  Supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 offer the first direct
evidence that supernovae are the progenitors of some GRBs. However, this burst
was unusually dim, smooth and soft compared to other bursts with known
afterglows. Whether it should be considered a prototype for cosmological GRBs
depends largely on whether the supernova explosion and burst were asymmetrical
or can be modeled as spherical. We address this question by treating the
acceleration of the supernova shock in the outermost layers of the stellar
envelope, the transition to relativistic flow, and the subsequent expansion
(and further acceleration) of the ejecta into the surrounding medium. We find
that GRB 980425 could plausibly have been produced by a collision between the
relativistic ejecta from SN 1998bw and the star's pre-supernova wind; the model
requires no significant asymmetry. This event therefore belongs to a dim
subclass of GRBs and is not a prototype for jet-like cosmological GRBs.
