Arcturus (...). Astr. Also 4 arthurus, arturis; arture, ariture, arctour. [L. arctrus a Gr. ..., f. ... the Bear + ...
guardian, ward (from its situation at the tail of the Bear); the forms arture, etc. were from Fr.] The brightest star in the constellation Bootes; formerly, also, the whole constellation, and sometimes the Great Bear itself. (OED
http://www.geocities.com/~dubricius/arthur.html about name "Arthur" and relationship to Arcturus