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

Lukas J. Furtak*, Ashish K. Meena, Erik Zackrisson, Adi Zitrin, Gabriel B. Brammer, Dan Coe, José M. Diego, Jan J. Eldridge, Yolanda Jiménez-Teja, Vasily Kokorev, Massimo Ricotti, Brian Welch, Rogier A. Windhorst, Abdurro’uf, Felipe Andrade-Santos, Rachana Bhatawdekar, Larry D. Bradley, Tom Broadhurst, Wenlei Chen, Christopher J. ConselicePratika Dayal, Brenda L. Frye, Seiji Fujimoto, Tiger Y.Y. Hsiao, Patrick L. Kelly, Guillaume Mahler, Nir Mandelker, Colin Norman, Masamune Oguri, Norbert Pirzkal, Marc Postman, Swara Ravindranath, Eros Vanzella, Stephen M. Wilkins

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of zphot ≃ 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 zspec = 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 Teff,B ≃ 15 000 K with either a redder F-type companion (Teff,F ≃ 6 250 K) or significant dust attenuation (AV ≃ 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 ≲ 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 ∼1.2 Gyr old showcases the ability of JWST to study early stars through extreme lensing.

Original languageAmerican English
Pages (from-to)L7-L13
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
StatePublished - 1 Jan 2024

Bibliographical note

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© 2024 Oxford University Press. All rights reserved.


  • galaxies: high-redshift
  • galaxies: star clusters: general
  • gravitational lensing: micro
  • gravitational lensing: strong
  • stars: individual: MACS0647-star-1
  • stars: massive


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