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

We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of z(phot) similar or equal to 4.8, previously detected in JWST/NIRCam imaging of the strong lensing (SL) cluster MACS J0647+7015 (z = 0.591). The spectroscopic observation allows us to precisely measure the redshift of the host arc at z(spec) = 4.758 +/- 0.004, and the star's spectrum displays clear Lyman- and Balmer-breaks commensurate with this redshift. A fit to the spectrum suggests a B-type super-giant star of surface temperature T-eff,T-B similar or equal to 15 000 K with either a redder F-type companion (T-eff,T-F similar or equal to 6 250 K) or significant dust attenuation (A(V) similar or equal to 0.82) along the line of sight. We also investigate the possibility that this object is a magnified young globular cluster rather than a single star. We show that the spectrum is in principle consistent with a star cluster, which could also accommodate the lack of flux variability between the two epochs. However, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, r less than or similar to 0.5 pc, could indicate that this scenario is somewhat less likely - albeit not completely ruled out by the current data. The presented spectrum seen at a time when the Universe was only similar to 1.2 Gyr old showcases the ability of JWST to study early stars through extreme lensing.