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WikiPedia: Mắt_thường http://astronomy-mall.com/Adventures.In.Deep.Space... http://www.hurtlingrock.com/naked-eye-astronomy.ht... http://www.skyandtelescope.com/resources/darksky/3... http://www.springerlink.com/content/q1696565458u32... http://messier.obspm.fr/xtra/supp/m81naked.txt http://simbad.u-strasbg.fr/simbad/sim-sam?Criteria... //dx.doi.org/10.1007%2F978-1-84628-736-7_4 http://messier.seds.org/m/m081.html https://web.archive.org/web/20130921050820/http://...