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

**Authors:** 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. Conselice  
**arXiv:** [2308.00042](https://arxiv.org/abs/2308.00042)  
**Date:** 2023-07-31  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.00042v1/

## Key points

- Observations of a highly magnified star candidate using the James Webb Space Telescope (JWST) and its Near-Infrared Spectrograph (NIRSpec)
- Photometric redshift of the star candidate estimated to be around $z_{\mathrm{phot}}\simeq4.8$
- Spectroscopic observation with NIRSpec provides precise measurement of the redshift of the host arc ($z_{\mathrm{spec}}=4.758\pm0.004$)
- Spectrum of the star displays Lyman- and Balmer-breaks consistent with this redshift
- Star likely a B-type super-giant with a surface temperature of $T_{\mathrm{eff,B}}\simeq15000$ K
- Alternative scenarios considered: redder F-type companion or significant dust attenuation along line of sight
- Investigation into whether object could be a magnified young globular cluster rather than a single star
- Spectrum provides insights into early stars at a time when Universe was about 1.2 billion years old
- Contributes to understanding high-redshift star formation and gravitational lensing in astronomy

## Summary

In this study, the authors present observations of a highly magnified star candidate using the James Webb Space Telescope (JWST) and its Near-Infrared Spectrograph (NIRSpec). The star candidate was previously detected in JWST/NIRCam imaging of the strong lensing cluster MACS J0647+7015. The photometric redshift of the star candidate is estimated to be around $z_{\mathrm{phot}}\simeq4.8$. The spectroscopic observation with NIRSpec allows for a precise measurement of the redshift of the host arc, which is found to be $z_{\mathrm{spec}}=4.758\pm0.004$. The spectrum of the star displays clear Lyman- and Balmer-breaks that are consistent with this redshift. By fitting the spectrum, the authors suggest that the star is likely a B-type super-giant with a surface temperature of $T_{\mathrm{eff,B}}\simeq15000$ K. They also consider two alternative scenarios: either a redder F-type companion with $T_{\mathrm{eff,F}}\simeq6250$K or significant dust attenuation ($A_V\simeq0.82$) along the line of sight. Furthermore, they investigate whether this object could be a magnified young globular cluster rather than a single star. They demonstrate that while the spectrum is compatible with such an interpretation, other factors make it somewhat less likely; specifically, there is no counter image observed and lensing symmetry imposes an upper limit on its size ($r \lesssim 0.5$ pc). However, these limitations do not completely rule out this possibility. The presented spectrum provides valuable insights into early stars as it was observed at a time when the Universe was only about 1.2 billion years old; this showcases JWST's ability to study distant objects through extreme lensing. Overall, this study contributes to our understanding of high-redshift star formation and gravitational lensing in astronomy.