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How did Maarten Schmidt identify the unusual spectral lines of 3C 273?

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six quasar host galaxies — Quasar Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).

In 1963, astronomer Maarten Schmidt solved the mystery of the unusual spectral lines observed in 3C 273 by recognizing that the emission lines were produced by hydrogen atoms that had undergone a significant redshift.1

Rather than representing unknown elements or exotic physics, the pattern of lines indicated that the light had been shifted toward longer, redder wavelengths due to the expansion of the universe. Specifically, Schmidt identified that the wavelength of each line was 1.158 times longer than the corresponding wavelength measured in a laboratory, corresponding to a redshift of 0.158. This interpretation revealed that 3C 273 was located more than two billion light-years away, establishing that quasars are among the most distant and luminous objects in the cosmos.12