APOD: NGC 2023 in the Horsehead's Shadow (2018 Mar 29)

[APOD Robot]

NGC 2023 in the Horsehead's Shadow

Explanation: Carved by a bright young star in Orion's dusty molecular clouds, NGC 2023 is often overlooked in favor of the nearby dramatic silhouette of the Horsehead Nebula. In its own right it is seen as a beautiful star forming emission and reflection nebula though, a mere 1500 light-years distant. Surprisingly colorful and complex filaments are detailed in this rare NGC 2023 portrait. Scattered points of emission are also from the region's Herbig-Haro objects, associated with the energetic jets from newborn stars. The sharp telescopic view spans about 10 light-years at the estimated distance of NGC 2023. Off the right edge of the frame lies the more familiar cosmic Horsehead.

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[Boomer12k]

Beautiful...

:---[===] *

[Ann]

This is indeed a very, very beautiful portrait of NGC 2023. The details are exquisite, and the colors are beautiful too.

Nevertheless, the colors baffle me. The nebula, which is indeed a (blue) reflection nebula, looks almost more red than blue.

APOD Robot wrote;

Surprisingly colorful and complex filaments are detailed in this rare NGC 2023 portrait. Scattered points of emission are also from the region's Herbig-Haro objects, associated with the energetic jets from newborn stars.

Well, those filaments are red, which is the normal color of emission nebulosity. The question is what causes the ionization. My software Guide gives conflicting information on the star that is the source of the blue reflection nebula, HD 37903. Its spectral type is given as both B3 and B1.5V. A star of spectral class B3 should not be hot enough to ionize a nebula, but a star of spectral class B1.5V is indeed hot enough to do just that.

According to Simbad Astronomical Database, the spectral class of HD 37903 is B3IV. That means, again, that the star isn't hot enough to ionize a nebula.

It is however possible that various outflows from the star and subsequent collisions in the surrounding nebula might cause some ionization.

I think, in any case, that the red color of NGC 2023 has been exaggerated in this APOD, so that we don't get a good idea of the nebula's overall dominant color. Perhaps an extra amount of Hα exposure has gone into making this picture?

Ann

[starsurfer]

This is indeed a very, very beautiful portrait of NGC 2023. The details are exquisite, and the colors are beautiful too.

Nevertheless, the colors baffle me. The nebula, which is indeed a (blue) reflection nebula, looks almost more red than blue.

APOD Robot wrote;

Surprisingly colorful and complex filaments are detailed in this rare NGC 2023 portrait. Scattered points of emission are also from the region's Herbig-Haro objects, associated with the energetic jets from newborn stars.

Well, those filaments are red, which is the normal color of emission nebulosity. The question is what causes the ionization. My software Guide gives conflicting information on the star that is the source of the blue reflection nebula, HD 37903. Its spectral type is given as both B3 and B1.5V. A star of spectral class B3 should not be hot enough to ionize a nebula, but a star of spectral class B1.5V is indeed hot enough to do just that.

According to Simbad Astronomical Database, the spectral class of HD 37903 is B3IV. That means, again, that the star isn't hot enough to ionize a nebula.

It is however possible that various outflows from the star and subsequent collisions in the surrounding nebula might cause some ionization.

I think, in any case, that the red color of NGC 2023 has been exaggerated in this APOD, so that we don't get a good idea of the nebula's overall dominant color. Perhaps an extra amount of Hα exposure has gone into making this picture?

Ann

It's nice to see this nebula in a more closer context. The red filaments are various Herbig Haro objects, which I think in this area were first discovered by David Malin. You might find this paper interesting.

[Ann]

It's nice to see this nebula in a more closer context. The red filaments are various Herbig Haro objects, which I think in this area were first discovered by David Malin. You might find this paper interesting.

I doubt that the red filaments are Herbig-Haro object.

APOD Robot wrote:

Surprisingly colorful and complex filaments are detailed in this rare NGC 2023 portrait. Scattered points of emission are also from the region's Herbig-Haro objects, associated with the energetic jets from newborn stars.

The caption described the "scattered points of emission" as products of Herbig Haro objects, but it didn't say the same about the filaments. Admittedly, it said that the scattered points of emission are also from Herbig-Haro objects, but I still find this wording very unclear.

Malin, D. F.; Ogura, K.; Walsh, J. R. wrote:

Two groups of Herbig-Haro objects have been discovered in the vicinity of the reflection nebula NGC 2023 and the Horsehead nebula (Barnard 33). Prime focus photographs, objective prism and low-dispersion spectra, and high-resolution line profiles have been obtained for the HH objects and proper motions have been derived for the brighter knots.

The abstract here talks about knots, not filaments.

I don't believe that the filaments are caused by Herbig-Haro objects.

Ann

[De58te]

Ann, in the link (Herbig-Haro objects) it says, "NGC 2023 is illuminated by HD 37903, a B1.5V star (Sharpless 1952) and appears blue on colour photographs."

[orin stepanek]

NGC2023! Nice;

[Ann]

Ann, in the link (Herbig-Haro objects) it says, "NGC 2023 is illuminated by HD 37903, a B1.5V star (Sharpless 1952) and appears blue on colour photographs."

Okay, thanks! I should have checked that!

A star of spectral class B1.5V is hot enough to ionize an emission nebula, but not a bright emission nebula. So the rather faint red filaments in NGC 2023 may indeed be ionized by HD 37903.

Click to view full size image

The Iris Nebula, illuminated and ionized by B2Ve-type star HD 200775.
Photo; Adam Block/Caelumobservatory

Click to view full size image

O9.5Vvar-type star AE Aurigae ionizing and illuminating the Flaming Star nebula.
Photo: Rolf Geissinger.

Another well-known blue reflection nebula, the Iris Nebula or NGC 7023, is rather brightly illuminated and faintly ionized by B2Ve star HD 200775. So there is a prominent blue reflection nebula and faint wisps of emission nebulosity in NGC 7023.

The rather well-known Flaming Star Nebula is primarily an emission nebula, ionized by an O9.5Vvar type of star, AE Aurigae. AE Aurigae represents the very coolest O-type stars, but O-type stars are nevertheless hotter than any B-type star, and AE Aur is definitely hot enough to ionize a prominent red emission nebula. The blue reflection nebula next to the star is actually more remarkable, because O-type stars are so rarely surrounded by blue reflection nebulas. The reason why there is one here is that AE Aurigae is a runaway star that is just passing through, and its harsh O-type stellar wind and hard ultraviolet radiation have not yet had time to scatter and disintegrate the grains of dust in the molecular cloud that reflects the star's blue light.

So whether you get an emission or a reflection nebula is due to the temperature of the illuminating or ionizing star. At spectral class B1.5V, HD 37903, the ionizing star in today's APOD, is hot enough to do some ionizing, but not a lot of it. The nebula NGC 2023 is therefore mostly a reflection nebula, and its color is predominantly blue.

Ann

[neufer]

---

Beautiful...

This is indeed a very, very beautiful portrait of NGC 2023.
The details are exquisite, and the colors are beautiful too.

It's nice to see this nebula in a more closer context.

[Chris Peterson]

I think, in any case, that the red color of NGC 2023 has been exaggerated in this APOD, so that we don't get a good idea of the nebula's overall dominant color.

This object has no color, so any image which shows color can reasonably be said to exaggerate it.

[Ann]

I think, in any case, that the red color of NGC 2023 has been exaggerated in this APOD, so that we don't get a good idea of the nebula's overall dominant color.

This object has no color, so any image which shows color can reasonably be said to exaggerate it.

I don't agree with you there.

Yes, it is true inasmuch as it is impossible for humans to spot color in any reflection nebula. (For that matter, we can't even spot any color in emission nebulas.)

Click to view full size image

The California Nebula at left and the Pleiades at right, with IC 348 in between.
Photo: Rogelio Bernal Andreo.

But it is wrong to say that nebulas aren't colored if we use electronic equipment to measure the predominant wavelengths of the light that is emitted or reflected from nebulas.

The nebulas of the Pleiades are richer in wavelengths around 400 nm than in wavelengths around 650 nm, there is just no denying it.

Similarly, the light of the California Nebula is strongly dominated by 656 nm wavelengths, with a non-negligible addition of wavelengths of 486 nm.

To me, that means that the Pleiades nebulas are blue and the California nebula is red. You may or may not agree with me, but I will not change my mind about this.

Ann

[Chris Peterson]

I think, in any case, that the red color of NGC 2023 has been exaggerated in this APOD, so that we don't get a good idea of the nebula's overall dominant color.

This object has no color, so any image which shows color can reasonably be said to exaggerate it.

I don't agree with you there.

Yes, it is true inasmuch as it is impossible for humans to spot color in any reflection nebula. (For that matter, we can't even spot any color in emission nebulas.)

But it is wrong to say that nebulas aren't colored if we use electronic equipment to measure the predominant wavelengths of the light that is emitted or reflected from nebulas.

The nebulas of the Pleiades are richer in wavelengths around 400 nm than in wavelengths around 650 nm, there is just no denying it.

Similarly, the light of the California Nebula is strongly dominated by 656 nm wavelengths, with a non-negligible addition of wavelengths of 486 nm.

To me, that means that the Pleiades nebulas are blue and the California nebula is red. You may or may not agree with me, but I will not change my mind about this.

Color is a physiological phenomenon, not a physical one. These objects have no color. Wavelengths aren't color. We are not using display devices that are capable of reproducing narrowband wavelengths. So every color image we see of them is reasonably seen as exaggerating color in some way. I think you're just biased by your preferences for the way some imagers choose to present this information. It's an aesthetic decision, not a scientific one.

[Ann]

Color is a physiological phenomenon, not a physical one. These objects have no color. Wavelengths aren't color. We are not using display devices that are capable of reproducing narrowband wavelengths. So every color image we see of them is reasonably seen as exaggerating color in some way. I think you're just biased by your preferences for the way some imagers choose to present this information. It's an aesthetic decision, not a scientific one.

Indeed, "color" is a physiological response to external stimuli.

Let me put it like this, then. The reason why we can't spot color in the nebulas is because their light is far, far too faint to stimulate any response from the cones in our retinas.

What I mean, when I say that these objects have "color", is that if we could "turn up the intensity" of their emitted or reflected light, we might, indeed, spot color in them. And if we did, the Pleiades nebulas would be blue to us and the California nebula would be reddish-pink.

That is how I think of nebulas. And if you don't agree, let's agree to disagree.

Ann

[Chris Peterson]

Color is a physiological phenomenon, not a physical one. These objects have no color. Wavelengths aren't color. We are not using display devices that are capable of reproducing narrowband wavelengths. So every color image we see of them is reasonably seen as exaggerating color in some way. I think you're just biased by your preferences for the way some imagers choose to present this information. It's an aesthetic decision, not a scientific one.

Indeed, "color" is a physiological response to external stimuli.

Let me put it like this, then. The reason why we can't spot color in the nebulas is because their light is far, far too faint to stimulate any response from the cones in our retinas.

What I mean, when I say that these objects have "color", is that if we could "turn up the intensity" of their emitted or reflected light, we might, indeed, spot color in them. And if we did, the Pleiades nebulas would be blue to us and the California nebula would be reddish-pink.

That is how I think of nebulas. And if you don't agree, let's agree to disagree.

But keep in mind that color is dependent upon intensity. Look through a Ha filter, and a dim source will appear a totally different color than a bright one. So yes, if you turn up the intensity of an astronomical object, you'll reach a point where you can see color. Turn it up a bit more, and you'll see a different color. So what is an imager to do when mapping data channels to red, green, and blue?

[Cousin Ricky]

It is ironic that NGC 2023 is often overlooked; I have seen NGC 2023, quite an easy object to see, but I have never been able to make out the Horsehead.

[Chris Peterson]

It is ironic that NGC 2023 is often overlooked; I have seen NGC 2023, quite an easy object to see, but I have never been able to make out the Horsehead.

I'm surprised. I've always found the Horsehead an easy visual target, although it only looks like a darker splotch, not any obvious shape. But it's quite contrasty, which is generally what makes something visible.