Very-long-baseline interferometer observations of the nucleus of M81 show an unresolved radio source with linear dimensions not exceeding about 1300 AU. This is the smallest extragalactic radio source whose size has been directly measured. Its luminosity is intermediate between that of the compact radio source at the galactic center and the luminosity of compact sources found in the centers of extended radio galaxies and quasars.
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Kellermann et al. (1976) studied this question.