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Yellow hypergiant

A yellow hypergiant is a rare class of evolved massive star, typically with an initial mass of 20–40 solar masses, exhibiting extreme luminosities exceeding $10^5 solar luminosities (log L/L_\odot > 5.4) and effective temperatures between 4000 K and 8500 K.^[1] These stars occupy spectral types from A to K and luminosity classes Ia0+ or I0, characterized by absolute visual magnitudes brighter than M_V < -8.5 and strong Hα emission indicative of high mass-loss rates ranging from $10^{-6} to $10^{-4} M_\odot/yr, with episodic events reaching up to $10^{-2} M_\odot/yr.^[1] Positioned near the Humphreys–Davidson luminosity limit on the Hertzsprung–Russell diagram, yellow hypergiants represent a short-lived transitional phase, lasting roughly 1000 years or less, in the blueward evolution of post-red supergiant stars, marked by dynamic instabilities, photometric and spectroscopic variability, and the formation of complex circumstellar envelopes with infrared excess from dust. Recent studies as of 2025 indicate that their outbursts are driven by vigorous pulsations causing temperature fluctuations over decades.^[1]^[2]^[3] Yellow hypergiants are among the most luminous and unstable objects in the Milky Way, with fewer than 20 confirmed examples known in our galaxy as of 2025. Their atmospheres are extended and often nebulous, featuring arcs, knots, and shells from episodic ejections, while spectra reveal strong emission lines of H, Ca II, [O I], and [Ca II], alongside excesses in nitrogen and sodium from CNO-cycle processing.^[1]^[2] These stars frequently show pulsations with radial velocities of 7–11 km/s and expanding winds up to 40 km/s, contributing to their erratic brightness changes and potential role in shaping supernova progenitors or black hole formation through extreme mass shedding.^[2] Many are binary systems, which may drive their evolution via interactions, and they can reach radii up to 1500 solar radii, making them comparable in size to some red supergiants despite their surface temperatures.^[4] Notable examples include ρ Cassiopeiae, a prototypical yellow hypergiant at about 3.5 kpc with luminosity log L/L_\odot = 5.48 and recurrent mass-loss outbursts; IRC +10420 (also known as V1302 Aql), an F8Ia+ to A2I star at ~5 kpc showing bipolar outflows and log L/L_\odot = 5.70; and HR 5171 A, the largest known at 1315 ± 260 solar radii and ~1 million times the Sun's brightness, located 12,000 light-years away in Centaurus.^[1]^[2]^[4] Other prominent members are V509 Cassiopeiae, undergoing extreme evolution with recent outbursts, and Var A in M33, which fluctuates between F0Ia and M types at log L/L_\odot = 5.70.^[1] These objects provide critical insights into the late stages of massive star evolution, bridging red supergiants to luminous blue variables or Wolf–Rayet stars, though their brief phase and scarcity limit observational samples.^[1]

Definition and Classification

Spectral and Luminosity Classes

Yellow hypergiants are classified within the spectral types ranging from late F to middle G, corresponding to effective temperatures between 5000 K and 8000 K.^[1] This temperature range positions their peak blackbody emission in the yellow portion of the optical spectrum, giving these stars their characteristic "yellow" color despite spanning cooler supergiant temperatures.^[5] In terms of luminosity classification, yellow hypergiants are designated as class 0, Ia-0, or Ia+, indicating they exceed the brightness of standard supergiants (class Ia).^[6] Their absolute visual magnitudes are brighter than -8.5 (M_V < -8.5), typically around -9 or brighter, with bolometric luminosities exceeding 250,000 times that of the Sun (log L/L⊙ > 5.4), typically 250,000 to 600,000 or more.^[6]^[1] The formal classification of hypergiants, including yellow subtypes, originated in the early 20th century with identifications of exceptionally luminous stars, but the term "hypergiant" was formalized in the late 20th century by C. de Jager to describe super-supergiants with M_Bol < -9.^[6] Modern criteria build on the Humphreys–Davidson limit, established in 1979 as an empirical upper luminosity boundary for cool supergiants, beyond which stars exhibit hypergiant characteristics due to instability and mass loss.^[7]^[8] Yellow hypergiants are distinguished from yellow supergiants primarily by their extended, unstable atmospheres and extremely low surface gravities (log g ≈ 0), which lead to high mass-loss rates and spectral peculiarities not seen in less luminous counterparts.^[9]

Identification Criteria

Yellow hypergiants are identified through a combination of spectroscopic and photometric observations that reveal their extreme luminosity, high mass-loss rates, and extended envelopes. Key spectral features include strong Hα emission with broad components and variable profiles, often indicating significant mass ejection, as observed in stars like ρ Cassiopeiae and V1302 Aql.^[2]^[6] The Balmer discontinuity is prominent in their spectra, reflecting the high electron density in the atmosphere, while P Cygni profiles in hydrogen lines and the Ca II infrared triplet signal outflowing winds with velocities up to 40 km/s.^[2]^[10] These features distinguish yellow hypergiants from less luminous supergiants, particularly when combined with low surface gravity indicators such as broadened absorption lines from the O I 7773 Å triplet, which yield equivalent widths of 1.86–2.86 Å.^[2] Photometric criteria further aid identification, with irregular variability typically showing amplitudes of 1–2 magnitudes in visual bands due to pulsations and episodic outbursts.^[10] High infrared excess is a hallmark, arising from circumstellar dust formed in ejected material, as evidenced by spectral energy distributions (SEDs) that peak strongly beyond 10 μm for objects like IRC +10420.^[6] Distance-independent indicators include line broadening from low gravity (log g ≈ 0–0.5) and inferences of extended atmospheres from radio photometry, where free-free emission traces envelope sizes exceeding 100 stellar radii in cases like V509 Cas.^[2] Classification challenges stem from the rarity of yellow hypergiants—fewer than 20 confirmed in the Milky Way—and their instability, which causes rapid spectral changes that blur boundaries with other classes.^[6] Particular care is needed to avoid confusion with luminous blue variables (LBVs), as both exhibit high mass loss and variability, but yellow hypergiants lack the strong X-ray emission typical of LBV winds and instead show Na overabundances indicative of post-red supergiant evolution.^[10] These diagnostics align with luminosity class Ia-0 for the most extended objects, emphasizing their hypergiant status.^[2]

Physical Characteristics

Atmospheric and Surface Properties

Yellow hypergiants possess extended atmospheres characterized by enormous physical radii, typically ranging from 300 to 1,500 times that of the Sun (R⊙), which positions them among the largest known stars.^[9] For instance, HR 5171 A has a measured radius of approximately 1,315 ± 260 R⊙, determined through interferometric observations that resolve its photospheric disk.^[10] Similarly, recent near-infrared interferometry of RW Cephei yields a radius of 1,100 ± 44 R⊙ during its rebrightening phase, highlighting the scale of these objects.^[11] Their surface temperatures generally fall between 4,000 and 8,000 K, contributing to yellow spectral appearances while maintaining high luminosities. Surface compositions reveal signatures of prior evolutionary processing, including nitrogen enhancements from the CNO cycle, alongside enhancements in sodium and other elements from dredge-up events. Some yellow hypergiants, such as V1302 Aql and ρ Cas, exhibit sodium overabundances, indicative of deep mixing events that dredge up processed material to the surface. These stars feature extremely low surface gravities, with log g values around 0, resulting from their vast sizes and low densities that foster extended, cool atmospheres susceptible to pulsations.^[9] This near-zero gravity enables high mass-loss rates and turbulent photospheres, as observed in examples like V509 Cas.^[9] Multiepoch interferometric monitoring of RW Cephei in 2023 confirms a stable stellar radius during periods of relative quiescence, with only minor variations (∼8% in diameter) tied to episodic brightness changes.^[11] These properties underpin their classification as luminosity class Ia-0 supergiants.^[9]

Variability and Outbursts

Yellow hypergiants exhibit irregular photometric variability characterized by semiregular pulsations with periods ranging from 100 to 1,000 days and amplitudes typically between 0.5 and 3 magnitudes.^[12]^[13] These pulsations are driven by radial oscillations in the stellar envelope, which arise from the stars' low surface gravity and extended atmospheres, contributing to their overall instability.^[9] The variability is often quasi-periodic, with amplitudes that can fluctuate by factors of 3 to 5 over timescales of about 33 pulsation periods, reflecting the complex dynamics of the outer layers. In addition to this baseline variability, yellow hypergiants undergo episodic outbursts involving shell ejections that cause temporary increases in brightness followed by significant dimming.^[14] For instance, the prototypical yellow hypergiant ρ Cassiopeiae experienced major outbursts in 1946 and 2000, during which its visual brightness dropped by 1.2 to 1.4 magnitudes due to the formation of an optically thick cool envelope.^[15] These events are associated with enhanced mass ejection rates reaching up to 10^{-4} M_\odot per year, far exceeding the quiescent rates and leading to the expulsion of substantial material into the circumstellar environment.^[16]^[17] Recent analyses highlight the heightened instability of yellow hypergiants compared to red supergiants, particularly within the "yellow void" phase of evolution, where rapid spectral type changes and episodic mass loss create pseudo-photospheres that amplify variability.^[18] This instability stems from the stars' position near the boundary of dynamical instability in the Hertzsprung-Russell diagram, resulting in more frequent and intense pulsational episodes than in preceding red supergiant stages.^[18] Such behaviors underscore the transitional nature of yellow hypergiants, with outbursts triggered by intensified pulsations that destabilize the envelope.^[19] A 2025 study of ρ Cas and similar stars found that pulsation periods lengthen and amplitudes increase before outbursts, offering a potential predictor for these events. Spectroscopically, these outbursts are marked by enhanced emission lines, such as variable wings in Hα, and the appearance of low-excitation metallic lines indicating a temporary drop in effective temperature.^[15] Post-outburst, infrared excess often signals dust formation in the ejected shells, as cooler material condenses and contributes to circumstellar envelopes.^[21] These signatures provide key evidence for the rapid atmospheric changes driving the events.^[22]

Evolutionary Pathway

Post-Red Supergiant Transition

Yellow hypergiants represent a fleeting evolutionary stage for massive stars that have completed their red supergiant (RSG) phase, typically those with initial masses between 20 and 40 M⊙. This post-RSG transition is remarkably brief, lasting roughly 1000 years or less,^[18] during which these stars shed substantial portions of their hydrogen-rich envelopes through intensified stellar winds and episodic outbursts. Such mass loss is driven by the stars' proximity to the Eddington limit, where radiation pressure destabilizes the outer layers, facilitating the rapid stripping necessary for blueward evolution.^[6]^[23] The core of this transition involves navigating the yellow evolutionary void, an unstable region in the Hertzsprung-Russell (HR) diagram characterized by dynamical instabilities that cause dramatic atmospheric responses. As the star contracts following the RSG phase, its effective temperature rises sharply from around 3,500 K to 6,000–8,000 K over short timescales, triggering extensive envelope expansion and potential loops in the HR diagram as the star oscillates between contraction and re-expansion. These instabilities manifest as enhanced mass-loss rates and photometric variability, preventing stable residence in the void and propelling the star toward hotter spectral types.^[24]^[25] Accompanying these structural changes is significant chemical evolution, where deep convective dredge-up episodes expose products of core helium burning to the surface, leading to abundance anomalies such as elevated helium and nitrogen enrichment relative to solar values. These alterations alter the atmospheric opacity and excitation conditions, contributing to the peculiar spectral features observed in yellow hypergiants, including strong emission lines and molecular bands.^[26]^[27] Binary interactions may accelerate this transition in some systems. A compelling example of this transitional phase is provided by the 2024 study of WOH G64 in the Large Magellanic Cloud, which documents its shift from an extreme RSG to a yellow hypergiant around 2014, marked by irregular variability, envelope stripping via a prolonged eruption and superwind, and the presence of a B-type companion in a symbiotic binary system. This case highlights how binary interactions may accelerate the transition, offering direct evidence of the mechanics linking RSGs to yellow hypergiants.^[28]

Long-Term Fate

Yellow hypergiants, as post-red supergiant stars with initial masses typically ranging from 20 to 40 solar masses, are expected to evolve further depending on their core masses and mass-loss histories, potentially transitioning into luminous blue variables (LBVs), Wolf-Rayet stars, or becoming direct progenitors of core-collapse supernovae.^[29]^[30] For stars around 30–40 solar masses, models indicate a pathway toward the LBV phase, characterized by further instability and enhanced mass ejection before potentially reaching the Wolf-Rayet stage, where the hydrogen envelope is largely stripped.^[31] In contrast, lower-mass yellow hypergiants may proceed directly to supernova explosion without re-entering the blue supergiant domain, influenced by rotational effects and metallicity.^[30] Intense mass loss during the yellow hypergiant phase plays a critical role in shaping these outcomes by cumulatively ejecting material from the envelope, which reduces the core mass significantly in many cases.^[32] This reduction is driven by strong stellar winds and episodic ejections in the yellow hypergiant and subsequent LBV regimes, creating a mass-loss runaway where decreasing envelope mass paradoxically enhances wind efficiency due to luminosity degeneracies.^[33] By lowering the core mass below thresholds for very massive stars, such processes help these progenitors avoid pair-instability supernovae, which require cores exceeding about 40–60 solar masses to trigger electron-positron pair production and explosive oxygen ignition.^[34] Instead, the stripped cores typically lead to iron-core collapse and standard Type IIb or Ib supernovae.^[35] Observational evidence supports the notion that some yellow hypergiants can stabilize within the "yellow void"—a sparsely populated region of the Hertzsprung-Russell diagram—indicating potentially prolonged phases before further evolution. For instance, the yellow hypergiant V509 Cassiopeiae has shown signs of surface stabilization since its 20th-century outbursts, with recent spectroscopic and photometric data revealing reduced variability and a more quiescent atmosphere, suggesting it may reside in this unstable transitional zone for an extended period.^[9] This stability implies that not all yellow hypergiants rapidly exit the yellow void toward hotter spectral types, allowing for longer observation of this evolutionary stage.^[36] Theoretical models of massive star evolution depict yellow hypergiants tracing "blue loops" in the Hertzsprung-Russell diagram, where they move from cooler red supergiant positions back toward hotter temperatures due to core contraction and envelope expansion dynamics.^[29] However, post-yellow hypergiant evolutionary tracks remain uncertain, with variations arising from incomplete treatments of convection, rotation, and mass loss in stellar interior models.^[37] A 2025 review highlights these ambiguities, noting that while some tracks predict multiple loops and eventual LBV or Wolf-Rayet phases, others suggest direct paths to supernova for certain mass ranges, underscoring the need for refined simulations incorporating recent observational constraints.^[18]

Internal Structure

Core and Envelope Layers

Yellow hypergiants possess a compact, convective helium-burning core that accounts for a very small fraction of the stellar radius and contains the majority of the star's mass, with central temperatures exceeding $10^8 K where the triple-alpha process and subsequent carbon-burning reactions dominate energy generation.^[38] This core is surrounded by a hydrogen-burning shell that sustains the star's extreme luminosity, typically on the order of $10^5 to $10^6 solar luminosities, as the star transitions through its post-red supergiant phase.^[1] The core's convective nature arises from the high temperatures and densities, facilitating efficient mixing of fusion products.^[39] The envelope of a yellow hypergiant is an extended layer primarily composed of hydrogen and helium, with significant metallicity gradients resulting from prior convective mixing during the red supergiant stage, including enhancements in CNO-cycle products such as nitrogen and sodium excesses.^[2] These gradients contribute to unstable pulsation modes by altering local opacities and densities, promoting dynamical instabilities in the outer layers.^[40] The envelope's tenuous structure, approaching the Eddington limit, results in low gas densities that amplify these effects.^[38] Energy transport within yellow hypergiants involves a combination of radiative diffusion in the inner hydrogen shell and partial convection in the envelope, where the \kappa-mechanism drives variability through opacity variations in helium ionization zones.^[40] Opacity peaks from iron-group elements at temperatures around $2 \times 10^5 K in the iron convection zone further enhance this mechanism, allowing super-Eddington fluxes to be transported adiabatically despite the envelope's marginal stability.^[38] In the outer envelope, turbulent pressure from supersonic convection cells supplements radiative transport, though it remains a minor contributor overall.^[38] Evolutionary models indicate helium core masses consistent with progenitors of initial masses above 25 M_\odot.^[1] These insights reveal internal mixing efficiencies but remain constrained by the scarcity of high-quality data for such luminous objects.^[39]

Mass Loss and Envelope Dynamics

Yellow hypergiants exhibit mass-loss winds that are often episodic and driven by pulsations and dynamical instabilities, with contributions from radiative and dust-driving mechanisms. Momentum transfer occurs through absorption in spectral lines, and observations of P Cygni profiles in optical lines indicate outflows from the extended atmospheres.^[6] These winds operate at rates typically ranging from 10^{-6} to 10^{-4} M_\odot yr^{-1}, significantly higher than those of less luminous supergiants due to the stars' exceptional luminosities exceeding 10^5 L_\odot.^[23] The envelopes of yellow hypergiants undergo spherical expansion at velocities of 10–50 km s^{-1}, driven by the low surface gravity that facilitates the ejection of material into extended, optically thick layers.^[1] During periods of instability, these expanding envelopes can form pseudo-photospheres, where cooler material temporarily obscures the hotter stellar surface, altering the observed spectrum and effective temperature.^[6] This dynamic expansion contributes to the overall instability in the yellow void region of the Hertzsprung-Russell diagram, as the low gravity promotes ongoing atmospheric ejection. Binary interactions may further influence envelope dynamics and mass loss in some systems.^[23] Cooling in the outer envelopes enables the formation of dust grains, including silicates and amorphous carbon, which are observed through prominent infrared excess in the spectral energy distributions of these stars.^[6] For instance, the prototypical yellow hypergiant IRC +10420 displays a substantial IR excess from 2 to over 20 μm, arising from warm dust in its circumstellar envelope formed during prior high mass-loss episodes.^[41] Molecular species such as CO and SiO also condense in these cool regions, further evidencing the chemical processing in the outflowing material.^[6] Recent studies of post-red supergiant candidates in the Large Magellanic Cloud have quantified enhanced mass-loss rates in yellow hypergiants compared to typical red supergiants, with rates up to 6 \times 10^{-5} M_\odot yr^{-1} inferred from infrared observations of circumstellar dust shells.^[23] This enhancement, observed in six luminous yellow supergiants, suggests that the transition from the red supergiant phase triggers intensified wind activity, leading to detached dust envelopes extending thousands of AU.^[23] Such findings underscore the role of these winds in shaping the late evolutionary paths of massive stars.^[6]

Cataloged Examples

Within the Milky Way

Only about a dozen yellow hypergiants are known within the Milky Way, representing a rare evolutionary phase for massive stars.^[42] These stars are characterized by their extreme luminosities, typically exceeding 100,000 solar luminosities, and spectral types ranging from late F to middle G, placing them in the unstable "yellow void" region of the Hertzsprung-Russell diagram.^[43] Distances to these objects vary from a few kiloparsecs to over 6 kpc, determined primarily through Gaia parallax measurements and spectroscopic analyses. Prominent examples include ρ Cassiopeiae, a prototypical yellow hypergiant at approximately 3.5 kpc distance, known for major outbursts in 1946–1947 and 2000–2001 that involved significant mass ejection forming cool, optically thick shells. Interferometric observations indicate its angular diameter corresponds to a radius of 564–700 R⊙, making it one of the largest known stars during quiescence. HR 5171 A (also known as V766 Centauri), located about 3.6 kpc away, holds the record for the largest yellow hypergiant observed, with a radius of 1315 ± 260 R⊙ measured via Very Large Telescope interferometry in 2014, revealing it as part of a binary system in a common envelope phase.^[10] This star exhibits high mass-loss rates and photometric variability consistent with its transitional status.^[10] IRC +10420, situated at 4–6 kpc in Aquila, exemplifies extreme mass loss among yellow hypergiants, with rates up to 10^{-3} M⊙ yr^{-1} over the past few centuries, leading to a dusty bipolar nebula that marks it as a precursor to a protoplanetary nebula. Its spectral type varies from F8 to A2, with episodic high-velocity outflows detected in CO emission. V509 Cassiopeiae, at around 5 kpc, has been confirmed as stably residing in the yellow void through long-term monitoring, showing low-amplitude pulsations and no recent major outbursts since the 20th century, as detailed in 2024 spectroscopic and photometric analyses.^[9] Recent additions to the catalog include IRAS 18357-0604, a yellow hypergiant at ~6 kpc displaying spectral and wind properties analogous to IRC +10420, with pronounced Balmer emission and high mass loss indicative of its post-red supergiant phase.^[44] HD 179821, located approximately 5.3 kpc away, is classified as a yellow hypergiant with a detached dust shell and semi-periodic variability, though debates persist on whether it is a true massive example or a lower-mass post-AGB object; recent studies affirm its inclusion based on luminosity and envelope dynamics.^[45] Additionally, RW Cephei exhibited a notable rebrightening in 2023–2024 following its great dimming event, with interferometric imaging revealing surface changes consistent with hypergiant behavior.

In Nearby Galaxies

Yellow hypergiants in nearby galaxies are challenging to detect due to their distance and the need for high-resolution spectroscopy to identify their characteristic spectral features, such as strong emission lines and variable profiles indicative of unstable atmospheres.^[46] In the Large Magellanic Cloud (LMC), post-red supergiant candidates include WOH G64, which underwent a dramatic transition from a red supergiant to a yellow hypergiant in observations reported in 2024, revealing a symbiotic system with a B-star companion and significant mass ejection. Approximately five such yellow hypergiants have been confirmed in the LMC, often exhibiting high mass-loss rates and circumstellar dust shells that obscure optical signatures.^[28]^[23] In M33, luminous A/F-type hypergiants have been identified through monitoring of yellow massive stars, with Kourniotis et al. (2017) reporting three candidates surrounded by hot dust and displaying photometric variability consistent with yellow hypergiant behavior. A prominent example is Var A, which fluctuates between F0Ia and M types at log L/L_\odot = 5.70.^[47]^[48]^[1] Detections in M31 are rarer, relying on Hubble Space Telescope imaging and spectroscopy to resolve luminous yellow stars near luminous blue variable regions, where their high extinction from dust complicates ground-based observations. These candidates show spectral similarities to warm hypergiants, with effective temperatures around 6000–8000 K and evidence of episodic mass loss akin to local examples.^[46] Comparative studies indicate that the lower metallicity in the LMC (about 0.5 Z⊙) results in reduced mass-loss rates for yellow hypergiants compared to Galactic counterparts, potentially leading to less pronounced envelope instabilities and slower evolutionary transitions, as detailed in analyses of evolved stars in the Magellanic Clouds.^[49]

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[44]
https://www.aanda.org/articles/aa/abs/2014/01/aa22772-13/aa22772-13.html
[45]
IRAS 18357-0604 – an analogue of the galactic yellow hypergiant ...
Yellow hypergiants represent a short-lived evolutionary episode experienced by massive stars as they transit to and from a red supergiant phase. As such, their ...
[46]
ρ Cas, HR 8752, and HR 5171A, with notes on HD 179821
We investigate various types of photometric and physical instabilities in the yellow hypergiant (YHG) ρ Cas as well as its evolution between 1885 and 2023.
[47]
Luminous and Variable Stars in M31 and M33. IV ... - IOP Science
In this series of papers we have presented the results of a spectroscopic survey of luminous stars in the nearby spirals M31 and M33. Here, we present ...Missing: detections | Show results with:detections
[48]
[1612.06853] Monitoring luminous yellow massive stars in M33 - arXiv
Dec 20, 2016 · The yellow hypergiants (YHGs) appear to be the warm counterparts of post-RSG classes located near the Humphreys-Davidson upper luminosity limit, ...Missing: F 2022 identifications
[49]
Monitoring luminous yellow massive stars in M 33
Yellow hypergiants (YHGs; de Jager 1998) are post-RSG stars that occupy a region in the HR diagram between 4000−7000 K. Their luminosity is confined above log L ...
[50]
Mass-loss Rate of Highly Evolved Stars in the Magellanic Clouds
Nov 25, 2024 · We utilized the extinction map for the LMC and SMC obtained by B.-Q. Chen et al. (2022) and applied the extinction law of J. A. Cardelli et ...