Optical spectroscopy of supernova 1993J during its first 2500 days

We present 42 low-resolution spectra of supernova (SN) 1993J, our complete collection from the Lick and Keck observatories, from day 3 after explosion to day 2454, as well as one Keck high-dispersion spectrum from day 383. SN 1993J began as an apparent SN II, albeit an unusual one. After a few weeks, a dramatic transition took place, as prominent helium lines emerged in the spectrum. SN 1993J had metamorphosed from a SN II to a SN IIb. Nebular spectra of SN 1993J closely resemble those of SNe Ib and Ic, but with a persistent Hα line. At very late times, the Hα emission line dominated the spectrum, but with an unusual, boxlike profile. This is interpreted as an indication of circumstellar interaction.