APOD: Windblown NGC 3199 (2021 May 06)

[APOD Robot]

Windblown NGC 3199

Explanation: NGC 3199 lies about 12,000 light-years away, a glowing cosmic cloud in the nautical southern constellation of Carina. The nebula is about 75 light-years across in this narrowband, false-color view. Though the deep image reveals a more or less complete bubble shape, it does look very lopsided with a much brighter edge along the top. Near the center is a Wolf-Rayet star, a massive, hot, short-lived star that generates an intense stellar wind. In fact, Wolf-Rayet stars are known to create nebulae with interesting shapes as their powerful winds sweep up surrounding interstellar material. In this case, the bright edge was thought to indicate a bow shock produced as the star plowed through a uniform medium, like a boat through water. But measurements have shown the star is not really moving directly toward the bright edge. So a more likely explanation is that the material surrounding the star is not uniform, but clumped and denser near the bright edge of windblown NGC 3199.

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[Ann]

APOD Robot wrote:

Near the center is a Wolf-Rayet star, a massive, hot, short-lived star that generates an intense stellar wind.

So... which of the stars near the center in the image is the Wolf-Rayet star?

Wolf-Rayet star HD 89358.

F-type star Mirfak. Photo: Sergio Diaz.

In the picture at left you can see the Wolf-Rayet star, HD 89358.

Note the fiery orange color of HD 89358 in the APOD. The light from this blisteringly hot and blue star has been considerably reddened by dust. Yes, but the light that reaches us from HD 89358 is still much bluer in color than the Sun.

In fact, the light that reaches us from HD 89358 is similar in color to bright star Mirfak, Alpha Persei, an F-type star. The beautiful image by Sergio Diaz at right gives you a good idea of the color of Mirfak, and of the apparent color of the light that reaches us from blisteringly hot Wolf-Rayet star HD 89358.

It pains me when stars whose apparent color - that is, the wavelengths in the optical part of the spectrum that reaches us from a particular star - is made to look bright orange, even though the optical light that we can see from this star is so much bluer than that.

And in particular, it pains me when this is done with no explanation whatsoever.

Ann

[De58te]

It pains me when stars whose apparent color - that is, the wavelengths in the optical part of the spectrum that reaches us from a particular star - is made to look bright orange, even though the optical light that we can see from this star is so much bluer than that.

And in particular, it pains me when this is done with no explanation whatsoever.

Ann

Well, they did say it was a false-color view. But Ann, before your commentary I had thought the false color was the bright blue of the nebula. It never occurred to me that the false color was the Wolf-Rayet star.

[Ann]

It pains me when stars whose apparent color - that is, the wavelengths in the optical part of the spectrum that reaches us from a particular star - is made to look bright orange, even though the optical light that we can see from this star is so much bluer than that.

And in particular, it pains me when this is done with no explanation whatsoever.

Ann

Well, they did say it was a false-color view. But Ann, before your commentary I had thought the false color was the bright blue of the nebula. It never occurred to me that the false color was the Wolf-Rayet star.

Yes, the bright blue of the nebula is false color too.

NGC 3199. Image Credit & Copyright: Mike Selby and Roberto Colombari

NGC 3199. Photo: ESO.

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Let's compare today's APOD with a more-or-less true color picture of the same nebula by ESO.

These are the filers used be ESO:

480 nm, mapped as blue.

480 nm, mapped as green.

625 nm, mapped as red.

659 nm, redshifted hydrogen alpha, mapped as red (and it is red).

770 nm, invisible infrared, mapped as red.

The colors mapped by ESO correspond more or less with how our eyes react to the wavelengths detected by their filters.

As you can see, the bright nebular arc of NGC 3199 is pink in the ESO image. I would say that the color is a mixture of blue (480 nm) and red hydrogen alpha (659 nm). Clearly, the color of the bright nebular arc is not "extremely red", but it is clearly not "bright blue" either.

But yes, the blue-green nebular arc seen in today's APOD is shown in false color. I'll have a guess and say that the blue-green color of the arc in the APOD is a mixture of false-color OIII (OIII), mapped as blue, and false-color hydrogen alpha, mapped as green.

Ann

[orin stepanek]

I wish they would have pointed out the location of the star that
created this beautiful NGC3199 Nebula!

[Sa Ji Tario]

Ann, does the narrow band not modify the star colors?

[Chris Peterson]

It pains me when stars whose apparent color - that is, the wavelengths in the optical part of the spectrum that reaches us from a particular star - is made to look bright orange, even though the optical light that we can see from this star is so much bluer than that.

And in particular, it pains me when this is done with no explanation whatsoever. :cry: :evil:

Ann

Well, they did say it was a false-color view. But Ann, before your commentary I had thought the false color was the bright blue of the nebula. It never occurred to me that the false color was the Wolf-Rayet star.

And in comparing this image to an RGB view, we can see why imaging in these bands is so popular and so useful. So much more detail and information is exposed than broadband imaging makes apparent.

[Ann]

Ann, does the narrow band not modify the star colors?

Let me try to explain a little.

The human retina has three kids of color-sensitive cone cells, blue-sensitive (short wavelength-sensitive), green-sensitive (medium wavelength-sensitive) and red-sensitive (long wavelength-sensitive) cone cells. Illustration: BenRG at English Wikipedia.

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Ideal(?) filter sensitivity of RGB photography. Illustration: I don't know.

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The human eye has three color-sensitive cones, or receptors, blue, green and red, in our retinas. In order to get "true-color" images, RGB (Red, Green, Blue) imagery is preferred, as it reacts to color in a relatively similar way to the human eye. RGB photos are usually achieved by using red, green and blue broadband filters.

OIII, Hα and SII filters used for the most common type of narrowband photography. Source: https://astrobackyard.com/narrowband-imaging/

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The OIII narrowband filter detects wavelengths around 501 nm. The Hα filter detects a wavelength of 656 nm, and the SII filter detects a wavelength of 658 nm.

As you can see from the illustration, 501 nm looks definitely green to most people. 656 nm, as in Hα, and 658 nm, as in SII, look absolutely identical to the human eye, and both look extremely red to us.

So OIII, Hα and SII filters really correspond to two colors visible to the human eye: green and red. But we humans prefer three-color images, because we have three different color receptors in our eyes. And anyway, what would be the point of taking pictures of deep space objects through both Hα and SII filters and then show both the Hα and the SII filter images as bright red, as they would appear to the eye? It would be impossible to tell the Hα and the SII features apart, if they both looked the same shade of bright red!

That is why narrowband images are made using false (or mapped) color. The green OIII features are typically mapped as blue, Hα is mapped as green and SII is mapped - make that shown - as red.

In reality, OIII is not blue - certainly not deep blue - and Hα is absolutely not green. Never!

Not only are the filter images mapped to colors that don't correspond to reality, but if I'm not mistaken, Hα, which is normally so dominant in nebulas, is often made to look fainter, so that the other two wavelengths can "shine". In other words: A nebula that looks very red from Hα in RGB photography is usually not going to look very green in narrowband photography, even though Hα is typically mapped as green. Hα is typically "played down" in narrowband photography, and OIII and SII are enhanced and made brighter.

The Rosette Nebula, looking very red from Hα in this RGB image. Photo: Evangelos Souglakos.

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The Rosette Nebula, looking blue, kind of yellow and a bit red in this OIII + Hα + SII narrowband image. Photo: Chuck Ayoub.

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Note how the color of the nebula changes in narrowband images. But also note how the color and brightness of the stars change in this kind of photography.

Ann

[Chris Peterson]

Note how the color of the nebula changes in narrowband images. But also note how the color and brightness of the stars change in this kind of photography.

It is worth noting that this is often seen as an advantage of narrowband imaging, the suppression of stars. Because stars are broadband sources, only a tiny fraction of their output makes it through narrowband filters. So their brightness is reduced several orders of magnitude in comparison with the emissive nebula structures. As bright stars (and their imaging artifacts) can hide nebular detail, this reduction can be valuable.

[VictorBorun]

ColombariNGC3199_1024.jpg

I wish they would have pointed out the location of the star that
created this beautiful NGC3199 Nebula!

Also please does anyone knows if there is some lighting from above the picture?
That brighter edge along the top covers in fact a large part of the surface far from the Wolf-Rayet star

[orin stepanek]

I can't imagine being able to see lightning being a light year or more across; but I really don't know!