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What is “OII”?

-- Susan | March 28, 2025
Color composite image of distant galaxy SXDF-NB1006-2

Question:

Can you explain Oll to me? I read it in the following paragraph but can’t find any information about it’s meaning:
This is NGC 2467, with the common moniker of Skull and Crossbones Nebula. Located in the constellation Puppis, it is a large star forming region, rich in ionizing hydrogen and even OII.

-- Susan

Answer:

“OII” is singly-ionized oxygen, in what is called spectroscopic notation.

-- Jeff Mangum