A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway [ ] .
Complex structures observed in the ejecta at late stages [ ] could result from interactions with the companion during the common envelope phase [ ] .
Alternatively , the explosion could be intrinsically bipolar , resulting from a localized ignition on the surface of the white dwarf [ ] or as a consequence of rotational distortion [ ] .
Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes [ ] .
Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013 , starting from one day after the explosion and continuing with extensive time coverage during the first 43 days .
Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope .
The optical depth of the ejected material changes as it expands .
We detect an ellipticity in the light distribution , suggesting a prolate or bipolar structure that develops as early as the second day .
Combining the angular expansion rate with radial velocity measurements , we derive a geometric distance to the nova of 4.54 \pm 0.59 kpc from the Sun .