I present an overview of optical observations (mostly spectra) of Type II,
Ib, and Ic supernovae (SNe). SNe II are defined by the presence of hydrogen,
and exhibit a very wide variety of properties. SNe II-L tend to show evidence
of late-time interaction with circumstellar material. SNe IIn are distinguished
by relatively narrow emission lines with little or no P-Cygni absorption
component and (quite often) slowly declining light curves; they probably have
unusually dense circumstellar gas with which the ejecta interact. Some SNe IIn,
however, might not be genuine SNe, but rather are ``impostors'' ---
specifically, super-outbursts of luminous blue variables. SNe Ib do not exhibit
the deep 6150 Angstrom absorption characteristic of ``classical'' SNe Ia;
instead, their early-time spectra have He I absorption lines. SNe Ic appear
similar to SNe Ib, but lack the helium lines as well. Spectra of SNe IIb
initially exhibit hydrogen, yet gradually evolve to resemble those of SNe Ib;
their progenitors seem to contain only a low-mass skin of hydrogen.
Spectropolarimetry thus far indicates large asymmetries in the ejecta of SNe
IIn, but much smaller ones in SNe II-P. As one peers deeper into the ejecta of
core-collapse SNe, the asymmetry (indicated by the amount of polarization)
seems to increase. There is intriguing, but inconclusive, evidence that some
peculiar SNe IIn might be associated with gamma-ray bursts. The rates of
different kinds of SNe as a function of Hubble type are still relatively poorly
known, although there are good prospects for future improvement.Comment: 19 pages, 10 figures. To appear in "Young Supernova Remnants," ed. S.
S. Holt (New York: American Institute of Physics), 200
