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What causes Ha, Hb, OIII and SII emission lines?


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I presently researching narrow band filters and I'm keen to find out what actually causes H alpha, H beta, OIII and SII emission lines we see in nebula? I'm after a simple ex-explanation, not a doctoral thesis. I also want to know why some nebula emit in Hb and others in OIII or Ha.


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electrons whiz round the nucleus. They can do this in different "orbits" depending on how frisky the electron is. When the electron decides it has had enough it moves to a lower energy orbit. When it does this some energy is released. In some instances this energy is release as light at a very specific wavelength. The difference between hydrogen alpha and beta is the electron orbit which changes. The II in sulpher and III in oxygen must also relate to the particular electron layer.

I think it is the same process which can colour flames (apart from soot of course).

Why some have heavier elements than others will need someone with more knowledge of the formation of the cosmos than I do. I'm sure Narrowband Paul will be along shortly and all will be revealed!

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The emission lines are due to the fact that when the electrons in a particular atom are excited they jump from one energy state to the next and fall back again but only in discrete defined steps, so the energy emitted when the electrons falling back to a lower level is always the same, hence the discrete lines in the spectra.

Sodium (Na) has only 2 lines in visible yellow, hence sodium lamps look yellow to our eyes.

An anology would be climbing and jumping from different rungs of a step ladder, each time you jump off the same step you "release" the same energy and higher up the steps the more energy is released but you also climb up in discrete "steps" as well. These various steps (energy levels) are named H alpha H beta etc to descibe the "jump levels"

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Hydrogen makes up almost 99.999% of everything in the Universe ( including us!) when hydrogen gas is exposed to starlight, the energy is absorbed by the hydrogen gas which then re-emits this energy by shifting electrons around the nucleus. This gives rise to a series of "discrete" emmisions ( Balmer series etc) which are seen by us as bright lines of light at different wavelengths. The brightest emission is the H alpha ( in the red) and the H beta ( in the green) etc.

Similarly when Oxygen is energised it can become ionised ( ie have more electrons than usual) and when these are lost and the atom returns to it natural state, the light emitted gives the noticable OIII line in the green ( OIII means an atom of Oxygen "triple ionised" with extra electrons)

Sulpher can have the same type of emission from it's double ionised state and gives of a red light ( SII)

Most nebulae are made up from hydrogen ( and some minor elements, sometimes Sulpher) and dust hence the Ha / H beta emission. Planetary nebulae are very strong in OIII and the brighter emission areas in distant star forming galaxies can give the SII emissions.

Hope this helps......

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Basically yes!

If it wasn't for the "re-cycled star stuff" we're made of. Remember as well, we're 80% water which contains 2:1 mix of hydrogen. Through the actions of Super Novae converting the Hydrogen into heavier elements we get this "star stuff" otherwise we wouldn't be here!

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The simple answer is that nearby stars radiate their energy into a nearby gas cloud, the nebula. This then ionizes the gas in the nebula (Electrons gain energy). The nebula then re-emits the energy in the form of light (Photons) now at specific wavelengths determined by the ionized gas. Hydrogen being the most common.

Interestingly, young blue stars that emit lots of ultraviolet light ionize gas the most efficiently.

So depending on the gas and the amount of energy to ionize it, will all determine the emissions that we can detect.

Narrow band filter are simply tuned to let a specific frequency / wavelength of light pass through them.

OIII = Triple ionized oxygen


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Are you sure about that?

I always thought that eg OI was atomic oxygen, OII was Oxygen minus 1 electron, and OIII was oxygen minus 2 electrons. The "normal" rules of chemistry don't always apply in the high-energy / low-density realm of gaseus nebulae.


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Your correct!

The astronomers haven't changed the rules of Chemistry (yet!)

In Keith Robinson's very good book on the subject "Spectroscopy - The key to the Stars", page 28

A base element ( in this case Oxygen) is known as OI ( Chemical symbol+Roman numeral I); first ionisation, OII and second ionisation, OIII where two electrons have been lost.

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To clarify....in "normal" everyday chemistry, oxygen ions are O2- ie oxygen atoms plus 2 electrons (To make a closed shell of eight), whereas what we're talking about in spectroscopy are Oxygen atoms that have had 2 electrons knocked out. The oxygen that you and I breathe, O2 should really be called DiOxygen.



Had to change 8 for eight to avoid smiley

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Thanks for suggesting myself martinB. Sorry I turned up late. I feel the points have been answered.

Electrons orbit the nuclues of an atom in shells. The more postive charge in the shells the more strongly the electron is bound , and the more energy required to remove it.

hydrogen is by far the most abundant element in the universe, so it natural to look at the emission lines of hydrogen. The different emission lines series are named after the scientist who discovered, and are defined by the final state of the electron. As has already been mentioned, emission lines originate from a higher energy state and fall down to a lower energy level. The name of the series is defined by the lower energy level

If an emission from Hydrogen results with the electron ending up in the ground state (lowest energy, or n=1) then the emission line belongs to the Lyman series. The first emission is from the second level (n=2) to the first (n=1). This is called Lyman alpha, and has a wavelength of 121.5nm. This lies in the ultraviolet part of the spectrum. Cameras wont detect this wavelength. The next line is n=3 to n=1, this is called Lyman beta and has a shorter, higher energy wavelength

if the emission ends with the electron in the second level (n=2) the emission is called Balmer. The first line is n=3 to n=2. The wavelength is 656.3nm and is called Balmer (or Hydrogen) alpha. Normally hydrogen alpha. This is red. Cameras quite like it. The next emission is n=4 to n=2. This is called Hydrogen beta, and has a wavelength of 486.1nm. This is a turquoise blue colour. Your eyes like it.

if hydrogen is present (it always is in the universe) then it will emit all hydrogen wavelengths. It will emit Lymna alpha (you wont see it though, or with a CCD) and hydrogen alpha, and hydrogen beta, and even longer wavelngth ones.

The only difference is the relative intensity of the emission. Thanks to physics, the amounts of each emission you get are well defined. For example the Hydrogen alpha emission is 2.92 times stronger than the Hydrogen beta emission.

This means you can take a Hydrogen alpha image, divide the pixel intensity by 2.92 and hey presto...hydrogen beta for free. No filter required.

This is not the case for O[iII] and S[iI], which I have written correctly here. As already mentioned, the III means doubly ionised. This means two electrons have been stripped away. Oxygen has 7 more protons than hydrogen, so the electrons are much more tightly bound. Therefore O[iII] comes from areas that are hot or near stars emitting high energy photons, that are absorbed by the electrons.

So an image taken with an O[iII] filter should map high energy areas. These are typically found near young clusters in the centre of some nebulae. Young stars are hot, and hot stars emit UV radiation which can knock electrons out from their happy orbits.

The square brackets denote a 'forbidden wavelength'. This is an emission that can only occur in low temperature and low density areas like nebula, where collisions are infrequent. In fact the O[iII] wavelength was attributed to an element called nebulium until the 70's. This wavelength cannot be seen on earth.

The wavelength of O[iII] is 495 and 500.7nm...it is a doublet. The 500.7nm emission is usually a bit stronger. Nature loves lower energy stuff.

The S[iI] seems really wierd. Why sulphur? For reasons which fathom me, S[iI]shows up well in areas of compression, like where the expanding nebula hits the interstellar medium, which compresses the nebula.

So for me, H alpha acts as a backdrop, with O[iII] providing high energy details, and S[iI] providing compression info (like the wall in North american).

The H alpha and H beta lines are said to be correlated. That is, once you know the brightness of H alpha, you know the brightness f H beta. That is Ha =2.92 Hbeta...

but H alpha O[iII] and S[iI] are all uncorrelated. A nebula can have completely arbitrary amounts of all three. Each object has different ratios of each component.

O[iII] shows up well in planetary nebula, since they shine through a white dwarf with very high temperatures....which can ionise the oxygen.

The combo of Ha O[iII] and S[iI] creates a 3d looking image.

Which is cool.

Hope this helps.


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