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How does spectroscopy confirm that a blue object is a quasar?

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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).

Spectroscopy is the definitive method for confirming that a blue object is a quasar. While astronomers initially identify potential candidates by their unusually blue color, these objects can be confused with stars; therefore, subsequent spectroscopic analysis is required for verification.12

Spectroscopy confirms a quasar by revealing a distinct pattern of emission lines that are shifted toward longer, redder wavelengths, a phenomenon known as redshift. These emission lines originate from hydrogen atoms and other ions located in gas clouds surrounding the quasar's central accretion disk.13

When astronomers analyze the spectra, they look for these specific signatures of high-velocity gas clouds, which indicate that the object is not a nearby star but a distant, highly luminous active galactic nucleus powered by a supermassive black hole.13