Australia's Northern Star heads for best week since early August

** Shares of Northern Star Resources NST.AX up 9.8% so far this week, set for best week since the week ended August 7 if current gains hold

** Gold miner said on Monday it rejected an unsolicited takeover offer from Gold Fields GFIJ.J, saying it undervalued the company

** Offer valued Northern Star at about A$38.7 billion ($26.80 billion)

** A successful bid would have created the world's second-biggest gold producer after Newmont NEM.N

** Stock rises as much as 2.4% to A$24.330 on Friday

** Stock down over 9% YTD, including session's moves

($1 = 1.4440 Australian dollars)