APOD: Stars, Dust, and Gas Near Antares (2022 Jan 26)

[APOD Robot]

Stars, Dust, and Gas Near Antares

Explanation: Why is the sky near Antares and Rho Ophiuchi so dusty yet colorful? The colors result from a mixture of objects and processes. Fine dust -- illuminated from the front by starlight -- produces blue reflection nebulae. Gaseous clouds whose atoms are excited by ultraviolet starlight produce reddish emission nebulae. Backlit dust clouds block starlight and so appear dark. Antares, a red supergiant and one of the brighter stars in the night sky, lights up the yellow-red clouds on the lower right of the featured image. The Rho Ophiuchi star system lies at the center of the blue reflection nebula on the top left. The distant globular cluster of stars M4 is visible above and to the right of Antares. These star clouds are even more colorful than humans can see, emitting light across the electromagnetic spectrum.

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[Ann]

RhoOphAntares_Cogo_1024_annotated[1].jpg

Stars, Dust, and Gas Near Antares.
Image Credit & Copyright: Mario Cogo (Galax Lux)

APOD January 26 2022 Antares Rho Ophiuchi annotated.png

Why are the colors the way they are in the APOD?

1) The yellow Antares reflection nebula. Antares is, despite being called a red supergiant, not actually a red star. Its B-V index is around +1.9, which makes it a yellow-orange star. (If you want a redder star, check out Mu Cephei, with a B-V index of cirka +2.2, or, better yet, carbon star T Lyrae, with a variable B-V index which is almost always larger than +5.)

So Antares isn't actually a red star, but yellow-orange. Its reflection nebula is even less orange and more yellow than the star itself, because the dust particles in reflection nebulas preferentially scatter shorter wavelengths. The Antares reflection nebula is by far the largest of the yellow reflection nebulas visible in the sky.

2) The blue Rho Ophiuchi reflection nebula. The star Rho Ophiuchi itself is a binary star made up of two stars of spectral class B2, which is ideal for creating a blue reflection nebula. Such stars pump out huge amounts of blue and violet photons, while at the same time just falling short of emitting enough ultraviolet photons to ionize an emission nebula. In the case of Rho Ophiuchi, there is just the right amount of dust behind the stellar pair to scatter their blue light back at us. The Rho Ophiuchi nebula is one of the largest blue reflection nebulas in the sky.

3) The pink Sigma Scorpii nebula. Sigma Scorpii is another binary star, and I have seen many different suggestions as to what their spectral types are: B1III, B2III+O9.5V, and O9.5V+B7V. Well, whichever combination is the correct one, Sigma Scorpii definitely produces more ultraviolet photons than Rho Ophiuchi does, and therefore Sigma Scorpii ionizes a pink emission nebula. This is to say that a sufficiently large number of energetic photons from Sigma Scorpii knocks electrons in hydrogen atoms in the surrounding gas cloud into a "higher orbit" around their protons, or rather, the energetic photons knock electrons into a higher electron shell. As the electron "falls down" again, it rids itself of the extra energy it gained when it was kicked into another electron shell, and it does so by emitting a photon of 656.281 nm. This is the wavelength of hydrogen alpha, which is very red. So why does the emission nebula look pink when all those "kicked-upstairs-and-falling-down" electrons emit all that red light?

The pink color is caused by the attenuation of the very red 656 nm light by the simultaneous emission of bluish cyan hydrogen beta 486 nm photons, emitted by electrons that were knocked "two levels up" by extra energetic photons. But please note that Sigma Scorpii is also lighting up a blue reflection nebula, because there is a significant amount of dust particles in the gas cloud near Sigma Scorpii, and the blue reflection nebulosity further attenuates the red light of hydrogen alpha.

4) The very dark brown dust tendrils. Here the dust particles are sufficiently numerous to (almost) completely block light from behind them. The intrinsic color of the dust grains themselves is dark brown, so we see the color of the dust itself in these dark tendrils.

Dust reddening. Image: 2MASS.

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5) The light brown or orange patches. Here dust partly blocks and reddens light from behind.

Look at the picture at right. You can see that the edges of the dark cloud allows yellow light to pass through, and the starlight looks yellower that the surrounding star field. Further in, the yellow light is also blocked, and only orange light can penetrate. Then only red light is let through, until finally all light is blocked by the dust cloud.

The orange and light brown patches in the Antares-Rho Ophiuchi cloud complex are places where some light is blocked by dust, and the light that passes through has been reddened. Actually the rather pale yellow color of the Antares nebula is made into a deeper, more orange shade of yellow in places by dust reddening.


Ann

[orin stepanek]

Nice work Ann!

RhoOphAntares_Cogo_1024.jpg

APOD; These star clouds are even more colorful than humans can see

I can't imagine as this is pretty colorful! Kudos to Mario Kogo

HumanSpaceship2_TsevisHubbleRJN_960.jpg

A new super Hero!

[Guest]

In general, it’s not always clear if colors in Astro photos are natural or enhanced or even altered to show various things in the photo. It would be nice if the editors mention that subject of colors in each write-up (no, I’m not referring to this write-up).

[NCTom]

Thanks, Ann, for the color formation process, an explanation I haven't studied in decades.

[Chris Peterson]

In general, it’s not always clear if colors in Astro photos are natural or enhanced or even altered to show various things in the photo. It would be nice if the editors mention that subject of colors in each write-up (no, I’m not referring to this write-up).

If you look at this region with binoculars, you will see some pale blue and pale orange in some of the stars, and everything else will be gray. This is true of all astronomical views. Imaging allows us to see what our eyes cannot. So every color astroimage has had its colors enhanced, or we would not see color. The choice of saturation is largely aesthetic, as the actual visual saturation is zero, and saturation has no physical meaning here. "Altered colors" really only makes sense in the context of false color or pseudocolor palettes, and when those are used it is generally made clear in the description.

[Sa Ji Tario]

The "beautiful pictures" are not, they are made with the RGB color palette and some important characters are highlighted. Sometimes, the same image appears with different colors depending on what you want to show

[Chris Peterson]

4) The very dark brown dust tendrils. Here the dust particles are sufficiently numerous to (almost) completely block light from behind them. The intrinsic color of the dust grains themselves is dark brown, so we see the color of the dust itself in these dark tendrils.

This dust also fluoresces under UV stimulation, and the fluorescence is in the red part of the spectrum. So the actual brown color we see is some combination of reflected light (i.e. the approximate color of the dust itself) and emitted fluorescence. Mostly the former in most cases, but the two can't be separated in an image like this.

[neufer]

1) The yellow Antares reflection nebula. Antares is, despite being called a red supergiant, not actually a red star. Its B-V index is around +1.9, which makes it a yellow-orange star. (If you want a redder star, check out Mu Cephei, with a B-V index of cirka +2.2, or, better yet, carbon star T Lyrae, with a variable B-V index which is almost always larger than +5.)

So Antares isn't actually a red star, but yellow-orange. Its reflection nebula is even less orange and more yellow than the star itself, because the dust particles in reflection nebulas preferentially scatter shorter wavelengths. The Antares reflection nebula is by far the largest of the yellow reflection nebulas visible in the sky.

https://skyandtelescope.org/astronomy-b ... 203201401/

[Ann]

In general, it’s not always clear if colors in Astro photos are natural or enhanced or even altered to show various things in the photo. It would be nice if the editors mention that subject of colors in each write-up (no, I’m not referring to this write-up).

If you look at this region with binoculars, you will see some pale blue and pale orange in some of the stars, and everything else will be gray. This is true of all astronomical views. Imaging allows us to see what our eyes cannot. So every color astroimage has had its colors enhanced, or we would not see color. The choice of saturation is largely aesthetic, as the actual visual saturation is zero, and saturation has no physical meaning here. "Altered colors" really only makes sense in the context of false color or pseudocolor palettes, and when those are used it is generally made clear in the description.

Chris is right that all nebulas in the sky (except some planetary nebulas) are much too faint for us to perceive any colors in them.

But that doesn't mean that the colors aren't there, just that we can't see them. A camera and a suitable set of filters, however, can.

A broadband R filter would detect a lot of "yellow-green to red signal" in the Antares nebula. A broadband G filter would detect a lot of yellow-green to yellow signal in the Antares nebula. A broadband B filter would detect barely any signal at all from the Antares nebula. Together, the three filters would detect a lot of yellow to yellow-orange light in the Antares nebula.

Similarly, a broadband B filter would detect a lot of blue signal in the Rho Ophiuchi nebula, while the response from the G and the R filter would be much weaker. The overall color detected would be dominated by blue hues.

And an R filter would detect the red H-alpha emission nebula near Sigma Scorpii, and a B filter would detect the blue reflection nebula produced by light-scattering dust particles in the vicinity of this star. The overall color detected by the filters near Sigma Scorpii would be pink, magenta and blue.

These colors are real, because they represent wavelengths that are real and that are either emitted or scattered by these nebulas. Of course, if we choose other filters, we are going to detect other wavelengths, and some of those wavelengths may not represent visual colors at all. For example, the James Webb Space Telescope will detect mostly (or exclusively?) infrared wavelengths. The JWST images will have to be shown as mapped (or "false") color in order for us to see its images at all, since we can't see (all or most of) the wavelengths that the JWST will detect.

The Pleiades in visual light. Photo: Keesscherer.

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The Pleiades in infrared light. John Stauffer (Spitzer Science Center, Caltech) credits: Credit: NASA/JPL-Caltech/J. Stauffer (SSC/Caltech)

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To see what I mean about different filters, compare these two pictures of the Pleiades. The Pleiades is a group of young stars, whose brightest members emit a lot of blue light. This blue light is scattered by dust particles in the vicinity of the stars, making the Pleiades nebulosity look very blue in visible light. If our eyes were sensitive enough, we would be able to see this blue light, which would look much like it does in the picture at left.

In the picture at right, however, the Pleiades has been imaged in infrared light. Our eyes can't see infrared light, so no matter how sensitive our (red-green-and-blue-sensitive) retinas were, we wouldn't be able to see infrared light.

The infrared image is dominated by green and orange hues. But green and orange are colors that our eyes can detect, and they don't correspond to any infrared wavelengths detected by Spitzer. Invisible infrared wavelengths were detected by the infrared-sensitive Spitzer Space Telescope, and the invisible wavelengths detected were "mapped" as wavelengths that we can see, mostly green and orange.

Note that the stars of the Pleiades appear a little fainter than we are used to in the infrared image. That is because the bright stars of the Pleiades are hot and blue, and they emit a lot of blue light but comparatively little infrared light.

But while the colors in the infrared portrait of the Pleiades are "false", the wavelengths detected are "real".

Ann

[Chris Peterson]

In general, it’s not always clear if colors in Astro photos are natural or enhanced or even altered to show various things in the photo. It would be nice if the editors mention that subject of colors in each write-up (no, I’m not referring to this write-up).

If you look at this region with binoculars, you will see some pale blue and pale orange in some of the stars, and everything else will be gray. This is true of all astronomical views. Imaging allows us to see what our eyes cannot. So every color astroimage has had its colors enhanced, or we would not see color. The choice of saturation is largely aesthetic, as the actual visual saturation is zero, and saturation has no physical meaning here. "Altered colors" really only makes sense in the context of false color or pseudocolor palettes, and when those are used it is generally made clear in the description.

Chris is right that all nebulas in the sky (except some planetary nebulas) are much too faint for us to perceive any colors in them.

But that doesn't mean that the colors aren't there, just that we can't see them. A camera and a suitable set of filters, however, can.

A broadband R filter would detect a lot of "yellow-green to red signal" in the Antares nebula. A broadband G filter would detect a lot of yellow-green to yellow signal in the Antares nebula. A broadband B filter would detect barely any signal at all from the Antares nebula. Together, the three filters would detect a lot of yellow to yellow-orange light in the Antares nebula.

Similarly, a broadband B filter would detect a lot of blue signal in the Rho Ophiuchi nebula, while the response from the G and the R filter would be much weaker. The overall color detected would be dominated by blue hues.

And an R filter would detect the red H-alpha emission nebula near Sigma Scorpii, and a B filter would detect the blue reflection nebula produced by light-scattering dust particles in the vicinity of this star. The overall color detected by the filters near Sigma Scorpii would be pink, magenta and blue.

These colors are real, because they represent wavelengths that are real and that are either emitted or scattered by these nebulas. Of course, if we choose other filters, we are going to detect other wavelengths, and some of those wavelengths may not represent visual colors at all. For example, the James Webb Space Telescope will detect mostly (or exclusively?) infrared wavelengths. The JWST images will have to be shown as mapped (or "false") color in order for us to see its images at all, since we can't see (all or most of) the wavelengths that the JWST will detect.

Ann

To be precise, "color" is a physiological property, not a physical one. So in a very real sense, these objects have no color. Colloquially, of course, we can treat "color" as fairly synonymous with "wavelength distribution" or some similar physical concept.

That said, I hope nothing in my comment is taken to suggest that the "colors" aren't "real". Only that if we're seeing color in an image, we're seeing enhancement of some kind. Three images may show three different sets of color (in particular, in terms of saturation), and none can be said to be more or less "real" or "accurate" than the others.