SN 2005df was a Type Ia supernova in the barred spiral galaxy NGC 1559, which is located in the southern constellation of Reticulum.

[8][5] It was positioned at an offset of 15.0″ east and 40.0″ north of the galaxy's nucleus,[2] reaching a maximum brightness of 12.3 on August 18.

[7] The supernova luminosity appeared unreddened by dust from its host galaxy.

[9] The progenitor was a carbon-oxygen white dwarf close to the Chandrasekhar limit, making a merger scenario unlikely.

Modelling of the explosion shows a low central density for a hydrogen accretion scenario, suggesting the donating companion was a helium star or a tidally-disrupted white dwarf.