It was discovered on September 27, 2005, by the Sloan Digital Sky Survey and the Nearby Supernova Factory.

[2][3] 2005gj was noted because it had qualities of both type Ia and type IIn supernovae, and because hydrogen emission lines were found in its spectrum (see hydrogen spectral series).

[3] These hydrogen lines, which were found on the spectrum at redshift z=0.0613, are thought to be indicative of interactions with a circumstellar medium (CSM; a donut-shaped, nebula-like ring of matter around a star) by the supernova's ejected matter or white dwarf progenitor.

The mass-loss history 2005gj's hydrogen lines suggest has been cited as evidence that luminous blue variable (LBV) hypergiants can be progenitors of thermonuclear supernovae.

[8] Scientists Denis Leahy and Rachid Ouyed from the University of Calgary contend that the incidence of a quark nova, a very luminous process involving the degeneration of neutrons into their constituent quarks, could explain the unusual magnitude of the luminosity.