The accretion of hydrogen onto a white dwarf star ignites a classical nova eruption1,2—a thermonuclear runaway in the accumulated envelope of gas, leading to luminosities up to a million times that of the Sun and a high-velocity mass ejection that produces a remnant shell (mainly consisting of insterstellar medium). Close to the upper mass limit of a white dwarf3 (1.4 solar masses), rapid accretion of hydrogen (about 10−7 solar masses per year) from a stellar companion leads to frequent eruptions on timescales of years4,5 to decades6. Such binary systems are known as recurrent novae. The ejecta of recurrent novae, initially moving at velocities of up to 10,000 kilometres per second7, must ‘sweep up’ the surrounding interstellar medium, creating cavities in space around the nova binary. No remnant larger than one parsec across from any single classical or recurrent nova eruption is known8,9,10, but thousands of successive recurrent nova eruptions should be capable of generating shells hundreds of parsecs across. Here we report that the most frequently recurring nova, M31N 2008-12a in the Andromeda galaxy (Messier 31 or NGC 224), which erupts annually11, is indeed surrounded by such a super-remnant with a projected size of at least 134 by 90 parsecs. Larger than almost all known remnants of even supernova explosions12, the existence of this shell demonstrates that the nova M31N 2008-12a has erupted with high frequency for millions of years.
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All relevant data generated or analysed during this study are available from the corresponding author on reasonable request.
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The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the Science and Technology Facilities Council (STFC). This work is based on observations made with the NASA/ESA HST, obtained from the Data Archive at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA) under NASA contract NAS 5-26555. These observations are associated with programmes GO:14125 and GO:14651 for which financial support for R.H., M.H., M.M.S. and A.W.S. was provided by NASA through grants from STScI. This work is based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. The Hobby–Eberly Telescope (HET) is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. HET is named in honour of its principal benefactors, William P. Hobby and Robert E. Eberly. We thank Z. Levay for creating a colour composite image of the nova super-remnant, I. A. Steele for assistance with the Liverpool Telescope spectra, K. L. Page for assistance with XSPEC, K. A. Misselt and D. Baer for assistance with the Steward 2.3-m observations, also M. Link and C. Proffitt, and W. Eck and K. Long, the programme coordinators and contact scientists for HST GO:14125 and GO:14651, respectively. M.J.D. and M.W.H. acknowledge financial support and a PhD studentship, respectively, from the STFC. N.M.H.V. acknowledges support from the European Commission through the Horizon 2020 Marie Sklodowska-Curie Actions Individual Fellowship 2014 programme (grant agreement number 659706). V.A.R.M.R. acknowledges financial support from the Fundação para a Ciência e a Tecnologia in the form of an exploratory project (reference IF/00498/2015), from the Center for Research and Development in Mathematics and Applications (strategic project UID/MAT/04106/2013), and from Enabling Green E-science for the Square Kilometer Array Research Infrastructure (ENGAGE SKA), POCI-01-0145-FEDER- 022217, funded by Programa Operacional Competitividade e Internacionalização (COMPETE 2020) and the Fundação para a Ciência e a Tecnologia, Portugal.
Nature thanks S. Shore and J. Sokoloski for their contribution to the peer review of this work.