The Type II-P supernova 2004dj in the nearby galaxy NGC 2403 occurred at a position coincident with object 96 in the list of luminous stars and clusters in this galaxy published by Sandage in 1984 .
The coincidence is established definitively through astrometric registration of our ground-based archival images of NGC 2403 with our recent images showing the SN .
The archival images show that Sandage 96 is slightly resolved from the ground .
Pre-outburst blue spectrograms obtained by Humphreys and Aaronson reveal that Sandage 96 has a composite spectrum , dominated in the blue region by A- and B-type stars , while infrared photometry shows that Sandage 96 also contains red supergiants .
These results demonstrate that Sandage 96 is a young compact cluster .
We have studied the stellar population of Sandage 96 , using published photometric measurements combined with a chi-square-fitting code .
We derive a cluster age of 13.6 Myr , a reddening of \mbox { $E ( 4405 - 5495 ) $ } = 0.172 , and a total stellar mass of 24,000 M _ { \odot } .
For this cluster age , the SN progenitor had a main-sequence mass of 15 M _ { \odot } .
Post-outburst photometry of Sandage 96 may establish whether the progenitor was a red or blue supergiant .