The word "αστέρι" (asteri)is used literally to describe the common celestial body.In former periods or still nowadays, in astronomy issues, the word "αστερας" (asteras)is being used, i.e polar star in greek is called "πολικός αστέρας" (polikos asteras).
The word is also used in the sense of a movie star or star of the show business in general. Here in greek you say "αστέρι"(asteri).
In special cases of very famous actresses or actors (or singers etc)who enjoy the glory of stardom we also use the word "σταρ"(star)in greek, which is the transliteration of the english star in greek letters.In several cases the word "σταρ" has negative connotations.

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Note added at 2002-06-19 01:10:21 (GMT)
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There is also the word \"άστρο\" (astro)