Evolutionary Tracks and Spectral Properties of Quasi-stars and Their Correlation with Little Red Dots

Andrew D. Santarelli, Ebraheem Farag, Earl P. Bellinger, Priyamvada Natarajan, Rohan P. Naidu, Claire B. Campbell, Matthew E. Caplan

First listed 2025-10-20 | Last updated 2026-01-09

Abstract

JWST has revealed a population of red, compact, high-redshift (${z\sim3-10}$) objects referred to as ``Little Red Dots'' (LRDs). These objects exhibit unusual spectral features reminiscent of stellar spectra with blackbody-like SEDs, large hydrogen Balmer breaks, Balmer line absorption, and classical stellar absorption features such as calcium H&K and the calcium triplet. Following the recent suggestion that these may be actively accreting direct-collapse black holes in the process of assembly, i.e. quasi-stars, we present evolutionary models of quasi-stars using our recently released, publicly available MESA-QUEST modeling framework. We compute a grid of models spanning a range of black hole masses and predict the luminosities, temperatures, surface gravities, and lifetimes of these objects. We find that these models lie along a Hayashi track once they hit their ``late-stage'' which constitutes the majority of their lives ($\sim 20$~Myr). We present scaling relations for estimating the mass of a quasi-star as a function of the bolometric luminosity, as well as the bolometric luminosity as a function of the effective temperature for the Hayashi track. The short lifetimes in tandem with the observed number density of LRDs imply the possibility that every supermassive black hole was once a quasi-star. We compare synthetic spectra of our quasi-star models to observations of LRDs, and show that these models are broadly capable of reproducing the continuum spectra of observed LRDs. These results indicate that quasi-stars are promising candidates for the origin of supermassive black holes via direct collapse in the early universe.

Short digest

Using the MESA-QUEST framework, the authors build evolutionary tracks and spectra for quasi-stars as candidates for the red, compact Little Red Dots seen at z≈3–10. The models settle onto a late-stage Hayashi track lasting about 20 Myr and yield practical scaling relations linking quasi-star mass to Lbol and Lbol to Teff. Synthetic spectra from these models broadly match the continuum of exemplar LRDs (MoM-BH*-1 and UNCOVER-45924), including blackbody-like SEDs with strong Balmer breaks and stellar absorption features. Coupling the short lifetimes with the observed LRD number densities, they argue quasi-stars could be a common pathway to assembling early supermassive black holes.

Key figures to inspect

• Evolution tracks on the HR and Kiel diagrams: verify when the model enters the late-stage Hayashi track, how long it stays there, and how BH mass growth maps onto Teff–log g; note the ATLAS9 coverage region and constant-radius lines.
• Scaling relations panel: inspect the fitted M_QS–Lbol and Lbol–Teff relations along the Hayashi track and how to use them to estimate quasi-star masses from observed LRD luminosities/temperatures.
• Observed vs synthetic spectra for MoM-BH*-1: check the continuum shape, Balmer break amplitude, and presence of Ca H&K/Ca II triplet absorption against the model envelope spectrum.
• Observed vs synthetic spectra for UNCOVER-45924: assess how well the quasi-star model reproduces the NIR SED slope and Balmer absorption, and whether any galaxy light component is required.
• Dust attenuation test (if shown): look for comparisons with simple dust corrections to confirm the conclusion that attenuation is minimal for LRD continua.