The number of tree stars is O*(1.357k).

Bernhard Fuchs Walter Kern Xinhui Wang The number of tree stars is O*(1.357k). 183-185 2006 25 Electronic Notes in Discrete Mathematics http://dx.doi.org/10.1016/j.endm.2006.06.070 db/journals/endm/endm25.html#FuchsKW06