The early evolution of a supernova (SN) can reveal information about the environment and the progenitor star. When a star explodes in vacuum, the first photons to escape from its surface appear as a brief, hours-long shock-breakout flare^1,2, followed by a cooling phase of emission. However, for stars exploding within a distribution of dense, optically thick circumstellar material (CSM), the first photons escape from the material beyond the stellar edge and the duration of the initial flare can extend to several days, during which the escaping emission indicates photospheric heating³. Early serendipitous observations^2,4 that lacked ultraviolet (UV) data were unable to determine whether the early emission is heating or cooling and hence the nature of the early explosion event. Here we report UV spectra of the nearby SN 2023ixf in the galaxy Messier 101 (M101). Using the UV data as well as a comprehensive set of further multiwavelength observations, we temporally resolve the emergence of the explosion shock from a thick medium heated by the SN emission. We derive a reliable bolometric light curve that indicates that the shock breaks out from a dense layer with a radius substantially larger than typical supergiants.

超新星（SN）的早期演化可以揭示有关环境和祖星的信息。当一颗恒星在真空中爆炸时，第一个从其表面逸出的光子会出现短暂的、长达数小时的冲击爆发耀斑^1,2，然后是发射的冷却阶段。然而，对于在致密、光学厚的星周物质（CSM）分布内爆炸的恒星来说，第一个光子从恒星边缘以外的物质中逸出，并且初始耀斑的持续时间可以延长到几天，在此期间逸出的发射表明光球供暖³ .早期的偶然观测^2,4由于缺乏紫外线 (UV) 数据，无法确定早期排放是加热还是冷却，从而无法确定早期爆炸事件的性质。在这里，我们报告了梅西耶 101 (M101) 星系中附近 SN 2023ixf 的紫外光谱。利用紫外数据以及一系列全面的进一步多波长观测，我们暂时解决了由超短波发射加热的厚介质中爆炸冲击的出现。我们得出了可靠的辐射热光曲线，表明激波是从半径远大于典型超巨星的致密层爆发的。