| Abstract
| - Context. The formation of a penumbra is crucial for our understanding of solar magnetism, but it has not been observed in detail. Aims. We aim to enhance our knowledge of how a sunspot penumbra forms and how sunspots grow in size. Methods. We present a data set of the active region NOAA 11024 acquired at the German VTT with speckle-reconstructed images in the G-band and Ca ii K. The data set includes spectropolarimetric profiles from GFPI in Fe i 617.3 nm and TIP in Fe i 1089.6 nm. Results. On 2009 July 4, at 08:30 UT, a leading spot without penumbra and pores of opposite polarity were present in the active region. For the next 4:40 h, we observed the formation of a penumbra in the leading spot at a cadence of 5 images per second. We produced speckle reconstructed images of 0 $\farcs$3 spatial resolution or better, interrupted by one large gap of 35 min and a few more small gaps of about 10 min. The leading spot initially has a size of 230 arcsec 2 with only a few penumbral filaments and then grows to a size of 360 arcsec 2. The penumbra forms in segments, and it takes about 4 h until it encircles half of the umbra, on the side opposite the following polarity. On the side towards the following polarity, elongated granules mark a region of magnetic flux emergence. Conclusions. This ongoing emergence appears to prevent a steady penumbra from forming on this side. While the penumbra forms, the umbral area is constant; i.e., the increase in the total spot area is caused exclusively by the growth of the penumbra. From this we conclude that the umbra has reached an upper size limit and that any new magnetic flux that joins the spot is linked to the process of penumbral formation.
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