  There is considerable controversy surrounding the nature of M1-78, a compact
nebula located beyond the Perseus arm. It was first classified as a planetary
nebula and is nowadays generally considered to be a compact HII region. To
investigate the nature M1-78 further, we present a detailed spectroscopic study
of M1-78 in the optical and near-infrared. M1-78 is a high-density nebula with
substantial physical differences between its two main morphological zones: a
bright arc to the SW and a blob of emission in the NE. Specifically, the blob
in the NE has a higher electron temperature and visual extinction than the SW
arc. The most important result, however, is the confirmation of a nitrogen
enrichment in M1-78. This enrichment is stronger at the location of the NE blob
and is correlated with a defficiency in the O abundance and a (dubious) He
enrichment. Such an abundance pattern is typical of ejecta nebulae around
evolved massive stars such as Wolf-Rayet and Luminous Blue Variable stars. The
spatial variations in the physical conditions and chemical abundances and the
presence of more than one possible ionizing source indicates, however, that
M1-78 is better described as a combination of a compact HII region + ejecta.
Finally, we detect H2 emission that extends over a large (~30 arcsec) area
around the ionized nebula. Analysis of the near-infrared H2 lines indicates
that the excitation mechanism is UV fluorescence.
