Source : Webster's Revised Unabridged Dictionary (1913)

Earthstar \Earth"star`\, n. (Bot.)
   A curious fungus of the genus {Geaster}, in which the outer
   coating splits into the shape of a star, and the inner one
   forms a ball containing the dustlike spores.

Source : WordNet®

earthstar
     n : any fungus of the family Geastraceae; in form suggesting a
         puffball whose outer peridium splits into the shape of a
         star