Supermassive Stars Match the Spectral Signatures of JWST's Little Red Dots

Abstract: The James Webb Space Telescope (JWST) has unveiled a population of enigmatic, compact sources at high redshift known as ''Little Red Dots'' (LRDs), whose physical nature remains a subject of intense debate. Concurrently, the rapid assembly of the first supermassive black holes (SMBHs) requires the formation of heavy seeds, for which supermassive stars (SMSs) are leading theoretical progenitors. In this work, we perform the first quantitative test of the hypothesis that LRDs are the direct observational manifestation of these primordial SMSs. We present a novel, first-principles pipeline generating synthetic spectra for a non-rotating, metal-free $10^6 \, M_\odot$ SMS. We establish that its luminosity ($L_\lambda \approx 1.7 \times 10^{44} \, \text{erg} \, \text{s}^{-1} \, \mu\text{m}^{-1}$ at 4050 Angstroms) provides a decisive constraint, matching prominent LRDs. Our model self-consistently reproduces their defining spectral features: the extreme, V-shaped Balmer break is an intrinsic photospheric effect, while the complex line phenomenology, strong H$\beta$ in emission with other Balmer lines in absorption arises from non-LTE effects in a single stellar atmosphere. Applying physically motivated broadening, our spectrum provides an excellent quantitative match to LRDs at both high ($z=7.76$) and low ($z=3.55$) redshift. Our model provides a simple, self-consistent physical picture for LRDs, offering a compelling alternative to multi-component obscured AGN scenarios and suggesting we may be directly witnessing the final, luminous moments of an SMBH progenitor before its ultimate collapse.