As I understand, the heaviest bonded element found in space to date is iodine, in the form of iodide in sodium iodide. This is the only element heavier than iron on this list, and that makes sense to me because the nuclear fusion inside stars only continues until iron. The only way that heavier elements are formed is when a star is near death and the energy is so high in the center of the star that more nuclear fusion occurs, and then the elements are flung into space.

If this is true, then wherever they found the sodium iodide should be traces of more heavier-than-iron elements. Why then hasn't an element such as, say, bromine been found? Especially when it behaves so similarly to iodine.

This had stumped me for a while and information on the topic is sparse, so I'd just like some input on if it may be a limitation on observation of space or an astrochemical scenario where somehow iodine is more likely to form bonds in space than its heavier-than-iron counterparts

  • $\begingroup$ Why "bonded"? Is any atom not bonded except the noble gases (and some hydrogen)? $\endgroup$
    – DHMO
    Sep 10, 2016 at 2:24

2 Answers 2


I disagree with the assertion that Sodium Iodide has been found in circumstellar or interstellar space.

Sodium Iodide is not on the list Molecules in Space kept updated by University of Köln.

Instead it is likely that someone misunderstood an article such as Interstellar NaI absorption towards the stellar association ARA OB1, where NaI does not mean sodium iodide, but instead means neutral atomic sodium.

Also, LaO (lanthanum oxide) has been found in stellar atmospheres, and lanthanum is heavier than iodine. See table 5 here: http://www.ucl.ac.uk/phys/amopp/people/jonathan_tennyson/papers/311.pdf

as well as the 1948 article On the Occurrence of Lanthanum Oxide in S-Type Stars.


By "bonded", you mean in the form of a molecule or molecule ion? Only gaseous molecules can be found in space, because they have sharp spectroscopic lines. Heavy compounds condense on-/into stellar dust, which makes them mostly indistinguishable.

Iodine is heavy, but also forms only weak bonds, and such has high vapour pressure, simply speaking. Also true for any iodine compound, check the boiling points of the sodium halides.


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