So what exactly is HA?

[irtuk]

I was just sending a pic to my dad, yes, don't be fooled by the beard I'm not Santa's grandad, and I said "this is Hydrogen Alpha data" and I was then going to explain what that was when it occurred to me that I don't know.

I know that its "light" at a particular wavelength of 656 or so nm but is is emitted by hydrogen or filtered through hydrogen?

so in a HA sub, is the recorded signal starlight that has passed through a region dense in hydrogen, or light that has been emitted by hydrogen ...erm ...doing something? I want to say combusting but my knowledge sort of hits a brick wall after the spelling of "Hydrogen" and I am certain combusting is not the right word.

 

Ed.

 

[Stu]

So, the bit I do know is that it is caused by hydrogen atoms emitting photons of light in the Ha frequency, not by light travelling through a region of hydrogen.

The bit I could claim to know in detail but don’t really is this bit, which causes it:

https://en.m.wikipedia.org/wiki/H-alpha

 

[ONIKKINEN]

Hydrogen alpha is a type of emission from the element hydrogen at the very specific wavelength of 656nm and some decimals. By emission in this case i mean that the hydrogen itself is "glowing in the dark" so to speak and not just reflecting light that hits it. Emission from emission nebulae happen when a bit of gas gets ionized by a high energy photon. Typically in popular nebulae just a handful or even just one star is responsible for "powering up" the nebula and the emission of elements. Once the stars die or drift away, the nebula will also dim down. For example the Orion nebula is being powered by the trapezium, a tight-ish cluster of very bright stars that ionize a large area around them to produce the H-alpha rich nebula of M42. These stars will probably die before they leave the Orion nebula because they are so massive and so have short lifespans. In this way H-alpha is a convenient way to see where there is star formation and bright young stars, even if the stars themselves are still embedded in dust or gas that blocks visible light. When imaging other galaxies, like the Triangulum galaxy you can start to see pink-ish glow from bright clusters at very short integration times. This just means that there is extreme star formation going on in there if the Ha signal is strong enough to be picked up all those lightyears away in a short amount of time.

Now in astrophotography scenarios most of the light in a Ha filtered shot is from Ha, but some of it is light that also just happens to be that wavelength and has nothing to do with the ionized gas you're trying to capture. This is because stars emit light across the entire spectrum, and some of it also hits the 7nm (or 3nm, or whatever) band that the Ha filter lets trough, even if it was not emitted by ionized hydrogen. This is also why narrower narrowband filters work better, since Ha is always that exact same wavelength (plus or minus doppler shift, which really doesn't matter that much in most targets).

Heres a nice video from a good youtube channel that touches on the subject. Sixty symbols has a lot more interesting videos to watch on various astronomy topics, if you're interested.

https://www.youtube.com/watch?v=CH880_VrxxU

 

[IB20]

Going back to my a-level chemistry and trying to simplify it, is it not the energy emission from a hydrogen electron being excited by energy, moving away and then moving back towards the nucleus? Different “energy shell” or orbits = different wavelength?

[vlaiv]

Hydrogen Balmer series are created when single electron in excited hydrogen atom (one proton + one electron) - makes jump to ground state (lowest energy orbital) and releases photon of specific energy (energy difference between levels).

https://en.wikipedia.org/wiki/Balmer_series

4 of those wavelengths are in visible part of spectrum.

[Stu]

1 minute ago, IB20 said:

Going back to my a-level chemistry and trying to simplify it, is it not the energy emission from a hydrogen electron being excited by energy, moving away and then moving back towards the nucleus? Different “energy shell” or orbits = different wavelength?

From the Wiki link:

H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air; it occurs when a hydrogen electron falls from its third to second lowest energy level.

[vlaiv]

4 minutes ago, Stu said:

From the Wiki link:

H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air; it occurs when a hydrogen electron falls from its third to second lowest energy level.

Yes, from higher to second orbital.

Going to ground state is Lyman series but that is all in UV part of spectrum.

[IB20]

My brain starts to melt when we get to the quantum mechanics. 😅 Can’t help feel that people were a lot smarter back in the day, maybe they’d struggle to send a tiktok though so swings and roundabouts.

[dmki]

2 hours ago, irtuk said:

I was just sending a pic to my dad, yes, don't be fooled by the beard I'm not Santa's grandad, and I said "this is Hydrogen Alpha data" and I was then going to explain what that was when it occurred to me that I don't know.

I know that its "light" at a particular wavelength of 656 or so nm but is is emitted by hydrogen or filtered through hydrogen?

so in a HA sub, is the recorded signal starlight that has passed through a region dense in hydrogen, or light that has been emitted by hydrogen ...erm ...doing something? I want to say combusting but my knowledge sort of hits a brick wall after the spelling of "Hydrogen" and I am certain combusting is not the right word.

 

Ed.

 

the light that you are recording is emitted by hydrogen atoms in space.

because of the the way the universe works electrons around atoms behave like waves and can exist in specific resonant orbitals, which we label s, p, d and f, these are more clouds than planetary orbitals and levels that we label as 1,2,3....

each of these levels and orbitals is associated with a precise energy, and the electron whilst it is under the influence of the proton in the nucleus must have enough energy to be in one of this orbital levels.

now with hydrogen we are usually thinking of s orbital which is a spherical cloud around the proton, and when the hydrogen atom is at minimum energy its ground state, the electron is in the 1s orbital.

there are numerous ways in which an electron can be knocked into an excited state, which would be any level higher than 1, but once there is will be drop down to the ground state and it does not have to do it in one step.

each time it drops down a level, it must loose the energy difference between the two levels. it does this by radiating an photon whos energy determines its wavelength

so in the case of hydrogen-alpha its a transition from level 3 to level 2, hydrogen-beta is 4 to 2 and blue/green, hydrogen-gama 5-2 and blue, level infinity to 2 is violet

we dont really discuss transitions to level 1 as they are in the ultraviolet and space telescope stuff and transition to level 3 or higher are infarred or lower

you can get a more detailed overview of the subject on wikipedia

 

Now the question of dense hydrogen clouds is interesting, in these clouds hydrogen atoms can absorb hydrogen alpha photons and then re-emitted as a new photon in any direction. so as hydrogen alpha light travels through a dence hydrogen cloud  towards earth its at risk of being scattered in all directions, thus less photons reach our telescope. so actually dense hydrogen clouds can be opaque to hydrogen alpha and its series siblings whist transparent to other light frequencies