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We present JWST/NIRSpec observations of a highly magnified star candidate ata photometric redshift of $z_{\mathrm{phot}}\simeq4.8$, previously detected inJWST/NIRCam imaging of the strong lensing (SL) cluster MACS J0647+7015($z=0.591$). The spectroscopic observation allows us to precisely measure theredshift of the host arc at $z_{\mathrm{spec}}=4.758\pm0.004$, and the star'sspectrum displays clear Lyman- and Balmer-breaks commensurate with thisredshift. A fit to the spectrum suggests a B-type super-giant star of surfacetemperature $T_{\mathrm{eff,B}}\simeq15000$ K with either a redder F-typecompanion ($T_{\mathrm{eff,F}}\simeq6250$K) or significant dust attenuation($A_V\simeq0.82$) along the line of sight. We also investigate the possibilitythat this object is a magnified young globular cluster rather than a singlestar. We show that the spectrum is in principle consistent with a star cluster,which could also accommodate the lack of flux variability between the twoepochs. However, the lack of a counter image and the strong upper limit on thesize of the object from lensing symmetry, $r\lesssim0.5$ pc, could indicatethat this scenario is somewhat less likely -- albeit not completely ruled outby the current data. The presented spectrum seen at a time when the Universewas only $\sim1.2$ Gyr old showcases the ability of JWST to study early starsthrough extreme lensing.

Reaching for the stars -- JWST/NIRSpec spectroscopy of a lensed star candidate at $z=4.76$