We briefly review the young field of spectropolarimetry of core-collapse
supernovae (SNe). Spectropolarimetry provides the only direct known probe of
early-time supernova (SN) geometry. The fundamental result is that asphericity
is a ubiquitous feature of young core-collapse SNe. However, the nature and
degree of the asphericity vary considerably. The best predictor of
core-collapse SN polarization seems to be the mass of the hydrogen envelope
that is intact at the time of the explosion: those SNe that arise from
progenitors with large, intact envelopes (e.g., Type II-plateau) have very low
polarization, while those that result from progenitors that have lost part (SN
IIb, SN IIn) or all (SN Ib) of their hydrogen (or even helium; SN Ic) layers
prior to the explosion tend to show substantial polarization. Thus, the deeper
we probe into core-collapse events, the greater the asphericity seems to be,
suggesting a fundamentally asymmetric explosion with the asymmetry damped by
the addition of envelope material.Comment: Invited review at Supernovae as Cosmological Lighthouses, 16-19 June,
Padua, IT. 6 page