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In page Active galactic nucleus:
"A major breakthrough was the measurement of the redshift of the quasar 3C 273 by Maarten Schmidt, published in 1963.[14] Schmidt noted that if this object was extragalactic (outside the Milky Way, at a cosmological distance) then its large redshift of 0.158 implied that it was the nuclear region of a galaxy about 100 times more powerful than other radio galaxies that had been identified. Shortly afterward, optical spectra were used to measure the redshifts of a growing number of quasars including 3C 48, even more distant at redshift 0.37.[15]
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