HII Regions

[Kenza]

The term H II refers to regions of interstellar dust abundant in Hydrogen which have been ionized by UV radiation from nearby hot stars and have had their electrons stripped off. Also, the roman numeral next to the element refers to the number of electrons that an atom is missing, in this case two electrons. How can a Hydrogen atom lose 2 electrons??? What am I missing here?

[JulianO]

Its confusing terminology -  but its what we're stuck with. In astronomy you have

• H₂ - so called molecular hydrogen - what we see on Earth normally.
• H I - which is called atomic hydrogen - there is enough radiation to split the two atoms apart, but not enough to ionise it in the main.
• H II - is ionised hydrogen - stripped of it's electron, needing sufficient energy to kick the electrons away.

Confusing - certainly, always annoyed me, but its what everyone uses in the field.

[Kenza]

I get it now! That is confusing. So basically the rule doesn't apply for Hydrogen as it does for other ionized atoms like S II or O III.

[JulianO]

No - its similar for O as well.

[cosmiquery]

Basically you subtract one from the roman numeral and that give you the number of missing electrons.