Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs1,2. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun3—a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells3, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns4. These properties can be inferred from geometric model fitting5,6,7, but this indirect method does not provide information about the physical origin of the convective cells5,6,7. Here we report interferometric images of the surface of the evolved giant star π1 Gruis, of spectral type8,9 S5,7. Our images show a nearly circular, dust-free atmosphere, which is very compact and only weakly affected by molecular opacity. We find that the stellar surface has a complex convective pattern with an average intensity contrast of 12 per cent, which increases towards shorter wavelengths. We derive a characteristic horizontal granule size of about 1.2 × 1011 metres, which corresponds to 27 per cent of the diameter of the star. Our measurements fall along the scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection10,11,12.
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C.P. acknowledges the support of the Fonds National de la Recherche Scientifique (F.R.S.-FNRS), Belgium. C.S., G.S. are supported by the PRODEX office, Belgium. This research has been funded by the Belgian Science Policy Office under contract BR/143/A2/STARLAB (S.S., A.J., S.V.E.). K.K. is supported by a FRIA grant (Belgium). F.B. acknowledges funding by the National Science Foundation, NSF-AST numbers 1445935 and 1616483. J.K. acknowledges the Philip Leverhulme Prize (PLP-2013-110). S.V.E. thanks the Fondation ULB for its support. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement 730890 (OPTICON) and from the Austrian Science Fund (FWF) under project AP23006-N16. We thank all the ESO/VLTI staff for supporting our observations.