The progenitor of type II-plateau supernova ( SN ) 2004dj is identified with a supergiant in a compact star cluster known as ” Sandage Star 96 ” ( S96 ) in the nearby spiral galaxy NGC 2403 , which was fortuitously imaged as part of the Beijing-Arizona-Taiwan-Connecticut ( BATC ) Multicolor Sky Survey from Feb 1995 to Dec 2003 prior to SN 2004dj .
The superior photometry of BATC images for S96 , taken with 14 intermediate-band filters covering 3000 - 10000 Å , unambiguously establishes the star cluster nature of S96 with an age of \sim 20 Myr , a reddening of \hbox { E } ( B - V ) \sim 0.35 mag and a total mass of \sim 96 , 000 M _ { \odot } .
The compact star cluster nature of S96 is also consistent with the lack of light variations in the past decade .
The SN progenitor is estimated to have a main-sequence mass of \sim 12M _ { \odot } .
The comparison of our intermediate-band data of S96 with the post-outburst photometry obtained as the SN has significantly dimmed , may hopefully conclusively establish the nature of the progenitor .