A blue ring nebula from a stellar merger several thousand years ago

Stellar mergers are a brief but common phase in the evolution of binary star systems^1,2. These events have many astrophysical implications; for example, they may lead to the creation of atypical stars (such as magnetic stars³, blue stragglers⁴ and rapid rotators⁵), they play an important part in our interpretation of stellar populations⁶ and they represent formation channels of compact-object mergers⁷. Although a handful of stellar mergers have been observed directly^8,9, the central remnants of these events were shrouded by an opaque shell of dust and molecules¹⁰, making it impossible to observe their final state (for example, as a single merged star or a tighter, surviving binary¹¹). Here we report observations of an unusual, ring-shaped ultraviolet ('blue') nebula and the star at its centre, TYC 2597-735-1. The nebula has two opposing fronts, suggesting a bipolar outflow of material from TYC 2597-735-1. The spectrum of TYC 2597-735-1 and its proximity to the Galactic plane suggest that it is an old star, yet it has abnormally low surface gravity and a detectable long-term luminosity decay, which is uncharacteristic for its evolutionary stage. TYC 2597-735-1 also exhibits Hα emission, radial-velocity variations, enhanced ultraviolet radiation and excess infrared emission-signatures of dusty circumstellar disks¹², stellar activity¹³ and accretion¹⁴. Combined with stellar evolution models, the observations suggest that TYC 2597-735-1 merged with a lower-mass companion several thousand years ago. TYC 2597-735-1 provides a look at an unobstructed stellar merger at an evolutionary stage between its dynamic onset and the theorized final equilibrium state, enabling the direct study of the merging process.