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3C 273 is a bright quasar whose star-like optical appearance initially concealed its extragalactic character. In 1963, Maarten Schmidt identified its emission lines as a familiar series shifted by a common redshift of about $z=0.158$.

The identification connected the spectrum to a large cosmological distance and hence an enormous luminosity. The lesson is methodological: an object's unresolved angular size does not establish that it is an ordinary nearby star. Interpreting spectral-line patterns can change the inferred physical scale.^[ref]

Maarten Schmidt identified the large redshift of the quasar 3C 273 in his 1963 report. Recognizing familiar spectral lines at a common shifted wavelength scale made the seemingly star-like source a cosmological object, changing the interpretation of its luminosity.^[ref]

A quasar is an extremely luminous active galactic nucleus whose compact central emission can outshine its host galaxy and be observed across cosmological distances. Its apparent resemblance to a star originally concealed its nature: a small angular image does not establish a small physical size or a nearby distance. In 1963, Maarten Schmidt identified familiar Balmer series emission lines in the unusual optical spectrum of 3C 273, shifted together to a redshift of 0.158. The optical spectra came from a spectrograph on the 200-inch telescope at Palomar. Interpreting the shift as cosmological implied a distant, extraordinarily luminous galactic nucleus. Modern black-hole accretion models provide the compact power source for that luminosity.^[ref]