I can confirm it's luminous in each and every one of those filters. The star itself would appear quite white if given equal treatment from each filter, which would say nothing about the nebula surrounding it. The only way to make it appear blue would be to pump up the blue filters like crazy which there is absolutely no reason to do for this star because none of the structures around it are particularly interesting in that wavelength. The darker foreground dust is a little easier to notice for the bluer filters but there isn't enough light behind the outer area of the nebula to fully outline it. I think it's weird that you call the OIII filter a "blue-green" filter. Its purpose isn't to be blue-green but to show where doubly ionized oxygen emissions are.

[geckzilla]

Here you go, Ann, just for you. Since I just finished processing this myself it was no big deal to go in and tone down the green, red, and infrared stuff and seriously crank up the blue portions. Ultimately, the image is less informative like this.

[Ann]

Thank you, geckzilla!

The blue color of the star is probably not so easily proved by the filters used for this image, since the bluest filter is a blue-green OIII filter. The surrounding nebulosity and all sorts of emission lines will make the star bright at all wavelengths, although I would guess that a careful lightcurve for this star would show that its energy output rises very noticeably towards short wavelengths, particularly wavelengths shorter than 500 nm.

The point I was trying to make is that the star is blue, although it is obviously dust-reddened. Obviously the surrounding nebula is mostly non-blue.

Ann