  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\AA, 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.

Astrophysics