The supernova SN 1978K has been noted for its lack of emission lines broader than a few thousand km s ^ { -1 } since its discovery in 1990 .
Modeling of the radio spectrum of the peculiar SN 1978K indicates the existence of H ii absorption along the line of sight .
To determine the nature of this absorbing region , we have obtained a high-dispersion spectrum of SN 1978K at the wavelength range 6530–6610 Å .
The spectrum shows not only the moderately broad H \alpha emission of the supernova ejecta but also narrow nebular H \alpha and [ N ii ] emission .
The high [ N ii ] \lambda 6583/H \alpha ratio , 0.8–1.3 , suggests that this radio absorbing region is a stellar ejecta nebula .
The expansion velocity and emission measure of the nebula are consistent with those seen in ejecta nebulae of luminous blue variables .
Previous low-dispersion spectra have detected a strong [ N ii ] \lambda 5755 line , indicating an electron density of 3–12 \times 10 ^ { 5 } cm ^ { -3 } .
We argue that this stellar ejecta nebula is probably part of the pre-shock dense circumstellar envelope of SN 1978K .
We further suggest that SN 1997ab may represent a young version of SN 1978K .