I suspect they determined the heavy metal using spectrometry. Essentially, you break the light from an artifact, like a star, or a nebula, and you will get rainbow bars, and the bars that are illuminated tell you the elements that are there. Your question is still valid, how did they determine there are heavy elements? Did telescopes just magically improved and we found new elements? These heavy elements should have been detectable 50 years ago... What changed?