APOD: Wisps Surrounding the Horsehead Nebula (2019 Apr 03)

[Ann]

My bad. The nebula is, of course, IC 432. Forget everything I said about IC 435.

Ann

Don't be too hard on yourself, Ann, they are just funny coloured puffs of almost nothing, after all. I was also unsure which was which when I was writing my earlier posts, so I only referred to the image colours in general.

But both puffs are said to be reflection nebulae, and I suppose the interesting thing about them, in this context, is that their reflected starlight is largely filtered out from the narrowband APOD, revealing the elemental nature of the puffs themselves.

The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann

[Nitpicker]

The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann

Thanks Ann. Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.

I wonder what proportion of gas and dust lurks in the galaxy, undetected by science?

[Chris Peterson]

The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann

Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.

Maybe, maybe not. At the least you'd need to have some photometric information, which you can't get from this processed image. The nebula may have a faint emission, normally masked by broadband reflection. But in such a bright environment as this, we could still be seeing nothing but pure reflections of the surrounding emission sources.

[Ann]

The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann

Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.

Maybe, maybe not. At the least you'd need to have some photometric information, which you can't get from this processed image. The nebula may have a faint emission, normally masked by broadband reflection. But in such a bright environment as this, we could still be seeing nothing but pure reflections of the surrounding emission sources.

Click to view full size image

Purple emission/reflection nebula IC 432 (center) and blue reflection nebula
IC 431 (at right, next to a white star). Source: www.deepskyastro.com

Certainly you may be right, and I'm aware that SIMBAD calls IC 432 a reflection nebula, but I'd like to argue my point anyway. The picture at right is not the only color picture that shows that IC 432 is not the same color as neighboring reflection nebula IC 431. There is a red component to IC 432 that is totally lacking in IC 431.

Other images that show the same thing are this picture by RayM0506, this small image by sky-map, as well as this image by Jim Janusz.

Personally I find this 8 MB Hα-rich portrait of the Horsehead region to be particularly interesting. There can be no doubt that the B2IV-type star is sitting in the middle of a Hα nebula, which appears to be quite separate from other Hα nebulas in the field.

So if it looks like a duck, and quacks like a duck, should we then assume that it is a duck? Or if it is a hot star sitting in the middle of what appears to be an emission nebula, should we assume that the hot star is the ionizing source of the nebula?

Occam would say that we should.

Ann

[Chris Peterson]

Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.

Maybe, maybe not. At the least you'd need to have some photometric information, which you can't get from this processed image. The nebula may have a faint emission, normally masked by broadband reflection. But in such a bright environment as this, we could still be seeing nothing but pure reflections of the surrounding emission sources.

Certainly you may be right, and I'm aware that SIMBAD calls IC 432 a reflection nebula, but I'd like to argue my point anyway.

I'm not particularly arguing against your point, just trying to make it clear that when you have a narrowband image, it can be very ambiguous whether you're seeing emissions or reflections.

[Ann]

I forgot to credit the photographer for the stunning 8 MB Hα-rich picture of the Horsehead region. The photographer is Mark Wielgosz (2011). The technical detail that I find interesting is how much Hα has been used, and it appears to be a lot:

HaLHaRGB (4.5,4:4.5,4:4:4) Hrs

I don't see how a simple reflection nebula reflecting the blue light of a hot star could be so full of Hα. Couldn't we just as well expect the Pleiades reflection nebula to be Hα-red?

Click to view full size image

Red Pleiades?
WISE, IRSA, NASA; Processing by Francesco Antonucci

Click to view full size image

Red Pleiades?

Perhaps you mean that a dusty nebula, even in the presence of hot blue stars, would reflect the red light of an Hα nebula that is more distant the hot star seemingly sitting in the middle of the red and blue nebula. Can you name another such Hα reflection nebula, presuming that IC 432 is an example of one?

An alternative explanation might be that IC 432 is a reflection nebula superimposed on an emission nebula of unknown origin. But to me that is a rather contrived explanation, when an ionizing source is right there in the form of HD 37776. Bear in mind that there are other examples of stars that are ionizing an emission nebula and lighting up a reflection nebula at the same time. The most famous example is AE Aurigae.

Thanks for the brilliant photo, Mark!

Ann