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 .
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]
3C 273: A Star-Like Object with Large Red-Shift by Maarten Schmidt. 1963, spectral identification and redshift
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