---
title: 'Supermassive PopIII Stars: Formation & Fate'
url: https://www.emergentmind.com/topics/supermassive-popiii-stars-smss
type: topic
---

# Supermassive PopIII Stars: Formation & Fate

Searching arXiv for the cited SMS/Pop III literature and closely related work on formation, evolution, rotation, instability, and observability.
Supermassive Population III stars are metal-free, rapidly accreting primordial stars that occupy the extreme high-mass end of Pop III star formation, typically at \(M_\ast \sim 10^{4-5}\,M_\odot\) and, in some direct-collapse models, up to \(\lesssim 10^6\,M_\odot\). They are studied primarily as progenitors of heavy black-hole seeds in the early universe, because their direct collapse can yield black holes of \(\sim 10^5{-}10^6\,M_\odot\), alleviating the growth-time problem posed by the existence of quasars at \(z\gtrsim 6{-}7\) [1807.06355, 2209.02790]. Their formation, structure, stability, and observability are controlled by the coupled physics of atomic-cooling haloes, suppressed \(\mathrm{H_2}\) cooling, extreme accretion, radiation-pressure support, angular-momentum transport, and general-relativistic instability [2102.08963, 2011.02480].

## 1. Cosmological setting and formation environments

Supermassive Pop III stars are associated with rare metal-free atomic-cooling haloes whose virial temperatures reach \(T_{\rm vir}\sim 10^4\) K, enabling atomic hydrogen cooling while suppressing the usual minihalo mode of Pop III star formation. Across the literature summarized here, the relevant hosts lie in the approximate halo-mass range \(M_{\rm halo}\sim 10^{7-8}\,M_\odot\) and formation-redshift range \(z\sim 10{-}20\), with some studies examining even later-forming pristine haloes that were delayed by external ionizing fields [1402.4777, 2102.08963, 2501.12986]. In the standard direct-collapse picture, strong Lyman–Werner backgrounds dissociate \(\mathrm{H_2}\), the gas collapses nearly isothermally at \(T\sim 8000{-}10^4\) K, fragmentation is reduced, and central inflow rates become high enough to assemble a supermassive protostar [1803.04527, 1807.06355].

This environment is not unique to externally irradiated haloes. Dynamical heating during rapid halo assembly can also delay cooling and maintain warm, massive clouds until collapse, yielding a subset of atomic-cooling haloes with very high central supply rates [2206.14459]. At the same time, internal Lyman–Werner feedback from one Pop III source inside an atomic-cooling halo does not appear to be an efficient way to create a second supermassive star after a dense core has already formed. In simulations with two nearby collapsing clumps, a realistic internal field of \(J_{\rm LW,int}\sim 10^4 J_{21}\) reduces \(\mathrm{H_2}\) in low-density gas but does not heat the core to the atomic-cooling regime; accretion rates and final stellar masses decrease rather than increase, and SMS formation is not facilitated [2501.12986].

The rarity of the required conditions is a recurring result. Strong LW fields, dynamical heating, or both can create suitable primordial clouds, but the pathway is highly selective. This suggests that SMSs represent a special mode of Pop III star formation rather than the generic outcome of metal-free collapse [1803.04527, 2206.14459].

## 2. Assembly by rapid accretion and disk-mediated inflow

The decisive control parameter is the mass accretion rate onto the protostar. In atomic-cooling gas, characteristic inflow rates are \(\dot M \sim 0.1{-}1\,M_\odot\,{\rm yr^{-1}}\), and several stellar-evolution studies adopt a critical rate near \(\dot M_{\rm crit}\approx 0.04\,M_\odot\,{\rm yr^{-1}}\) for maintaining the SMS state; recent work also quotes a broader range \(\dot M_{\rm crit}\sim 0.01{-}0.04\,M_\odot\,{\rm yr^{-1}}\) [1803.04527, 2011.02480, 2501.12986]. Above this threshold, the protostar remains bloated and cool; below it, sustained contraction toward a hot main-sequence Pop III configuration becomes possible.

Three-dimensional and two-dimensional calculations consistently show that the inflow is neither smooth nor monolithic. Primordial disks around forming SMSs are self-gravitating and fragment when the Toomre parameter
\[
Q_{\rm T} = \frac{c_s \Omega}{\pi G \Sigma}
\]
falls to \(Q_{\rm T}\lesssim 1\), producing spiral arms and massive clumps [2011.02480]. In detailed disk calculations, raw accretion can fluctuate by 9 orders of magnitude, but the \(10^3\)-yr averaged rates remain typically \(\sim 0.1\,M_\odot\,{\rm yr^{-1}}\), with quiescent intervals shorter than the surface Kelvin–Helmholtz timescale,
\[
t_{\mathrm{KH,surf}} \approx 10^{3}\,\mathrm{yr}\,\left( \frac{M_\ast}{500\,M_\odot} \right)^{1/2},
\]
so the star does not have time to contract into a strong ionizing source [2011.02480]. This result is central: disk fragmentation does not automatically destroy the SMS channel; rather, clump migration and tidal disruption can feed the central object in bursts.

Cosmological simulations add an important dynamical qualification. In haloes irradiated by \(1000\,J_{21}\), supercritical accretion can persist for \(\gtrsim 2.5\times10^5\) yr and produce stars of \(\sim 10^5\,M_\odot\), but mild fragmentation and N-body interactions can eject the most massive objects from the halo center, shutting off subsequent growth [1803.04527]. By contrast, somewhat less extreme \(100\,J_{21}\) haloes produce central Pop III stars of \(\sim 10^4{-}2\times 10^4\,M_\odot\) that may remain coupled to the inflow and later feed their black-hole remnants more effectively [1803.04527]. A plausible implication is that “best SMS-forming” conditions and “best central BH-seed” conditions do not always coincide.

A separate 3D RHD study of dynamically heated atomic-cooling haloes reached a similar bifurcation. One halo sustained the cool bloating phase and grew unimpeded to \(M_\ast \gtrsim 10^4\,M_\odot\), whereas another supplied the protostar more weakly; there the star spent most of its life as a hot main-sequence source and its growth was terminated around \(500\,M_\odot\) by photoevaporation of the circumstellar disk [2206.14459]. In those simulations, the resulting primordial IMF over \(M_\ast \simeq 100{-}10^5\,M_\odot\) is approximately top-heavy, following \(\propto M_\ast^{-1.3}\) with a steeper decline at \(M_\ast \gtrsim 2\times 10^4\,M_\odot\) [2206.14459].

## 3. Stellar structure, inflated envelopes, and weak ionizing feedback

At sufficiently high accretion rates, SMSs enter the “supergiant protostar” stage. The defining structural relation is
\[
R_\ast \approx 2.6\times 10^3 R_\odot \left(\frac{M_\ast}{100 M_\odot}\right)^{1/2},
\]
which yields radii exceeding \(7000\,R_\odot\) after the star accretes more than \(10^3\,M_\odot\), and radii of order \(\sim 100\) AU at \(M_\ast\gtrsim 3\times 10^4\,M_\odot\) [1807.06355, 1803.04527]. The luminosity is near the Eddington limit, while the effective temperature stays low, typically \(T_{\rm eff}\sim 5000{-}5500\) K, because the inflated atmosphere is regulated by opacity physics in the cool outer envelope [1807.06355, 2011.02480]. The result is a star with enormous bolometric luminosity but very weak ionizing output.

This weak ionizing output is the main reason radiative feedback fails to halt SMS growth under sustained high accretion. In the bloated state, the ionizing photon production is negligible compared to that of a hot ZAMS-like Pop III star of the same mass; H II regions remain compact or absent, and accretion continues [1807.06355, 2206.14459]. Even when accretion is variable, if low-\(\dot M\) episodes remain shorter than \(t_{\rm KH,surf}\), the star re-inflates before contracting to \(T_{\rm eff}\sim 10^5\) K [2011.02480]. This makes intermittent accretion compatible with long-term SMS growth.

The envelope structure is not that of a fully relaxed classical \(n=3\) polytrope. In rapidly accreting models it consists of a compact nuclear-burning core, an extended radiative envelope, and a low-mass convective outer layer, with significant structural diversity when cosmological accretion histories are imposed directly [2102.08963]. In KEPLER models fed by realistic halo inflows, SMSs can range from highly bloated configurations to nearly fully thermally relaxed, almost fully convective objects, depending on the time history of the accretion [2102.08963]. This suggests that effective temperature and feedback may be more heterogeneous than constant-\(\dot M\) models imply.

Mass loss by radiation-driven winds does not eliminate the SMS channel in metal-free gas. Steady, optically thick wind calculations for rapidly accreting super-giant protostars show that the wind velocity never reaches escape speed, because once the temperature falls below \(\sim 10^4\) K the opacity drops sharply owing to hydrogen recombination and the acceleration ceases [1611.09601]. In realistic non-steady cases such outflows would fall back, so net mass loss is negligible compared to \(\dot M_{\rm acc}\sim 0.1{-}1\,M_\odot\,{\rm yr^{-1}}\) [1611.09601]. A distinct line of work on porous, super-Eddington atmospheres instead finds continuum-driven winds and reduced ionizing emission, but still concludes that non-rotating SMSs can collapse to supermassive black holes, whereas rotationally stabilized ones may be eroded down to stars of a few \(100\,M_\odot\) [1203.4372]. The coexistence of these results reflects differing assumptions about atmospheric porosity, rotation, and evolutionary phase.

## 4. Rotation, GR stability, and collapse to black-hole seeds

SMSs are radiation-pressure dominated and evolve close to the Eddington limit, with \(\Gamma=L/L_{\rm Edd}\sim 0.99\) in rotating Pop III models [2209.02790]. Their surface rotation is then constrained by the \(\Omega\Gamma\)-limit,
\[
\Omega_\Gamma \simeq \Omega_K \sqrt{1 - \Gamma},
\]
so that \(\Omega_\Gamma \approx 0.1\,\Omega_K\) for \(\Gamma\sim 0.99\) [2209.02790]. Even this “slow” surface rotation is dynamically important for GR stability. To keep the star below the \(\Omega\Gamma\)-limit while accreting from a disk, the specific angular momentum of inflowing gas must satisfy
\[
j_{\rm acc} = f\, j_{\rm Kep},\quad f \sim 10^{-3}\text{–}10^{-2},
\]
meaning that more than \(99\%\) of the Keplerian angular momentum must be removed before gas joins the star [2209.02790]. The centrifugal barrier is therefore one of the central theoretical constraints on SMS formation.

Rotation stabilizes SMSs against GR collapse. In hylotropic and GENEC-based analyses, even \(f\sim 10^{-3}\) shifts the GR instability threshold upward; for Pop III SMSs under typical atomic-cooling conditions, realistic values \(f\lesssim 0.1{-}0.2\%\) still allow final masses of a few \(\times 10^5{-}10^6\,M_\odot\), while more extreme accretion and rotation can in principle produce much larger masses in merger-driven environments [2209.02790]. By contrast, KEPLER models evolved in realistic cosmological flows but without rotation reach \(1{-}2\times10^5\,M_\odot\) before undergoing direct collapse during or at the end of main-sequence H burning at \(1{-}1.5\) Myr, largely independent of halo mass, spin, or merger history [2102.08963]. This indicates that the collapse mass is model-dependent: it varies with the treatment of accretion history, rotation, and internal angular-momentum transport.

When collapse occurs, the generic outcome in the SMS regime is direct black-hole formation. In full GRMHD simulations of uniformly rotating, marginally unstable SMSs modeled as \(\Gamma=4/3\) polytropes, collapse yields a black hole with \(M_{\rm BH}\simeq 0.9M\) and spin \(a_{\rm BH}/M_{\rm BH}\simeq 0.7\), plus a hot, massive torus [1704.04502]. After \(\Delta t\sim 400{-}550M\), an incipient jet is launched; the jet luminosity is \(L_{\rm EM}\sim 10^{51-52}\) erg s\(^{-1}\), and the GW signal peaks at \(\sim 15(10^6 M_\odot/M)\) mHz, placing \(10^6\,M_\odot\) collapses in the LISA band and \(10^4\,M_\odot\) collapses in the DECIGO/BBO band [1704.04502]. In the more recent rotational analysis, the amount of mass left outside the horizon at collapse depends strongly on the spin profile: for atomic-cooling Pop III SMSs with realistic \(f\sim 0.2\%\), little or no torus is expected, whereas higher-\(f\) merger-driven cases can retain \(\sim 10\%\) of the stellar mass in orbit [2209.02790].

## 5. Rare thermonuclear explosions and their observable transients

Direct collapse is not the only endpoint. A narrow mass range near \(M\sim 5.5\times10^4\,M_\odot\) admits thermonuclear disruption instead of black-hole formation. Early modeling presented a \(55{,}500\,M_\odot\) Pop III progenitor as a supermassive PI SN with explosion energy \(\sim 10^{55}\) erg and near-IR detectability to \(z\sim 10{-}20\) [1211.1815]. Subsequent calculations identified the trigger more precisely: a non-rotating \(55{,}500\,M_\odot\) primordial star becomes unstable because the general-relativistic contribution of radiation to gravity causes the core to contract during He burning; explosive helium burning then reverses collapse and completely unbinds the star [1402.4777]. In those models, the same star collapses to a black hole if GR corrections are ignored, and a \(56{,}000\,M_\odot\) model collapses directly even with GR included, implying a very narrow explosion window [1402.4777].

The explosion energetics are extreme. KEPLER gives \(E_{\rm exp}^{\rm 1D}\approx 6.52\times10^{54}\) erg, while 2D CASTRO yields \(E_{\rm exp}^{\rm 2D}\approx 8.82\times10^{54}\) erg, well above the binding energy \(E_{\rm bind}\approx 5.76\times10^{53}\) erg [1402.4777]. The star is completely disrupted, leaves no compact remnant, and ejects nearly all of its mass. The nucleosynthesis is distinctive: about half of the stellar mass is expelled as heavy elements, but the yield is dominated by \(\alpha\)-chain nuclei from C to Si, with \({}^{56}\mathrm{Ni}\ll 1\,M_\odot\) [1402.4777]. This differs sharply from ordinary pair-instability supernovae, which are triggered by pair creation in \(150{-}260\,M_\odot\) stars and can synthesize \(0.1{-}50\,M_\odot\) of \({}^{56}\mathrm{Ni}\) [1402.4777].

Such explosions are also observationally unusual. They are not powered by radioactive decay but by shock heating, radiative diffusion, and ejecta–CSM interaction. Semi-analytic models informed by stellar-evolution and GR collapse simulations predict bolometric luminosities of \(\sim 10^{45\mathrm{-}47}\,\mathrm{erg\,s^{-1}}\), source-frame durations of \(10{-}200\) yr, and observer-frame durations of \(250{-}3000\) yr after cosmological time dilation [2507.15556]. The light curves are therefore quasi-persistent. Bright SMS explosions are predicted to be observable in long-wavelength JWST filters up to \(z\leq 20\) at \(24{-}26\) mag, pulsating SMSs up to \(z\leq 15\), and Euclid or the Roman Space Telescope can detect SMS explosions at \(z<11{-}12\) [2507.15556]. Deep Euclid fields could constrain the SMS rate down to \(10^{-11}\,\mathrm{Mpc^{-3}\,yr^{-1}}\), substantially deeper than current JWST bounds [2507.15556].

The spectroscopic expectation is H/He-rich, metal-poor, Type IIn-like emission from an optically thick shock propagating through primordial circumstellar material [2507.15556]. A plausible implication is that some very red, weakly variable high-\(z\) sources could overlap in color–magnitude space with little red dots or AGN, and would require spectroscopy to distinguish.

## 6. Role in early black-hole demographics and open problems

SMSs remain one of the leading heavy-seed channels for the first quasars. In the simplest version of the scenario, a Pop III SMS forms in a rare atomic-cooling halo, accretes rapidly for \(\sim\)Myr, and collapses to a \(\sim 10^5{-}10^6\,M_\odot\) black hole that can later grow into the SMBHs powering quasars by \(z\sim 6{-}7\) [1807.06355, 2209.02790]. The difficulty of growing \(10{-}100\,M_\odot\) Pop III remnants into \(10^9\,M_\odot\) SMBHs under feedback-limited accretion is one of the main motivations for the SMS route [2011.02480, 2102.08963].

At the population level, however, the pathway is not guaranteed to be efficient. Fragmentation and dynamical ejection can create wandering SMS remnants rather than central seeds [1803.04527]. Radiative feedback in some ACHs terminates growth near \(500\,M_\odot\) instead of allowing the SMS regime [2206.14459]. Internal LW feedback inside a protogalaxy appears unlikely to rescue a cooled core and convert it into an SMS-forming cloud [2501.12986]. These results indicate that the massive end of the Pop III IMF is broad and environmentally sensitive, spanning from \(\sim 100\,M_\odot\) to \(\sim 10^5\,M_\odot\), rather than producing a single canonical SMS mass [2206.14459].

Multiple-object outcomes are another major theme. Cosmological flows can produce more than one massive disk or protostar in a single halo, raising the possibility of SMS binaries, supermassive X-ray binaries, and direct-collapse black-hole mergers detectable by LISA [2102.08963]. In rotating collapse models, multimessenger signatures include ultra-long GRB-like jets, strong gravitational waves, and, in some cases, remnant tori that power extended electromagnetic emission [1704.04502, 2209.02790]. Rare thermonuclear explosions would add a separate class of long-lived infrared transients [2507.15556].

Several uncertainties remain structural rather than merely parametric. The required angular-momentum extraction efficiency is extreme [2209.02790]. Fully coupled 3D radiation-MHD simulations of SMS formation and evolution are still lacking [2209.02790]. Surface temperatures and feedback strengths differ across stellar-evolution codes, especially when highly variable cosmological accretion is imposed [2102.08963]. Wind physics is model-dependent, with one class of calculations finding fallback-dominated metal-free winds [1611.09601] and another emphasizing super-Eddington porosity and continuum-driven mass loss with low ionizing output [1203.4372]. Finally, the narrow GR-instability explosion window near \(5.5\times10^4\,M_\odot\) remains a special-case outcome rather than the dominant SMS fate [1402.4777].

Taken together, the current picture is internally coherent but not closed. Supermassive Pop III stars are best understood as a rare, rapidly accreting, weakly ionizing branch of primordial stellar evolution that can either collapse into heavy black-hole seeds or, in a narrow and physically distinctive regime, explode as extraordinarily energetic, Ni-poor thermonuclear transients. Their importance lies both in what they may have done—seeded the first quasars—and in how they might now be constrained: through the demographics of high-redshift black holes, the chemistry of early enrichment, long-duration infrared transients, and future gravitational-wave detections [2102.08963, 2507.15556].

Source: https://www.emergentmind.com/topics/supermassive-popiii-stars-smss