I have an ongoing dispute with everybody. I say an OIII nebula filter cannot be “oh-three,” since OIII stands for doubly ionized oxygen atoms. I call it an “oh-two” filter. Who’s right?

This composite image of the Bubble Nebula combines two images taken with Hydrogen-alpha (red) and an OIII (yellow) filters.
Bill Snyder / SkyandTelescope.com Photo Gallery

Sorry Philip, you lose. Not only is “oh-three” the universal usage; it makes sense.

Well, sort of. A neutral, non-ionized oxygen atom, which has lost zero electrons, is called O I (“oh-one”) because it’s in the first of many possible ionization states. An oxygen atom missing one electron is in the second state, O II, and so on. This is why bright nebulae are called H II (“H-two”) regions, even though a hydrogen atom has only one electron to lose!

— Alan M. MacRobert


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