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Luminous blue variable
Luminous blue variable
Luminous blue variable
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Luminous blue variable AG Carinae as seen by the Hubble Space Telescope

Luminous blue variables (LBVs) are rare, massive, evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Large Magellanic Cloud.

Discovery and history

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P Cygni profile of a spectral line

The LBV stars P Cygni and η Carinae have been known as unusual variables since the 17th century, but their true nature was not fully understood until late in the 20th century.

In 1922 John Charles Duncan published the first three variable stars ever detected in an external galaxy, variables 1, 2, and 3, in the Triangulum Galaxy (M33). These were followed up by Edwin Hubble with three more in 1926: A, B, and C in M33. Then in 1929 Hubble added a list of variables detected in M31. Of these, Var A, Var B, Var C, and Var 2 in M33 and Var 19 in M31 were followed up with a detailed study by Hubble and Allan Sandage in 1953. Var 1 in M33 was excluded as being too faint and Var 3 had already been classified as a Cepheid variable. At the time they were simply described as irregular variables, although remarkable for being the brightest stars in those galaxies.[1] The original Hubble Sandage paper contains a footnote that S Doradus might be the same type of star, but expressed strong reservations, so the link would have to wait several decades to be confirmed.

Later papers referred to these five stars as Hubble–Sandage variables. In the 1970s, Var 83 in M33 and AE Andromedae, AF Andromedae (=Var 19), Var 15, and Var A-1 in M31 were added to the list and described by several authors as "luminous blue variables", although it was not considered a formal name at the time. The spectra were found to contain lines with P Cygni profiles and were compared to η Carinae.[2] In 1978, Roberta M. Humphreys published a study of eight variables in M31 and M33 (excluding Var A) and referred to them as luminous blue variables, as well as making the link to the S Doradus class of variable stars.[3] In 1984 in a presentation at the IAU symposium, Peter Conti formally grouped the S Doradus variables, Hubble–Sandage variables, η Carinae, P Cygni, and other similar stars together under the term "luminous blue variables" and shortened it to LBV. He also clearly separated them from those other luminous blue stars, the Wolf–Rayet stars.[4]

Variable star types are usually named after the first member discovered to be variable, for example δ Sct variables named after the star δ Sct. The first luminous blue variable to be identified as a variable star was P Cygni, and these stars have been referred to as P Cygni type variables. The General Catalogue of Variable Stars decided there was a possibility of confusion with P Cygni profiles, which also occur in other types of stars, and chose the acronym SDOR for "variables of the S Doradus type".[5] The term "S Doradus variable" was used to describe P Cygni, S Doradus, η Carinae, and the Hubble-Sandage variables as a group in 1974.[6]

Physical properties

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Upper portion of H-R Diagram showing the location of the S Doradus instability strip and the location of LBV outbursts. Main sequence is the thin sloping line on the lower left.

LBVs are massive unstable supergiant (or hypergiant) stars that show a variety of spectroscopic and photometric variation, most obviously periodic outbursts and occasional much larger eruptions.

In their "quiescent" state they are typically B-type stars, occasionally slightly hotter, with unusual emission lines. They are found in a region of the Hertzsprung–Russell diagram known as the S Doradus instability strip, where the least luminous have a temperature around 10,000 K and a luminosity about 250,000 times that of the Sun, whereas the most luminous have a temperature around 25,000 K and a luminosity over a million times that of the Sun, making them some of the most luminous of all stars.

During a normal outburst the temperature decreases to around 8,500 K for all stars, slightly hotter than the yellow hypergiants. The bolometric luminosity usually remains constant, which means that visual brightness increases somewhat by a magnitude or two. A few examples have been found where luminosity appears to change during an outburst, but the properties of these unusual stars are difficult to determine accurately. For example, AG Carinae may decrease in luminosity by around 30% during outbursts; and AFGL 2298 has been observed to dramatically increase its luminosity during an outburst although it is not clear if that should be classified as a modest giant eruption.[7] S Doradus typifies this behaviour, which has been referred to as strong-active cycle, and it is regarded as a key criterion for identifying luminous blue variables. Two distinct periodicities are seen, either variations taking longer than 20 years, or less than 10 years. In some cases, the variations are much smaller, less than half a magnitude, with only small temperature reductions. These are referred to as weak-active cycles and always occur on timescales of less than 10 years.[8]

Some LBVs have been observed to undergo giant eruptions with dramatically increased mass loss and luminosity, so violent that several were initially catalogued as supernovae. The outbursts mean there are usually nebulae around such stars; η Carinae is the best-studied and most luminous known example, but may not be typical.[9] It is generally assumed that all luminous blue variables undergo one or more of these large eruptions, but they have only been observed in two or three well-studied stars and a handful of supernova imposters (such as SN 2009ip, which later evolved into a true supernova[10]). The two clear examples in the Milky Way galaxy, P Cygni and η Carinae, and the possible example in the Small Magellanic Cloud, HD 5980A, have not shown strong-cycle variations. It is still possible that the two types of variability occur in different groups of stars.[11] 3-D simulations have shown that these outbursts may be caused by variations in helium opacity.[12]

Many luminous blue variables also show small amplitude variability with periods less than a year, which appears typical of Alpha Cygni variables,[7] and stochastic (i.e. totally random) variations.[8]

Luminous blue variables are by definition more luminous than most stars and also more massive, but within a very wide range. The most luminous are more than a million L☉ (Eta Carinae reaches 4.6 million) and have masses approaching, possibly exceeding, 100 M☉. The least luminous have luminosities around a quarter of a million L☉ and masses as low as 10 M☉, although they would have been considerably more massive as main-sequence stars, due to their rapid mass loss. Their high mass loss rates could be due to outbursts and very high luminosity and show some enhancement of helium and nitrogen.[7]

Evolution

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The Homunculus Nebula, produced by the Great Outburst of η Carinae

Because of these stars' large mass and high luminosity, their lifetime is very short—only a few million years in total and much less than a million years in the LBV phase.[13] They are rapidly evolving on observable timescales; examples have been detected where stars with Wolf–Rayet spectra (WNL/Ofpe) have developed to show LBV outbursts and a handful of supernovae have been traced to likely LBV progenitors. Some models suggest the latter scenario, where luminous blue variable stars are the final evolutionary stage of some massive stars before they explode as supernovae, for at least stars with initial masses between 20 and 25 solar masses.[14] For more-massive stars, computer simulations of their evolution suggest the luminous blue variable phase takes place during the latest phases of core hydrogen burning (LBV with high surface temperature), the hydrogen shell burning phase (LBV with lower surface temperature), and the earliest part of the core helium burning phase (LBV with high surface temperature again) before transitioning to the Wolf–Rayet phase,[15] thus being analogous to the red giant and red supergiant phases of less massive stars.

There appear to be two groups of LBVs, one with luminosities above 630,000 times the Sun and the other with luminosities below 400,000 times the Sun, although this is disputed in more recent research.[16] Models have been constructed showing that the lower-luminosity group are post-red-supergiants with initial masses of 30–60 times the Sun, whereas the higher-luminosity group are population-II stars with initial masses 60–90 times the Sun that never develop to red supergiants, although they may become yellow hypergiants.[17] Some models suggest that LBVs are a stage in the evolution of very massive stars required for them to shed excess mass,[18] whereas others require that most of the mass is lost at an earlier cool-supergiant stage.[17] Normal outbursts and the stellar winds in the quiescent state are not sufficient for the required mass loss, but LBVs occasionally produce abnormally large outbursts that can be mistaken for a faint supernova and these may shed the necessary mass. Recent models all agree that the LBV stage occurs after the main-sequence stage and before the hydrogen-depleted Wolf–Rayet stage, and that essentially all LBV stars will eventually explode as supernovae. LBVs apparently can explode directly as a supernova, but probably only a small fraction do. If the star does not lose enough mass before the end of the LBV stage, it may undergo a particularly powerful supernova created by pair-instability. The latest models of stellar evolution suggest that some single stars with initial masses around 20 times that of the Sun will explode as LBVs as type II-P, type IIb, or type Ib supernovae,[14] whereas binary stars undergo much-more-complex evolution through envelope stripping leading to less predictable outcomes.[19]

Supernova-like outbursts

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Stars similar to η Carinae in nearby galaxies

Luminous blue variable stars can undergo "giant outbursts" with dramatically increased mass loss and luminosity. η Carinae is the prototypical example,[20] with P Cygni showing one or more similar outbursts 300–400 years ago,[21] but dozens have now been catalogued in external galaxies. Many of these were initially classified as supernovae but re-examined because of unusual features.[22] The nature of the outbursts and of the progenitor stars seems to be highly variable,[23] with the outbursts most likely having several different causes. The historical η Carinae and P Cygni outbursts, and several seen more recently in external galaxies, have lasted years or decades whereas some of the supernova imposter events have declined to normal brightness within months. Well-studied examples are:

Early models of stellar evolution had predicted that although the high-mass stars that produce LBVs would often or always end their lives as supernovae, the supernova explosion would not occur at the LBV stage. Prompted by the progenitor of SN 1987A being a blue supergiant, and most likely an LBV, several subsequent supernovae have been associated with LBV progenitors. The progenitor of SN 2005gl has been shown to be an LBV apparently in outburst only a few years earlier.[24] Progenitors of several other type IIn supernovae have been detected and were likely to have been LBVs:[25]

Modelling suggests that at near-solar metallicity, stars with an initial mass around 20–25 M☉ will explode as a supernova while in the LBV stage of their lives. They will be post-red-supergiants with luminosities a few hundred thousand times that of the Sun. The supernova is expected to be of type II, most likely type IIb, although possibly type IIn due to episodes of enhanced mass loss that occur as an LBV and in the yellow-hypergiant stage.[26]

List of LBVs

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Main article: List of luminous blue variable stars
See also: List of most luminous stars

The identification of LBVs requires confirmation of the characteristic spectral and photometric variations, but these stars can be "quiescent" for decades or centuries at which time they are indistinguishable from many other hot luminous stars. A candidate luminous blue variable (cLBV) can be identified relatively quickly on the basis of its spectrum or luminosity, and dozens have been catalogued in the Milky Way during recent surveys.[27]

Recent studies of dense clusters and mass spectrographic analysis of luminous stars have identified dozens of probable LBVs in the Milky Way out of a likely total population of just a few hundred, although few have been observed in enough detail to confirm the characteristic types of variability. In addition the majority of the LBVs in the Magellanic Clouds have been identified, several dozen in M31 and M33, plus a handful in other local group galaxies.[28]

η Carinae, a luminous blue variable as seen from the Chandra X-ray Observatory
HD 168607 is the right star of the pair below the Omega Nebula. The other is the hypergiant HD 168625.
A selection of LBVs and suspected LBVs with nebula, observed with the Spitzer Space Telescope.

Milky Way

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Candidates:

Several more LBV's have been found near or in the Galactic Center:

Large Magellanic Cloud

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Small Magellanic Cloud

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Andromeda Galaxy

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Triangulum Galaxy

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NGC 2403:

NGC 1156

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  • J025941.21+251412.2[44]
  • J025941.54+251421.8[44]

NGC 2366 (NGC 2363)

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NGC 4449

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  • J122809.72+440514.8[45]
  • J122817.83+440630.8[45]

NGC 4559

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  • AT 2016blu, which has had multiple outbursts since its discovery in the year 2012.[46][47]

NGC 4736 (Messier 94)

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PHL 293B

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  • Unnamed star that underwent an outburst from 1998 to 2008 in an unusual supernova-like event, and has now disappeared[49]

Sunburst galaxy

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Other

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A number of cLBVs in the Milky Way (and in the case of Sanduleak -69° 202, in the LMC) are well known because of their extreme luminosity or unusual characteristics, including:

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Luminous blue variable

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Luminous blue variables (LBVs) are a rare class of evolved, massive that rank among the most luminous objects in their host galaxies, typically exhibiting luminosities exceeding 105L⊙10^5 L_\odot and pronounced photometric and spectroscopic variability on timescales ranging from days to decades. These stars, also known as variables, represent a brief transitional phase in the post-main-sequence evolution of very massive stars with initial masses greater than 30 M⊙M_\odot, bridging the hydrogen-rich O-type stage and the helium-burning –Rayet phase through episodes of and envelope stripping. However, recent models suggest that many LBVs may arise from binary interactions rather than a universal single-star phase. Their high luminosity-to-mass ratios place them near or above the classical Eddington limit, where drives powerful stellar winds and mass-loss rates that can reach 10−510^{-5} to 10−4M⊙10^{-4} M_\odot yr−1^{-1} during quiescent phases, escalating dramatically during outbursts. The defining variability of LBVs includes two primary modes: the more common S Doradus cycles, characterized by cooler temperatures (spectral types shifting from early O to F or even G) at roughly constant bolometric , and rarer giant eruptions that eject shells of material equivalent to several solar masses, as seen in the 19th-century event of η\eta Carinae, which lost over 10 M⊙M_\odot in a decade. These eruptions produce P Cygni line profiles in their spectra due to expanding atmospheres and can create extended nebulae, complicating direct observations of the stellar cores. Prominent examples include the Galactic η\eta Carinae, a with luminosities surpassing 106L⊙10^6 L_\odot and a surrounding ; P Cygni, known for its historical brightness changes; in the ; and . Studies indicate that a significant fraction (around 60%) of LBVs reside in binary or multiple systems, where interactions with companions could trigger or amplify instabilities, influencing their evolutionary paths and potentially leading to outcomes like Type IIn supernovae rather than the traditional single-star progression to core-collapse events. As such, LBVs provide critical insights into the upper mass limit for stars, the role of binarity in massive star evolution, and the enrichment of through their prolific mass ejection.

Overview and Definition

Definition and Classification

Luminous blue variables (LBVs) are a class of evolved massive stars with initial masses greater than approximately 20–40 solar masses (M⊙M_\odot), luminosities exceeding 10510^5 solar luminosities (L⊙L_\odot), and high mass-loss rates on the order of 10−5M⊙10^{-5} M_\odot yr−1^{-1}. These stars exhibit irregular photometric and spectroscopic variability, marking them as a transitional phase in the evolution of very massive stars toward Wolf–Rayet stages. The term "luminous blue variables" was coined by Peter S. Conti in 1984 during a at the IAU 105, unifying previously disparate groups of variable stars such as variables, P Cygni-type stars, and η Carinae under a single category while excluding typical blue supergiants and Wolf–Rayet stars. Classification of LBVs relies on several key criteria, including their location in the Hertzsprung–Russell (HR) diagram near or above the Humphreys–Davidson limit, an observed upper luminosity boundary of approximately log⁡(L/L⊙)≈5.8\log(L/L_\odot) \approx 5.8 beyond which cool supergiants are rare and stars instead appear as hot supergiants or LBVs. In quiescence, LBVs display spectral types ranging from early B supergiants to Ofpe/WN9 or late WN stars, characterized by broad P Cygni profiles in Balmer and helium lines, along with strong Fe II emission. They are distinguished from Wolf–Rayet stars, which lack hydrogen lines and have higher ionization spectra, and from normal blue supergiants, which do not exhibit the characteristic variability or extreme mass loss of LBVs. A defining feature of LBVs is the S Doradus instability, named after the prototype in the , involving cyclic photometric and spectroscopic variations over years to decades. During these S Doradus phases, the star's visual brightness increases by 1–2 magnitudes due to atmospheric expansion and cooling, forming a cooler pseudo-photosphere, while the bolometric luminosity remains essentially constant as the drops from around 25,000 K to 8,000–10,000 K. This instability is attributed to the stars' proximity to the Eddington limit, where drives episodic wind changes and mass ejection.

Observational Characteristics

Luminous blue variables (LBVs) exhibit a distinctive multi-wavelength appearance dominated by a hot, blue continuum in the optical and spectra, arising from their high effective temperatures typically exceeding 20,000 K. Their optical spectra are characterized by strong emission lines of (), neutral (He I), and (such as [N II]), often displaying P Cygni profiles indicative of high-velocity outflows with terminal speeds up to several hundred km/s. In the , LBVs are frequently associated with circumstellar nebulae formed from past mass ejections, where dust grains condense in the cooling ejecta, producing extended emission visible in mid-IR bands as observed by Spitzer spectroscopy revealing silicates and carbon-based features. Photometrically, LBVs display irregular variability in their light curves, with changes occurring on timescales ranging from days to years, reflecting processes in their unstable envelopes. These variations typically have amplitudes of 1–2 magnitudes in the V-band during S Doradus-type cycles, though some events reach up to 3 magnitudes, accompanied by corresponding shifts in spectral type from to late F or G supergiants. The irregular nature of these light curves distinguishes LBVs from periodic variables, with no single dominant periodicity but rather a superposition of short-term fluctuations and longer-term trends. Spatially, LBVs are often surrounded by resolved circumstellar nebulae, which appear as ring-like, bipolar, or irregular structures in high-resolution imaging, such as those obtained with the . These nebulae, spanning arcseconds to arcminutes on the sky, consist of ionized gas and dust shells ejected during previous outbursts, with morphologies suggesting asymmetric mass loss influenced by binary interactions or rapid rotation. For instance, bipolar lobes are common, as seen in detailed Hubble observations of Galactic LBVs, highlighting the role of past ejections in shaping their environments. Detection of LBVs relies on large-scale photometric surveys that identify candidates through their high luminosity and variability, such as the mission, which provides precise and photometry for distance-constrained samples, or the All-Sky Automated Survey for Supernovae (ASAS-SN), which monitors bright variables for irregular patterns. Confirmation typically involves follow-up with ground-based telescopes like the (VLT) or Keck Observatory, where the presence of characteristic emission lines and P Cygni profiles in high-resolution spectra solidifies the classification. This combined approach has expanded the known Galactic and extragalactic LBV populations, enabling systematic studies of their empirical signatures.

Historical Context

Early Observations

One of the earliest recorded examples of a luminous blue variable-like star is P Cygni, first observed in when it abruptly brightened from obscurity to third magnitude, likely due to a sudden mass-loss event that ejected material into its surrounding envelope. This outburst lasted several years, followed by a secondary brightening around 1655, after which the star faded but remained a subject of interest for its persistent variability and unusual spectral features resembling those of novae. Initially dismissed as a temporary phenomenon akin to a nova, P Cygni's behavior puzzled astronomers, who lacked a framework to classify such irregular, high-luminosity changes in a non-supernova context. Similarly, η Carinae was first cataloged by European astronomers in the late 17th century, noted by in 1677 as a steady fourth-magnitude star visible to the in the southern sky. Its prominence grew dramatically during the Great Eruption of the 1840s, with a peak brightness in 1843 when it reached an of approximately -1, temporarily becoming the second-brightest star in the night sky after Sirius. This explosive event, which expelled over 10 solar masses of material, was extensively documented by observers including , who, while stationed in , recorded detailed positional and photometric measurements highlighting the star's rapid rise and subsequent oscillations. Like P Cygni, η Carinae's eruption was often misinterpreted as a nova or peculiar variable without a distinct category, contributing to early confusion about massive stars undergoing such extreme, non-periodic outbursts. By the 19th and early 20th centuries, spectroscopic investigations provided deeper insights into these enigmatic objects, though their unified nature remained unrecognized. For P Cygni, Canadian astronomer C. S. Beals conducted pioneering analyses in the 1930s, examining its prominent emission lines—such as those from and —and interpreting them as signatures of expanding atmospheric shells driven by high-velocity outflows, a key indicator of ongoing mass loss. Beals' work built on earlier spectra but emphasized the dynamic envelope structure, distinguishing P Cygni from typical supergiants or novae. Meanwhile, in the exhibited notable variability, with deep photometric minima recorded in the 1930s during Edwin Hubble's systematic surveys of the nebula, revealing irregular brightenings and fadenings that echoed the behaviors of P Cygni and η Carinae but were initially attributed to general activity without specific classification. Stars like further exemplified these pre-classification challenges, monitored photometrically in the 1920s as an erratic variable with no clear periodic pattern or evolutionary context, often lumped with other supergiants suspected of nova-like episodes. These early observations, spanning brightening events, spectroscopic oddities, and unexplained variability, laid the groundwork for later recognition but were hampered by limited instrumentation and the absence of a cohesive theoretical model for such luminous, unstable massive stars.

Recognition as a Stellar Class

The recognition of luminous blue variables (LBVs) as a distinct stellar class emerged from mid-20th century efforts to synthesize observations of irregularly variable massive stars, culminating in the 1970s and 1980s with theoretical and observational unification. Early individual studies of stars like P Cygni, η Carinae, and revealed shared traits such as high , strong mass loss indicated by P Cygni absorption-emission profiles, and photometric instability, but it was not until systematic analyses that these were grouped together. A pivotal milestone was the 1979 work by Humphreys and Davidson, which empirically established an upper luminosity limit for massive stars (log L/L_⊙ ≈ 5.9–6.0 in the ), emphasizing that stars exceeding this threshold exhibit extreme instability and mass loss, providing a key criterion for identifying the LBV class. In the 1980s, Peter S. Conti proposed unifying P Cygni, η Carinae, S Doradus, and similar variables into a single category based on their common "geyser-like" behavior, extreme luminosities near or above the Humphreys-Davidson limit, and spectral similarities during variability cycles; he formally coined the term "luminous blue variables" in 1984 to describe this group of evolved massive stars undergoing a transitional phase of instability. Concurrently, C. de Jager advanced the concept of a brief "supergiant B phase" in massive star evolution, where stars temporarily cool to B-type temperatures while maintaining high luminosity, leading to atmospheric instabilities and enhanced mass loss that align with LBV characteristics. The International Ultraviolet Explorer (IUE) satellite, operational from 1978 to 1996, played a crucial role by providing high-resolution UV that revealed consistent P Cygni profiles in ultraviolet lines (e.g., C IV λ1550 and N V λ1240) across LBV candidates, demonstrating uniform high-velocity winds and ionization structures despite optical spectral diversity. This instrumental advance helped confirm the class's coherence beyond isolated cases. Initial debates confused LBVs with B supergiants, which also display permitted emission lines from circumstellar disks and shells, but resolution came via the Humphreys-Davidson luminosity threshold—LBVs typically exceed log L/L_⊙ > 5.4 with variable spectra, whereas most B stars are less luminous and lack the characteristic S Doradus-type variability. In the , influential studies by H.J.G.L.M. Lamers and collaborators developed theoretical instability models attributing LBV behavior to proximity to the Eddington limit, where drives atmospheric pulsations, enhanced mass loss (Ṁ ≈ 10^{-5}–10^{-4} M_⊙ yr^{-1}), and spectroscopic variability without invoking binary interactions. These models predicted quasi-periodic microvariations and major eruptions as outcomes of dynamical instabilities in extended envelopes. Surveys in the during this decade, notably by Breysacher et al., cataloged dozens of candidates through spectroscopy of Ofpe/WN9 stars and variables, expanding the known population to over 20 confirmed or probable LBVs and highlighting their rarity (≈1 per 10^6 years in a galaxy like the ).

Physical Properties

Stellar Parameters

Luminous blue variables (LBVs) exhibit a range of current masses between approximately 10 and 40 solar masses (M⊙), reflecting significant prior mass loss during their , while their initial masses are estimated to span 25 to 120 M⊙ based on comparisons with stellar evolutionary tracks. These stars are characterized by extreme luminosities, with logarithmic values log(L/L⊙) typically ranging from 5.3 to 6.0, placing them among the most luminous objects in galaxies. During quiescent phases, their effective temperatures vary between 8,000 and 25,000 , but these can drop to around 4,000 during eruptive outbursts, leading to cooler appearances. The radii of LBVs are extended, generally spanning 50 to 200 solar radii (R⊙), which contributes to their low surface gravities, with log g values approximately 0.5 to 1.5 (in cgs units). This low gravity indicates the presence of inflated envelopes, consistent with their proximity to limits. A key physical constraint for LBVs is their approach to the limit, given by the equation LEdd=4πGMcκ,L_{\rm Edd} = \frac{4\pi G M c}{\kappa}, where GG is the , MM is the , cc is the , and κ\kappa is the opacity. LBVs operate at 0.5 to 1 times LEddL_{\rm Edd}, facilitated by reduced opacity κ\kappa due to ionized , which lowers electron scattering thresholds and enables high mass-loss rates. These parameters are primarily derived from (SED) fitting and non-LTE hydrostatic atmosphere models, such as those using the CMFGEN code, which account for effects and states. Uncertainties in these estimates, often on the order of 20-30%, arise largely from challenges in determining accurate distances and handling the stars' intrinsic variability.

Spectral and Atmospheric Features

Luminous blue variables (LBVs) exhibit spectral types that typically range from B8Ia to A3Ia in the visual spectrum during their quiescent phases, characterized by prominent emission lines of Fe III and Fe IV, which play a crucial role in driving their stellar winds. These iron lines dominate the and optical spectra, reflecting the high states in the extended atmospheres of these massive stars. During hotter quiescent states, some LBVs display spectra resembling those of nitrogen-rich Wolf-Rayet (WN) stars, with strong He II and N III/V lines, indicating a transition toward more evolved compositions while maintaining significant hydrogen content. The winds of LBVs are expansive and optically thick, with terminal velocities generally between 100 and 300 km/s, as measured from the absorption troughs in P Cygni profiles of Balmer and He I lines. Mass-loss rates are exceptionally high, ranging from 10^{-5} to 10^{-4} M_\odot yr^{-1}, derived from the strength of these P Cygni profiles and radio continuum observations, enabling the ejection of substantial envelopes that contribute to their surrounding nebulae. manifests through bi-stability jumps around an effective temperature of 25,000 K, where the recombination of Fe IV to Fe III enhances line opacity, leading to a fivefold increase in mass-loss rates and shifts in wind structure. Chemical anomalies in LBV atmospheres and ejecta reveal nitrogen-to-carbon (N/C) enhancements due to CNO-cycle processing, with surface abundances showing elevated nitrogen and depleted carbon and oxygen. In low-metallicity environments, such as the (LMC), LBVs like R 127 exhibit reduced metal line strengths but retain these CNO signatures, influencing wind driving and variability. Key spectroscopic diagnostics include the Balmer decrement and He I line ratios, which indicate density-bounded envelopes where escapes, preventing full recombination and producing extended emission structures.

Variability and Mass Loss

Photometric and Spectroscopic Variability

Luminous blue variables (LBVs) display characteristic photometric variability known as S Doradus-type excursions, where pseudo-photospheric ejections lead to apparent decreases in and increases in visual brightness by up to 2 magnitudes over timescales of months to years. These events, first observed in itself, involve the formation of extended, cooler pseudo-photospheres that obscure the hotter stellar core, resulting in a net brightening despite the temperature drop. In addition to these larger cycles, LBVs exhibit irregular photometric variations with small amplitudes of 0.1–0.5 mag occurring on timescales of days to weeks. These fluctuations are often attributed to processes such as stellar pulsations or instabilities in wind clumping, which modulate the stellar radius and on short scales without triggering full eruptions. Spectroscopic monitoring reveals corresponding changes in lines, including broadening or narrowing of profiles in high-ionization species like He II, which track the photometric variations through shifts in wind velocity and density structure. For instance, during active phases, P Cygni profiles in emission lines may develop additional blue-shifted absorption components, reflecting enhanced mass ejection correlated with brightness increases. Long-term photometric campaigns, such as those conducted with the All-Sky Automated Survey for Supernovae (ASAS-SN) since the , have documented recurring duty cycles in LBV light curves, highlighting the quasi-periodic nature of their variability over decades. Recent studies in the have further identified potential periodicities in objects like R127, with cycles suggesting underlying dynamical instabilities on multi-year scales. The primary mechanism driving this non-eruptive variability is the line-driving instability in the radiatively accelerated stellar winds, where velocity perturbations amplify into density enhancements that alter the apparent stellar radius and photometry. This instability, exacerbated by the stars' proximity to the Eddington limit, promotes cyclic mass-loss modulations without requiring global envelope ejection.

Eruptive Outbursts

Luminous blue variables (LBVs) are known for their extreme eruptive outbursts, which mimic supernova explosions in luminosity but do not result in the star's destruction, allowing recovery to a quiescent state. These events involve rapid mass ejection and high energy release, often forming expansive nebulae, and are distinguished from true supernovae by the survival of the progenitor star. Giant eruptions, the most dramatic type, release energies on the order of 10^{49} erg, comparable to some supernova remnants, while less energetic outbursts and supernova impostors exhibit lower luminosities but similar spectral signatures of dense, optically thick winds. The prototypical giant eruption occurred in η Carinae during the 1840s, when the star brightened dramatically and ejected approximately 10-20 solar masses of material at velocities up to 650 km/s, forming the iconic bipolar spanning about 0.2 parsecs. This event's kinetic energy is estimated at around 10^{50} erg, with the ejecta displaying remarkable symmetry indicative of a highly ordered outburst mechanism, as revealed by detailed imaging and spectroscopy. Less energetic events, such as those classified as like SN 2008S, involve cooler super-Eddington winds from LBV-like progenitors, peaking at absolute magnitudes around -13 to -15 and fading over months without nebula formation on the scale of the Homunculus. These impostors, observed in galaxies like , share LBV characteristics but are fainter and shorter-lived, typically lasting days to years. Outbursts in LBVs span timescales from years to decades, with mass loss rates surging to 10^{-3} M_\sun yr^{-1} or higher, profoundly impacting the star's envelope and surrounding . Ongoing monitoring of in the 2020s, using facilities like the , has captured its post-outburst nebula from a major event around 10,000 years ago, revealing expanding shells with complex ionization structures and providing insights into recovery phases. Theoretical models attribute these eruptions to sub-Eddington instabilities driven by opacity peaks in the stellar envelope or binary interactions enhancing , though the exact triggers remain debated; crucially, they differ from core-collapse supernovae by lacking nuclear burning termination. A recent advancement addressing gaps in outburst chemistry came in 2025 with the detection of fullerenes (C_{60} and C_{70}) in the circumstellar shell of the candidate LBV WRAY 16-232, likely formed during a past eruption via shock processing of carbon-rich ejecta.

Evolutionary Significance

Place in Massive Star Evolution

Luminous blue variables (LBVs) represent a brief post-main-sequence evolutionary phase for massive stars with initial masses typically exceeding 60 M⊙, occurring shortly before the transition to the Wolf–Rayet (WR) stage, when the stars are around 3–5 million years old. During this phase, LBVs undergo significant hydrogen envelope stripping driven by intense mass loss, which exposes hotter layers and alters their spectral characteristics from those of OB supergiants. This stripping is crucial for evolving these stars toward nitrogen-rich WR (WN) subtypes, as the remaining hydrogen-rich envelope is gradually eroded. In stellar evolution tracks, LBVs emerge from OB supergiant progenitors, potentially following a blue loop excursion in the or, in some models, a brief interlude for lower-mass cases, though very massive stars often remain in the domain. Evolutionary models, such as those from the Geneva group, predict that the LBV phase lasts approximately 10⁴–10⁵ years, representing only a small fraction (∼1–5%) of the star's total lifetime. For instance, non-rotating models of a 60 M⊙ star indicate a duration of about 2 × 10⁵ years in this phase. The high mass-loss rates (Ṁ ≈ 10⁻⁵ to 10⁻⁴ M⊙ yr⁻¹) during the LBV phase play a pivotal role in envelope reduction, enabling the transition to WN stars by removing substantial material over the phase's duration. The cumulative mass lost can be estimated as the integral of the mass-loss rate over time: ΔM=∫M˙ dt≈10 M⊙\Delta M = \int \dot{M} \, dt \approx 10 \, \mathrm{M}_\odot for typical parameters, though eruptive events can contribute up to several tens of solar masses in extreme cases, such as the historical outburst of η Carinae. Uncertainties persist due to the phase's brevity, which renders LBVs statistically rare and challenging to observe in sufficient numbers for robust population studies. Recent models from the 2020s incorporating binary evolution suggest that stellar mergers or in binary systems may trigger or prolong the LBV state, challenging the traditional single-star paradigm. Multiplicity studies indicate a binary fraction of approximately 50–70% among Galactic LBVs, implying that interactions in close binaries could drive the observed instabilities and mass loss. Luminous blue variables (LBVs) have been identified as potential progenitors of Type IIn supernovae (SNe), which exhibit narrow hydrogen emission lines indicative of interaction between the SN ejecta and dense circumstellar material shed by the progenitor. A prominent example is SN 2005gl in NGC 266, where pre-explosion Hubble Space Telescope images revealed a luminous, blue point source consistent with an LBV progenitor having an absolute magnitude of -12.0 in the F300W filter and effective temperature around 20,000 K. Follow-up observations confirmed the disappearance of this progenitor after the explosion, supporting the direct link between LBVs and some core-collapse SNe. However, not all LBV-like events lead to true explosions; many are classified as , where the star undergoes a giant eruption mimicking an SN but survives. SN 1961V in NGC 1058 exemplifies this, with archival Hubble images showing a luminous precursor at the site, interpreted as an LBV outburst rather than a terminal explosion, as the star's position aligns with a surviving massive object. These impostors highlight the ambiguity in distinguishing LBV eruptions from genuine SNe, with pre-explosion imaging providing crucial evidence for survival in cases like SN 1961V. Recent studies in the 2020s have explored failed explosions among massive stars, including potential LBV progenitors, finding that such events are either rare or produce faint, undetected transients rather than bright SNe. The ultimate end states of LBVs depend on their initial masses and evolutionary paths. Stars with initial masses exceeding 130 M⊙ may undergo pair-instability supernovae, leading to complete disruption or formation without a remnant, driven by electron-positron in the oxygen core destabilizing the star. For even more massive progenitors, direct collapse to a is possible, bypassing a traditional SN explosion due to insufficient energy release. Debates persist on whether all LBVs culminate in explosions or transition to Wolf-Rayet (WR) stars after shedding their envelopes. A 2025 review proposes that binary interactions play a key role in LBV evolution, potentially leading to supernovae with dense circumstellar material rather than the traditional single-star progression to WR phases and core-collapse events. These factors suggest not all LBVs reach core collapse in their current state, with binary interactions potentially altering trajectories toward SNe or stable remnants. Central to these discussions is the Humphreys-Davidson limit, an empirical upper luminosity boundary (approximately log L/L⊙ = 5.8) above which massive stars become dynamically unstable, triggering the LBV phase and preventing stable evolution to cooler supergiant stages. This limit underscores why very massive stars, potential LBV progenitors, favor explosive or collapsive ends over quiescent evolution.

Catalog of Known Examples

Milky Way and Solar Neighborhood

Luminous blue variables (LBVs) in the are rare, with only a handful of confirmed examples identified through their characteristic spectral variability and high mass-loss rates. These stars are typically found in the Galactic disk, and their proximity allows for detailed multi-wavelength studies. Among the confirmed LBVs, η Carinae stands out as the prototype, located at a distance of 2.3 kpc with a luminosity of 5×106L⊙5 \times 10^6 L_\odot. Its Great Eruption in the 1840s expelled approximately 10 solar masses of material, forming the and briefly making it one of the brightest stars in the sky. P Cygni, another well-studied confirmed LBV, resides at approximately 1.6 kpc and exhibits a luminosity of around 4×105L⊙4 \times 10^5 L_\odot. Known for its historical , it underwent a major eruption in 1600 that mimicked a , followed by a slow decline in brightness over centuries, providing key insights into LBV eruptive behavior. Recent Data Release 3 measurements have refined distances for several Galactic LBVs, including P Cygni, helping to calibrate their luminosities and place them more accurately on the Hertzsprung-Russell diagram. AG Carinae, at a distance of about 4.7 kpc, is a confirmed LBV displaying S Doradus-type variability with photometric changes up to 2 magnitudes, linked to episodes of enhanced mass loss. Similarly, HR Carinae, located roughly 4.4 kpc away, shows spectroscopic evidence of LBV characteristics, including strong P Cygni profiles in Balmer lines and a surrounding nebula indicative of past eruptions. These stars highlight the diversity in outburst scales among Galactic LBVs, with high-resolution observations revealing nebular structures shaped by their winds. Promising LBV candidates in the include Wd 1-243 in the cluster, which exhibits spectral features akin to confirmed LBVs such as nitrogen enrichment and variability, suggesting it is in a transitional phase. Another candidate, WRAY 16-232, has recently shown of fullerenes (C60_{60}) in its circumstellar shell through Spitzer IRS , indicating complex processing from prior mass ejections. These detections underscore the role of LBV candidates in probing carbon chemistry in massive star environments. Recent studies suggest that binarity is common among LBVs, with approximately 60-70% of known Galactic examples showing evidence of companions, potentially influencing their mass loss and eruptions. For instance, ζ¹ Sco, a candidate LBV, displays variations hinting at a possible binary companion, consistent with models where interactions in binary systems trigger LBV-like activity. Such multiplicity may explain some observed variabilities without invoking purely single-star mechanisms.

Magellanic Clouds

The Large Magellanic Cloud (LMC) hosts approximately 20–23 confirmed luminous blue variables (LBVs), identified through spectroscopic surveys that emphasize P Cygni profiles and variability consistent with S Doradus-type behavior. Prominent examples include R 127 (also known as HD 269582), R 71 (HD 269006), and S Doradus, which exhibit luminosities in the range of 10^{5.5} to 10^6 L_⊙, placing them among the most luminous stars in the galaxy. These stars are primarily detected via Hubble Space Telescope (HST) imaging of their associated nebulae and ground-based spectroscopic campaigns, such as those from the Very Large Telescope (VLT), which have confirmed their status through radial velocity measurements and outburst monitoring from the 2010s onward. The VISTA Magellanic Clouds (VMC) survey, conducted in the near-infrared during the 2010s, has enhanced completeness by providing photometric light curves for candidate selection, revealing variability in about half of the known LMC LBVs. As of 2025, ongoing surveys like those using TESS data continue to monitor variability, potentially identifying additional confirmations. In contrast, the (SMC) contains only a handful of confirmed LBVs, with four identified to date, reflecting lower survey completeness due to greater distance and intrinsically rarer occurrences at subsolar metallicities. Key examples are R 40 (LHA 115-S 6) and Brey 2 (LHA 115-S 18), alongside HD 5980, which exhibited an LBV-like eruption in the ; these stars display luminosities comparable to LMC counterparts but with subdued variability. HST observations have mapped their compact nebulae, while VMC and Optical Gravitational Lensing Experiment (OGLE) photometry from the 2010s–2020s have monitored long-term light curves, identifying few S Doradus cycles. The lower (Z ≈ 1/5 Z_⊙ in the SMC versus 1/2 Z_⊙ in the LMC) results in hotter effective temperatures, weaker line-driven winds, and reduced mass-loss rates for these LBVs, leading to less pronounced photometric variability and no observed giant eruptions akin to those in the . Recent (TESS) studies from 2023–2025, building on VMC data, confirm this trend by analyzing Fourier power spectra of LMC and SMC LBVs, showing diminished red noise amplitudes indicative of stable, low-activity phases. This dependence highlights systematic differences in LBV evolution, with LMC surveys achieving near-complete coverage of bright candidates while SMC detections remain limited to the most luminous examples.

Other Local Group and Nearby Galaxies

In the (M31), several luminous blue variables (LBVs) and candidates have been identified through optical and . Notable examples include AE Andromedae and Var A-1, confirmed via multi-epoch observations showing characteristic P Cygni profiles and variability, as well as newer candidates like LAMOST J0037+4016, a confirmed LBV located approximately 22 kpc from the with a of approximately 4.4×105L⊙4.4 \times 10^5 L_\odot. Additional candidates, such as five studied in 2020, exhibit spectral features akin to Galactic LBVs but with lower detection rates due to M31's distance of about 780 kpc. In M33, a total of around 37 LBV candidates have been cataloged, including confirmed stars like Var B, Var C, and Var 83, often associated with young OB associations. Recent studies highlight a new candidate discovered in 2023 through photometric monitoring, displaying S Doradus-type variability with amplitude up to 1-2 magnitudes, and spatial analyses in 2025 reveal clustering patterns suggesting formation in dense star-forming regions. These detections underscore M33's relatively metal-rich environment compared to more distant dwarfs, facilitating identification despite the galaxy's 840 kpc distance. Beyond the immediate , LBVs are scarcer in nearby galaxies due to observational challenges from distances exceeding 3 Mpc. In NGC 2363, hosted by the NGC 2366 at about 5.4 Mpc, the prototypical LBV NGC 2363-V1 (also known as V1) extensively monitored since its 1997 eruption, revealing ongoing giant outbursts with from WN-type to cooler Fe II emission, indicative of envelope instability. Another candidate, D1-013, shows similar high-ionization lines and variability, though less studied. The IC 10, at 760 kpc, hosts at least three LBV candidates identified via Hα excess and broad emission lines in (HST) imaging and ground-based spectroscopy, with recent 2023 surveys adding B supergiant-LBV hybrids in its metal-poor environment (Z ≈ 1/3 Z_⊙). In the extremely metal-poor dwarf PHL 293B (Z ≈ 1/30 Z_⊙, distance ~14 Mpc), a single LBV dominated the galaxy's spectrum until its apparent disappearance between 2011 and 2019, possibly due to a or heavy obscuration, as evidenced by fading broad Hβ emission. Most extragalactic LBVs beyond the Milky Way and Magellanic Clouds are detected using HST's high-resolution imaging for point-source resolution and ultraviolet/optical spectroscopy to confirm P Cygni profiles and variability, though incompleteness arises from distance-related flux limits and short eruption timescales. In the 2020s, James Webb Space Telescope (JWST) mid-infrared observations have begun revealing obscured LBVs, such as potential candidates in starbursts where dust enshrouds optical signatures. Trends indicate fewer than one confirmed LBV per galaxy on average, reflecting both lower massive star populations and detection biases, with 2025 polarimetric evidence suggesting binarity in candidates resembling Hen 3-519, implying binary interactions may trigger outbursts. Catalog incompleteness is evident in outdated lists; recent studies as of 2025 continue to refine identifications in Local Group galaxies.

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