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Depiction of Achernar, the brightest Be star

Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as classical Be stars, is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer emission lines.

Definition and classification

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Many stars have B-type spectra and show hydrogen emission lines, including many supergiants, Herbig Ae/Be stars, mass-transferring binary systems, and B[e] stars. It is preferred to restrict usage of the term Be star to non-supergiant stars showing one or more Balmer series lines in emission. These are sometimes referred to as classical Be stars. The emission lines may be present only at certain times.[1]

Although the Be type spectrum is most strongly produced in class B stars, it is also detected in O and A shell stars, and these are sometimes included under the "Be star" banner. Be stars are primarily considered to be main sequence stars, but a number of subgiants and giant stars are also included.[2]

Discovery

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The first star recognized as a Be star was Gamma Cassiopeiae, observed 1866 by Angelo Secchi, the first star ever observed with emission lines.[3] Many other bright stars were found to show similar spectra, although many of these are no longer considered to be classical Be stars.[4] The brightest is Achernar, although it was not recognised as a Be star until 1976.[5][6]

Model

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With the understanding of the processes of emission line formation in the early 20th century it became clear that these lines in Be stars must come from circumstellar material ejected from the star helped by the rapid rotation of the star.[7] All the observational characteristics of Be stars can now be explained with a gaseous disk that is formed of material ejected from the star. The infrared excess and the polarization result from the scattering of stellar light in the disk, while the line emission is formed by re-processing stellar ultraviolet light in the gaseous disc.[2]

Shell stars

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Main article: Shell star

Some Be stars exhibit spectral features that are interpreted as a detached "shell" of gas surrounding the star, or more accurately a disc or ring. These shell features are thought to be caused when the disc of gas that is present around many Be stars is aligned edge on to us so that it creates very narrow absorption lines in the spectrum.

Variability

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Main articles: Gamma Cassiopeiae variable and Lambda Eridani variable

Be stars are often visually and spectroscopically variable. Be stars can be classified as Gamma Cassiopeiae variables when a transient or variable disk is observed. Be stars that show variability without clear indication of the mechanism are listed simply as BE in the General Catalogue of Variable Stars. Some of these are thought to be pulsating stars and are sometimes called Lambda Eridani variables.

References

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Further reading

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Be star

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A Be star is a non-supergiant B-type main-sequence star (spectral types typically O9–B9.5) that exhibits, or has exhibited, emission lines in its optical spectrum—most prominently the of —arising from a circumstellar decretion disk composed of material viscously spread from the stellar equator. These rapidly rotating , with equatorial velocities often reaching 70–100% of their critical speed, form about 20% of all B-type and are distinguished from supergiant Be by their evolutionary stage on or near the . The disks, which are geometrically thin (half-opening angle <20°) and Keplerian in , lack and have temperatures ranging from 7,000 to 20,000 , leading to photometric and spectroscopic variability on timescales from days to decades, including multi-periodic pulsations and V/R (violet/red) emission asymmetries due to density waves. Formation mechanisms primarily involve single-star evolution toward critical during core burning, potentially enhanced by non-radial pulsations or binary interactions that transfer , with the disk material ejected and sustained by turbulent (parameterized by Shakura-Sunyaev α ~ 0.1–1). First in 1866 with γ Cassiopeiae by Angelo Secchi, Be have been extensively studied through , (resolving disks at milliarcsecond scales), and multi-wavelength observations, revealing their role in understanding stellar , mass loss, and disk physics in massive . Many are binaries (with ~20–30% confirmed multiplicities among well-observed samples), influencing disk dynamics and outburst events like those in Be/ binaries, while their high rates (critical fraction W ≥ 0.7) make them the fastest-rotating non-degenerate , with implications for transport and evolutionary paths toward Wolf-Rayet or supernovae.

Definition and Classification

Definition

Be stars are non-supergiant B-type stars with spectral types ranging from B0 to B9.7 that exhibit, or have exhibited at some point, one or more emission lines in the of , most prominently the Hα line, originating from circumstellar material. This distinguishes them from typical B stars, which display absorption lines in these series due to their photospheric atmospheres. The presence of such emission is a defining observational signature, typically linked to a gaseous envelope or disk surrounding the star, though the exact mechanisms are not part of the basic classification. To maintain clarity in , Be stars are restricted to classes III through V, encompassing giants, subgiants, and main-sequence stars, while excluding supergiants (class I) to avoid overlap with other categories of emission-line stars such as B supergiants or . This restriction ensures that the Be designation specifically applies to stars in earlier evolutionary stages where the emission arises primarily from equatorial decretion disks rather than more complex mass-loss phenomena associated with supergiants. The emission features in Be stars are often transient, with lines strengthening, weakening, or completely disappearing over timescales of months to years, reflecting variability in the circumstellar environment. The notation "Be" derives from the Morgan-Keenan (MK) spectral classification system, where "B" indicates the spectral type and the suffix "e" denotes the presence of emission lines in the spectrum.

Classification

Be stars are classified within the framework of the Morgan-Keenan (MK) classification system, which assigns a spectral type from O to M based on and adds luminosity classes (I to V) to indicate evolutionary stage, with Be stars typically spanning luminosity classes III to V as non-supergiant objects. The "e" suffix denotes the presence of emission lines, primarily in the , distinguishing Be stars from normal B-type stars; subtypes further incorporate emission strength, such as standard Be stars with broad emission profiles or Be-shell stars exhibiting narrow absorption "shell" lines superimposed on the emission due to specific disk viewing geometries. Luminosity classes help refine categorization, with class V indicating main-sequence stars and higher classes (III–IV) denoting subgiants or giants, though supergiant Be stars (class I–II) are rare and often excluded from classical definitions. Be stars are broadly divided into classical Be stars, which are main-sequence or slightly evolved B-type stars (spectral types B0–B9.5), and Herbig Be stars, the pre-main-sequence counterparts found in young clusters and star-forming regions. Classical Be stars represent the mature phase where rapid rotation ejects material to form a circumstellar disk, while Herbig Be stars are intermediate-mass (roughly 2–10 M⊙) objects still accreting from their natal envelopes, often showing stronger excess and association with reflection nebulae. This distinction highlights evolutionary differences, with Herbig Be stars serving as transitional objects between low-mass stars and high-mass protostars. In the , approximately 20% of non-supergiant B-type stars exhibit Be characteristics, a fraction that increases to about 30% in the metal-poor environment of the due to enhanced rotational velocities from reduced disk coupling. Be stars are distinguished from related classes like Oe stars, which are earlier O-type main-sequence objects (O9–B0) displaying analogous Balmer emission from circumstellar disks but with stronger He II lines reflecting higher temperatures, and Ae stars, which extend the phenomenon to cooler A-type spectra (A0–A5) with similar emission but lower masses and often pre-main-sequence status. These distinctions maintain the core B-type focus for classical Be stars while acknowledging the continuum of emission-line phenomena across early-type stars.

Historical Context

Discovery

The first identification of a Be star occurred in 1866, when Italian astronomer Angelo Secchi observed bright emission lines in the spectrum of during spectroscopic studies conducted in . This observation marked the initial recognition of a distinct class of stars exhibiting emission features, particularly in the hydrogen Balmer series, amid the emerging field of stellar spectroscopy. In the , such emission lines were initially puzzling to astronomers, as they contrasted with the absorption spectra typical of most and were more commonly associated with transient phenomena like novae or gaseous nebulae, leading to early interpretations of as a peculiar or anomalous object rather than a representative of a new stellar category. By the early , systematic surveys advanced the identification of additional candidates; the Observatory, under Edward C. Pickering and with classifications by , cataloged dozens of showing emission lines in the Henry Draper Catalogue (published 1918–1924), establishing a foundation for recognizing the Be class through photographic . Many stars initially cataloged as irregular or eruptive variables in the late 19th and early 20th centuries were later reclassified as Be stars once their photometric variability—often due to circumstellar material—was linked to the spectral emission characteristics. For instance, the brightest known Be star, (Alpha Eridani), was suspected of peculiarities in earlier observations but was definitively confirmed as a Be star in through the detection of strong Hα emission by Andrews and Breger, resolving prior ambiguities in its classification.

Early Models and Observations

In 1931, proposed that the emission lines observed in Be stars originate from gaseous material ejected equatorially due to the stars' extremely rapid rotation, which could reach velocities of several hundred km/s, leading to the formation of a flattened around the star. This model explained the double bright lines in the spectra of B-type stars with broad, flat absorption features, attributing them to rotational broadening and circumstellar emission rather than intrinsic stellar properties. Struve's hypothesis shifted the understanding of Be stars from anomalous spectral peculiarities to dynamically driven phenomena, laying the groundwork for subsequent investigations into rotational effects. In the 1970s, photoelectric photometry confirmed the presence of infrared excess in Be stars, indicating thermal emission from warm circumstellar material, primarily due to free-free and bound-free processes in the ionized gas disk. Concurrently, measurements of , achieved through early photoelectric polarimeters, revealed intrinsic polarization levels up to several percent in Be stars, interpreted as of stellar by circumstellar electrons in an asymmetric envelope. These observations, such as those using the Dyck polarimeter on bright Be stars like γ Cassiopeiae, supported the existence of extended, flattened structures consistent with Struve's equatorial ejection model and distinguished Be star polarization from interstellar foreground effects. Early slit spectrographs, employed since the late but refined in the mid-20th century for higher resolution, prominently revealed double-peaked emission profiles in Balmer lines of Be stars, with the peak separation corresponding to projected rotational velocities of 200–400 km/s. These profiles indicated Keplerian motion in a rotating disk-like viewed at an inclination angle. In the 1970s, initial infrared detections at wavelengths beyond 10 μm, using ground-based telescopes like the 200-inch at Palomar, were attributed to free-free emission from the hot ionized circumstellar disk (temperatures around 7,000–20,000 K). Initial interpretations faced challenges, including confusion with binary systems due to apparent radial velocity shifts in emission lines and shell absorption features mimicking eclipsing effects or companion spectra. By the 1980s, kinematic studies using high-resolution spectrographs resolved these ambiguities, demonstrating that the double-peaked profiles and velocity gradients were due to disk rotation rather than orbital motion, with stable stellar radial velocities confirming most Be stars as single rapid rotators. These analyses, including detailed mapping of emission line asymmetries, clarified the circumstellar origin of shell absorptions as density enhancements in the inner disk.

Physical Characteristics

Spectral Features

Be stars exhibit prominent emission lines in the , most notably Hα, Hβ, Hγ, and Hδ, which arise from the circumstellar environment surrounding the central B-type star. These lines are a defining spectroscopic signature, distinguishing Be stars from normal B stars, and their presence indicates ongoing mass ejection or disk formation processes. The Hα line, in particular, is the strongest and most commonly observed, serving as a primary diagnostic for disk activity. The profiles of these Balmer emission lines frequently display double-peaked structures, with the two peaks separated by velocities corresponding to the rotational motion in the Keplerian disk. This symmetric or asymmetric double-peaked appearance reflects the azimuthal velocity distribution in the disk, where the blue-shifted peak originates from the approaching side and the red-shifted peak from the receding side relative to the observer. In edge-on systems, known as shell stars, the profiles can appear more complex with central absorption reversals superimposed on the emission. For example, in the Be star γ Cas, the Hα profile shows a clear double peak with a separation of approximately 200–300 km/s, varying with disk phase. In addition to Balmer lines, permitted metallic emission lines, such as those from Fe II (e.g., multiplets at 5018 , 5169 , and 5317 ), are commonly detected, particularly in stars with cooler or denser disks where recombination occurs at lower temperatures. These Fe II lines often exhibit similar double-peaked or shell-like profiles, tracing the inner disk regions. Forbidden lines, such as [O I] at 6300 and 6364 , are generally absent in classical Be stars due to the relatively high densities in their disks, which suppress such low-density transitions. P Cygni profiles, featuring broad emission with blue-shifted absorption components, appear in select high-velocity cases among Be stars, signaling the presence of outflowing material superimposed on the disk emission. These are less common than pure emission profiles and are typically seen during episodes of enhanced mass loss, as in ζ Tau where Hα occasionally shows P Cygni characteristics with absorption blueshifts up to 500 km/s. The of the Hα line serves as a key metric for activity levels, typically ranging from 10 to 50 in active phases, with stronger emission (larger absolute widths) correlating to larger or denser disks; for instance, measurements in a sample of 118 classical Be stars yielded Hα equivalent widths between -0.5 and -72.7 , with most active objects falling in the 10–50 range. The strength of these emission features also informs the Be star classification scheme based on emission intensity.

Circumstellar Disk and Rotation

Be stars exhibit rapid , with equatorial velocities typically reaching 70–90% of their critical breakup velocity, which for main-sequence B-type stars ranges from approximately 400 to 600 km/s. This near-critical is essential for the formation and sustenance of their circumstellar disks, as it enables the ejection of stellar material into an equatorial plane through mechanisms such as non-radial pulsations or . Observations indicate that while some Be stars approach 95% of critical , the majority cluster around 80%, distinguishing them from slower-rotating non-emission B stars. The circumstellar disk surrounding a Be star is a geometrically thin, Keplerian structure extending from roughly 1 to 10 stellar radii, where the gas orbits the central star with velocities decreasing as v∝r−1/2v \propto r^{-1/2}. The disk's radial profile follows ρ∝r−n\rho \propto r^{-n}, with n≈3.5n \approx 3.5 derived from spectroscopic and interferometric analyses of multiple systems, reflecting a balance between viscous and outward mass transport. This power-law decline ensures higher densities near the star, facilitating the observed emission features. Direct imaging via long-baseline , such as with the Interferometer (VLTI) using the instrument, has resolved the mid-infrared continuum emission from Be star disks, confirming angular diameters corresponding to physical extents of 5–15 stellar radii for nearby examples like α Arae and ζ Tau. These observations reveal nearly edge-on, axisymmetric structures with minimal dependence in size between 8 and 12 μm, supporting a flared but thin . Additionally, Data Release 3 proper motions have identified isolated field Be stars beyond cluster environments, validating that disk-bearing systems are prevalent in the general Galactic population rather than confined to young associations. The disk's composition is dominated by neutral hydrogen, comprising over 99% of the gas mass, with trace amounts of metals such as iron and influencing opacity and line formation. decreases radially from about 10410^4 in the inner regions—heated primarily by stellar and viscous processes—to around 10310^3 at outer edges, creating a that affects and emission properties. This thermal structure, modeled in isothermal approximations, aligns with the observed double-peaked emission lines from Keplerian motion.

Theoretical Frameworks

Decretion Disk Model

The decretion disk model posits that the circumstellar disks around Be stars form through the ejection of material from the star's equatorial region, followed by viscous diffusion that transports outward, allowing the disk to expand and build up over time. This , first proposed in the early , contrasts with accretion disks by having mass injection at the inner boundary rather than the outer, resulting in near-Keplerian with outward radial drift. The model assumes a geometrically thin, isothermal disk where , parameterized by the Shakura-Sunyaev α prescription, drives the evolution. In this framework, the radial velocity vrv_r governing the outward flow is approximated by vr=32αΩK(Hr)2,v_r = \frac{3}{2} \alpha \Omega_K \left( \frac{H}{r} \right)^2, where α\alpha is the dimensionless viscosity parameter (typically 10−210^{-2} to 10−110^{-1}), ΩK=GM/r3\Omega_K = \sqrt{GM/r^3}
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