From Hubble Space Telescope images with 0 { \farcs } 05 resolution we identify four stars brighter than V = 25 mag within 2 { \farcs } 5 of SN 1993J in M81 which contaminated previous ground-based brightness estimates for the supernova progenitor .
Correcting for the contamination , we find that the energy distribution of the progenitor is consistent with that of an early K-type supergiant star with M _ { V } \approx - 7.0 \pm 0.4 mag and an initial mass of 13–22 M _ { \odot } .
The brightnesses of the nearby stars are sufficient to account for the excess blue light seen from the ground in pre-explosion observations .
Therefore , the SN 1993J progenitor did not necessarily have a blue companion , although by 2001 , fainter blue stars are seen in close proximity to the supernova .
These observations do not strongly limit the mass of a hypothetical companion .
A blue dwarf star with a mass up to 30 M _ { \odot } could have been orbiting the progenitor without being detected in the ground-based images .
Explosion models and observations show that the SN 1993J progenitor had a helium-rich envelope .
To test whether the helium abundance could influence the energy distribution of the progenitor , we calculated model supergiant atmospheres with a range of plausible helium abundances .
The models show that the pre-supernova colors are not strongly affected by the helium abundance longward of 4000 Å , and abundances ranging between solar and 90 % helium ( by number ) are all consistent with the observations .