APOD: The Galactic Core in Infrared (2015 Jan 18)

[Chris Peterson]

What do you mean by "H2"? H II? H₂? Your notation is ambiguous.

I meant H2. If I had meant HII, I would have said HII.

Well, I think that deuterium in its oxide form has some near-IR peaks. Nothing in the several micron range that I know of, however.

[geckzilla]

What do you mean by "H2"? H II? H₂? Your notation is ambiguous.

I meant H2. If I had meant HII, I would have said HII.

Well, I think that deuterium in its oxide form has some near-IR peaks. Nothing in the several micron range that I know of, however.

See http://en.wikipedia.org/wiki/Hydrogen_s ... 2_.3D_3.29
Our nebula would be the one visible at 1875 nm. It's the first one on the list just like H-alpha is the first one on the list for the Balmer series.

...I have not learned enough physics to know what n' is.

[Chris Peterson]

I have not learned enough physics to know what n' is. :(

It's just the quantum energy level of the start (or end) of the transition resulting in the emission or absorption of a particular wavelength photon.

[geckzilla]

I have not learned enough physics to know what n' is.

It's just the quantum energy level of the start (or end) of the transition resulting in the emission or absorption of a particular wavelength photon.

So Balmer-alpha = H-alpha? All the series are for hydrogen but for some reason Balmer's is the only one that took hydrogen into the name in common use.

[Chris Peterson]

So Balmer-alpha = H-alpha? All the series are for hydrogen but for some reason Balmer's is the only one that took hydrogen into the name in common use.

The Balmer series was the first described, so its transitions commonly carry the name hydrogen, with alpha for the lowest energy transition, beta for the next, and so on. Next came the Lyman series, but it would have been confusing to use anything other than "Lyman" for those transitions. And the same for the other series discovered after that.

[Ann]

Here are some facts about the hydrogen absorption and emission lines. The Paschen series is likely to become ever more interesting as astronomy is turning more and more of its attention to the infrared part of the spectrum.

Ann

[Chris Peterson]

Here are some facts about the hydrogen absorption and emission lines. The Paschen series is likely to become ever more interesting as astronomy is turning more and more of its attention to the infrared part of the spectrum.

I'm not sure of that. These higher order series represent increasingly uncommon transitions, and are correspondingly weak. For the most part, narrowband emission images are used to trace specific elements or molecules, and you only need one emission line for that purpose. In the case of singly ionized hydrogen, that's almost always H-alpha. There are a few other fairly bright emission lines, but they're seldom examined. I'm not sure what the scientific value would be in looking at the higher order series.

[geckzilla]

Here are some facts about the hydrogen absorption and emission lines. The Paschen series is likely to become ever more interesting as astronomy is turning more and more of its attention to the infrared part of the spectrum.

I'm not sure of that. These higher order series represent increasingly uncommon transitions, and are correspondingly weak. For the most part, narrowband emission images are used to trace specific elements or molecules, and you only need one emission line for that purpose. In the case of singly ionized hydrogen, that's almost always H-alpha. There are a few other fairly bright emission lines, but they're seldom examined. I'm not sure what the scientific value would be in looking at the higher order series.

In this case, the Paschen series is useful for detecting structures hidden behind dust. It could be very useful for space telescopes but not something useful to anyone sitting under a significant amount of atmosphere. A quick search last night showed me that Paschen-alpha observations have been done at Atacama. Makes sense.

[Chris Peterson]

In this case, the Paschen series is useful for detecting structures hidden behind dust. It could be very useful for space telescopes but not something useful to anyone sitting under a significant amount of atmosphere. A quick search last night showed me that Paschen-alpha observations have been done at Atacama. Makes sense.

It does, although these are not very long wavelengths, so dust is still attenuates significantly. For really peering through dust, even higher order series with lines of several micrometers or more (or into the sub-millimeter and millimeter spectrum) ought to be even more useful, assuming they're strong enough to be detected at all.

[geckzilla]

It does, although these are not very long wavelengths, so dust is still attenuates significantly. For really peering through dust, even higher order series with lines of several micrometers or more (or into the sub-millimeter and millimeter spectrum) ought to be even more useful, assuming they're strong enough to be detected at all.

It was apparently good enough to do a large survey of the galactic core with.

[Chris Peterson]

It does, although these are not very long wavelengths, so dust is still attenuates significantly. For really peering through dust, even higher order series with lines of several micrometers or more (or into the sub-millimeter and millimeter spectrum) ought to be even more useful, assuming they're strong enough to be detected at all.

It was apparently good enough to do a large survey of the galactic core with.

Oh, sure. With a scattering wavelength dependence on the order of the inverse fourth power, even a little over a micrometer is going to be a lot better than shorter visible wavelengths. Still, it's a dusty universe (and the galactic core isn't all that dusty compared to many other interesting places). But it's also hard to make narrowband filters in the several micron wavelength range. They get quite broad (which means lousy S/N when looking at weak lines), and they tend to not be very transmissive at their peak. That's probably one reason why we don't see them used on longer wavelength infrared cameras too much.

[starsurfer]

I'm not familiar with what anyone is talking about considering I'm optically minded but this is a fantastic discussion!

[geckzilla]

I'm not familiar with what anyone is talking about considering I'm optically minded but this is a fantastic discussion!

It bothers me that "optical" is used so commonly to refer to just the visible spectrum. Optics involves all light, visible and non.

[Chris Peterson]

It bothers me that "optical" is used so commonly to refer to just the visible spectrum. Optics involves all light, visible and non.

That is generally true. But when you say it, what do you mean by "light"?

[geckzilla]

It bothers me that "optical" is used so commonly to refer to just the visible spectrum. Optics involves all light, visible and non.

That is generally true. But when you say it, what do you mean by "light"?

Photons.

[Chris Peterson]

It bothers me that "optical" is used so commonly to refer to just the visible spectrum. Optics involves all light, visible and non.

That is generally true. But when you say it, what do you mean by "light"?

Photons.

Well, "light" usually refers to the part of the electromagnetic spectrum where we apply optical methodologies. Somewhere below the short end of the UV range we don't use conventional optics, but particle manipulators. Somewhere between far IR and submillimeter we start using things like waveguides, and think more in terms of manipulating radio signals, not photons.

We don't generally use "optical" to refer to all parts of the spectrum. It's reasonably wider than just the visible spectrum, but most of the spectrum is outside of what we typically call "light".

[geckzilla]

And yet optical phenomena can be observed in radio signals... because it's light.

(Please correct me if this statement is totally out of sync with reality. It's just the way I understand things so far.)

[Chris Peterson]

And yet optical phenomena can be observed in radio signals... because it's light.

(Please correct me if this statement is totally out of sync with reality. It's just the way I understand things so far.)

I don't know what you mean by "optical phenomena". Radio is certainly part of the electromagnetic spectrum. Its energy is certainly carried by photons. But you will not usually see radio energy referred to as "light", nor will you usually see the methods used to produce or detect radio waves referred to as "optics".

[geckzilla]

I mean this:
https://www.facebook.com/photo.php?fbid=717684661611165

After I saw that picture I stopped thinking of radio as distinct from light. It was a bit of an epiphany for me. I know lenses and mirrors are no longer useful when dealing with x-rays and radio waves but I know of no word other than optics for the study of light. I see no reason why it should be separated and I know of no other word for the study of the very long or very short wavelengths.

[Chris Peterson]

I mean this:
https://www.facebook.com/photo.php?fbid=717684661611165

After I saw that picture I stopped thinking of radio as distinct from light. It was a bit of an epiphany for me. I know lenses and mirrors are no longer useful when dealing with x-rays and radio waves but I know of no word other than optics for the study of light. I see no reason why it should be separated and I know of no other word for the study of the very long or very short wavelengths.

I don't really see where that image led you, but it doesn't matter. As long as you understand that in common usage, "light" is generally used only for EM in the visible and a bit on either side, where we see it manipulated using refractive materials, which is mainly what "optics" applies to, as well.

[geckzilla]

It's hard to see because of the choice of coloration but there is a diffraction pattern. The part which most interests me is in the magenta. Before this picture, radio was just a thing that music broadcast over and was collected mysteriously by antennae.

[Chris Peterson]

It's hard to see because of the choice of coloration but there is a diffraction pattern. The part which most interests me is in the magenta. Before this picture, radio was just a thing that music broadcast over and was collected mysteriously by antennae.

Well, I remember studying antenna design. Diffraction is certainly part of that. An image like this lets us see that radio and light exhibit similar behavior. You can look at x-ray diffraction patterns, too, and see that even very short wavelengths also exhibit such behavior. It's a common property of all EM.

[geckzilla]

Well, that's fine. I'll be an annoying oddball, then. To me the cutoff point at somewhere beyond visible doesn't make a lot of sense. It seems arbitrary. Kind of like the definition of a planet, which made sense until some people put a lot of thought into it and then I started thinking about it too and then it stopped making total sense and now I don't really agree with the IAU either.

[Chris Peterson]

Well, that's fine. I'll be an annoying oddball, then. To me the cutoff point at somewhere beyond visible doesn't make a lot of sense. It seems arbitrary. Kind of like the definition of a planet, which made sense until some people put a lot of thought into it and then I started thinking about it too and then it stopped making total sense and now I don't really agree with the IAU either.

There is no clear cutoff, or even any formal definitions. There's just conventional usage, based very practically on how the radiation is produced, manipulated, and detected. Basically, technology places different parts of the spectrum into different convenient domains, each with their own nomenclature. From the standpoint of theoretical physics, the same equations govern the entire range of EM. From the standpoint of experimental physics, radio, submillimeter, light, x-rays, and high energy gamma rays are all very different things.

[geckzilla]

I totally agree the theory vs experimental thing and I understand the segregation for that purpose but having a simple name (light) to call all of it would make it a lot friendlier to people and remind them that they can use the analogy of visible light, which most people understand on a basic level, for all other types of electromagnetic radiation. Calling it radiation, submillimeter, radio, x-rays, etc. creates technical barriers for lay people to understand. It's scary, technical, and mysterious. Visible light is comfortable and familiar. The people at Chandra understood this first and made sure to always include a chart showing where the light being represented by the picture actually fits in the overall spectrum. That, along with image swaps allows interested individuals to comprehend what's being presented more easily.