by Ann » Wed Mar 13, 2024 7:01 am
That's an interesting and very red portrait of the Seagull Nebula.
I can't complain. The Seagull Nebula
is very red, since it is heavily dominated by 656 nm Hydrogen Alpha, which is of course this color,
███.
Only one other color is immediately obvious in the APOD apart form the red Hydrogen Alpha, and that is pale blue. At first I thought that the APOD might be a two-filter image, constructed from from a red Hα filter and a cyan-green OIII filter, mapped as blue, but that is not fully consistent with the appearance of the image. I found just such an Hα/OIII picture of the Seagull Nebula on the net, and as you can see, it doesn't look like the APOD:
In narrowband images of nebulas, such as the Seagull Nebula picture by Simon D., you "play down" the Hydrogen Alpha and "enhance" other emission lines, such as OIII. That is why Simon D.:s image looks so very blue compared with the APOD. Note however that the bluest areas in Simon D.:s image look faintly bluish in the APOD as well, which makes me wonder if a bit of OIII has been used for the APOD, too. Note that both the APOD and Simon D.:s image show the bow shock of the blue star FN CMa (at center right) very clearly.
Not all Seagull Nebula images show any blue nebulosity below the red ridge (or the bow shock of FN CMa):
So I'd say we are probably seeing some OIII in the APOD after all. Of course, Gianni Lacroce, the APOD photographer, has brought out "a ton of Hα", too!
I'd like to point out two stars to you:
Blue FN CMA with its bow shock reminds me of Mary Poppins with her umbrella!
I'll talk about W CMa later.
Of course, while Mary Poppis used her umbrella to descend from the sky, hot blue B-type FN CMa is pushing its way ahead through gas and dust forming an umbrella-shaped bow shock in front of it.
As for red-orange W CMa, it's a rare and remarkable carbon star. Its small Gaia parallax puts it a distance of ~2,250 light-years. And it is remarkably red. Its B-V index is not that much different from that of Betelgeuse, but W CMa is four magnitudes brighter in R than in B. That's a lot. Its absolute V (yellow-green) luminosity is some 900 times the Sun, but in R (red), it is more than 4,000 times brighter than the Sun. And in infrared J, corresponding to a wavelength of 1220 nm, it is almost 60,000 times brighter than the Sun! And in infrared K, corresponding to 2190 nm, it is 170,000 times brighter than the Sun!
Wikipedia wrote about carbon stars like W CMa:
A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen.[1] The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.
W CMA is a sooty star! That's why it's so red. It is self-reddened by particles in its own atmosphere, just like an extra red sunset.
Clearly W CMa needs a chimney sweep! (Speaking of Mary Poppins...)
Click to play embedded YouTube video.
Ann
[img3="The Seagull Nebula.
Credit & Copyright: Gianni Lacroce"]https://apod.nasa.gov/apod/image/2403/Seagull_Lacroce_1080.jpg[/img3]
That's an interesting and very red portrait of the Seagull Nebula.
I can't complain. The Seagull Nebula [b][i][size=110]is[/size][/i][/b] very red, since it is heavily dominated by 656 nm Hydrogen Alpha, which is of course this color, [color=#FF0000]███[/color].
Only one other color is immediately obvious in the APOD apart form the red Hydrogen Alpha, and that is pale blue. At first I thought that the APOD might be a two-filter image, constructed from from a red Hα filter and a cyan-green OIII filter, mapped as blue, but that is not fully consistent with the appearance of the image. I found just such an Hα/OIII picture of the Seagull Nebula on the net, and as you can see, it doesn't look like the APOD:
[img3="The Seagull Nebula in Hα and OIII, mapped as red and blue. Credit: Simon D. at Cloudy Nights."]https://www.cloudynights.com/uploads/monthly_03_2021/post-216270-0-19938800-1615683298_thumb.jpg[/img3]
In narrowband images of nebulas, such as the Seagull Nebula picture by Simon D., you "play down" the Hydrogen Alpha and "enhance" other emission lines, such as OIII. That is why Simon D.:s image looks so very blue compared with the APOD. Note however that the bluest areas in Simon D.:s image look faintly bluish in the APOD as well, which makes me wonder if a bit of OIII has been used for the APOD, too. Note that both the APOD and Simon D.:s image show the bow shock of the blue star FN CMa (at center right) very clearly.
Not all Seagull Nebula images show any blue nebulosity below the red ridge (or the bow shock of FN CMa):
[float=left][img3="The Seagull Nebula. We can see a 'fat red ridge of Hα', but hardly any blue nebulosity below the red ridge. Credit: NAOJ/DSS/Robert Gendler"]https://compote.slate.com/images/abbf0a9a-7106-4923-ac4a-e00f6ab509d0.jpg[/img3][/float][float=right][img3="In this image, faint nebulosity can be seen below the bright red ridge, and this faint nebulosity is red in color. No bow shock is seen above FN CMa. Credit: Wikimedia Commons/Keesscherer (CC BY-SA 4.0)"]https://www.constellation-guide.com/wp-content/uploads/2023/02/Seagull-Nebula-IC-2177.webp[/img3][/float]
[clear][/clear]
So I'd say we are probably seeing some OIII in the APOD after all. Of course, Gianni Lacroce, the APOD photographer, has brought out "a ton of Hα", too!
I'd like to point out two stars to you:
[float=left][attachment=0]APOD 13 March 2024 detail annotated.png[/attachment][c][size=85][color=#0080FF]Blue FN CMA with its bow shock reminds me of Mary Poppins with her umbrella![/color]
[color=#FF8000]I'll talk about W CMa later.[/color][/size][/c][/float][float=right][img3=""]https://static.greatbigcanvas.com/images/singlecanvas_thick_none/movie-goods/mary-poppins-1964,mg0081135.jpg[/img3][/float]
[clear][/clear]
Of course, while Mary Poppis used her umbrella to descend from the sky, hot blue B-type FN CMa is pushing its way ahead through gas and dust forming an umbrella-shaped bow shock in front of it.
As for red-orange W CMa, it's a rare and remarkable carbon star. Its small Gaia parallax puts it a distance of ~2,250 light-years. And it is remarkably red. Its B-V index is not that much different from that of Betelgeuse, but W CMa is four magnitudes brighter in R than in B. That's a lot. Its absolute V (yellow-green) luminosity is some 900 times the Sun, but in R (red), it is more than 4,000 times brighter than the Sun. And in infrared J, corresponding to a wavelength of 1220 nm, it is almost 60,000 times brighter than the Sun! And in infrared K, corresponding to 2190 nm, it is 170,000 times brighter than the Sun!
[quote][url=https://en.wikipedia.org/wiki/Carbon_star]Wikipedia[/url] wrote about carbon stars like W CMa:
A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen.[1] The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance. [/quote]
W CMA is a sooty star! That's why it's so red. It is self-reddened by particles in its own atmosphere, just like an extra red sunset.
[img3="An orange sunset. Credit: Stephen F. Corfidi"]https://www.spc.noaa.gov/publications/corfidi/sunset/picss1990nov7gjpgmedLABEL.jpg[/img3]
Clearly W CMa needs a chimney sweep! (Speaking of Mary Poppins...)
[youtube]https://www.youtube.com/watch?v=YSCdFVc6DoY[/youtube]
Ann