HIGHLIGHTS

DON'T STOP NOVAE

• Ancients coined the term “novae” to describe the appearance of new stars in the sky that faded away after a few weeks or months. As comets and eclipses, these were considered premonition for future events. Nowadays, astronomers use this term to designate an explosive event on the surface of a white dwarf star, a compact stellar remnant with the mass of our Sun compressed to the size of the Earth. In a nova event, the material accreted onto the surface of the white dwarf from a late type binary companion reaches a critical mass and, under the effects of the high gravity of the white dwarf, a thermonuclear runaway is ignited. Material is ejected at speeds of hundreds to even a few thousands km/s in a nova explosion. This material forms a nova shell that expands swiftly, and soon can be resolved by space and ground-based telescopes.
  
  The expansion of nova shells can be witnessed in time-scales of decades or even only a few years in some cases (e.g., Sab 142, Santamaría et al. 2019). Textbook models based on simple assumptions predict that the expansion velocity of a nova shell slows down to 50% of its initial value after 50—100 years. To test this classical prediction, high-quality images of nova shells in astronomical archives have been complemented with contemporary sub-arcsec images obtained at the Nordic Optical Telescope in the island of La Palma to investigate the present expansion of five nova shells with ages ranging from 50 to 130 years (Santamaría et al. 2020). Contrary to the theoretical expectations, the nova shells in our sample have a free expansion that will take effect until their very end, when they dilute into the interstellar medium (Santamaría et al. 2022). Apparently, the mechanical energy and momentum of the material in a nova shell is sufficiently large to sweep away the circumstellar material around the white dwarf. The expansion of the nova shells included in this study is notable in the “timelapse” movie of DQ Her, a cataclysmic variable system that experienced an outburst on December 1934.