Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. Their surface temperatures range from 30,000 K to around 200,000 K, hotter than almost all other stars. They were previously called W-type stars referring to their spectral classification.
Classic (or Population I) Wolf–Rayet stars are evolved, massive stars that have completely lost their outer hydrogen and are fusing helium or heavier elements in the core. A subset of the population I WR stars show hydrogen lines in their spectra and are known as WNh stars; they are young extremely massive stars still fusing hydrogen at the core, with helium and nitrogen exposed at the surface by strong mixing and radiation-driven mass loss. A separate group of stars with WR spectra are the central stars of planetary nebulae (CSPNe), post asymptotic giant branch stars that were similar to the Sun while on the main sequence, but have now ceased fusion and shed their atmospheres to reveal a bare carbon-oxygen core.
All Wolf–Rayet stars are highly luminous objects due to their high temperatures—thousands of times the bolometric luminosity of the Sun (L☉) for the CSPNe, hundreds of thousands L☉ for the Population I WR stars, to over a million L☉ for the WNh stars—although not exceptionally bright visually since most of their radiation output is in the ultraviolet.
In 1867, using the 40 cm Foucault telescope at the Paris Observatory, astronomers Charles Wolf and Georges Rayet discovered three stars in the constellation Cygnus (HD 191765, HD 192103 and HD 192641, now designated as WR 134, WR 135, and WR 137 respectively) that displayed broad emission bands on an otherwise continuous spectrum. Most stars only display absorption lines or bands in their spectra, as a result of overlying elements absorbing light energy at specific frequencies, so these were clearly unusual objects.
The nature of the emission bands in the spectra of a Wolf–Rayet star remained a mystery for several decades. Edward C. Pickering theorized that the lines were caused by an unusual state of hydrogen, and it was found that this "Pickering series" of lines followed a pattern similar to the Balmer series, when half-integer quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium; a chemical element that was discovered in 1868. Pickering noted similarities between Wolf–Rayet spectra and nebular spectra, and this similarity led to the conclusion that some or all Wolf Rayet stars were the central stars of planetary nebulae.
By 1929, the width of the emission bands was being attributed to Doppler broadening, and hence that the gas surrounding these stars must be moving with velocities of 300–2400 km/s along the line of sight. The conclusion was that a Wolf–Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is radiation pressure. It was well known that many stars with Wolf Rayet type spectra were the central stars of planetary nebulae, but also that many were not associated with an obvious planetary nebula or any visible nebulosity at all.
In addition to helium, emission lines of carbon, oxygen and nitrogen were identified in the spectra of Wolf–Rayet stars. In 1938, the International Astronomical Union classified the spectra of Wolf–Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively.
In 1969, several CSPNe with strong O VI emissions lines were grouped under a new "O VI sequence", or just OVI type. These were subsequently referred to as [WO] stars. Similar stars not associated with planetary nebulae were described shortly after and the WO classification was eventually also adopted for population I WR stars.
The understanding that certain late, and sometimes not-so-late, WN stars with hydrogen lines in their spectra are at a different stage of evolution from hydrogen-free WR stars has led to the introduction of the term WNh to distinguish these stars generally from other WN stars. They were previously referred to as WNL stars, although there are late-type WN stars without hydrogen as well as WR stars with hydrogen as early as WN5.
Wolf–Rayet stars were named on the basis of the strong broad emission lines in their spectra, identified with helium, nitrogen, carbon, silicon, and oxygen, but with hydrogen lines usually weak or absent. The first system of classification split these into stars with dominant lines of ionised nitrogen (N III, N IV, and N V) and those with dominant lines of ionised carbon (C III and C IV) and sometimes oxygen (O III - O VI), referred to as WN and WC respectively. The two classes WN and WC were further split into temperature sequences WN5-WN8 and WC6-WC8 based on the relative strengths of the 541.1nm He II and 587.5 nm He I lines. Wolf–Rayet emission lines frequently have a broadened absorption wing (P Cygni profile) suggesting circumstellar material. A WO sequence has also been separated from the WC sequence for even hotter stars where emission of ionised oxygen dominates that of ionised carbon, although the actual proportions of those elements in the stars are likely to be comparable. WC and WO spectra are formally distinguished based on the presence or absence of C III emission. WC spectra also generally lack the O VI lines that are strong in WO spectra.
The WN spectral sequence was expanded to include WN2 - WN9, and the definitions refined based on the relative strengths of the N III lines at 463.4-464.1 nm and 531.4 nm, the N IV lines at 347.9-348.4 nm and 405.8 nm, and the N V lines at 460.3 nm, 461.9 nm, and 493.3-494.4 nm. These lines are well separated from areas of strong and variable He emission and the line strengths are well correlated with temperature. Stars with spectra intermediate between WN and Ofpe have been classified as WN10 and WN11 although this nomenclature is not universally accepted.
The type WN1 was proposed for stars with neither N IV nor N V lines, to accommodate Brey 1 and Brey 66 which appeared to be intermediate between WN2 and WN2.5. The relative line strengths and widths for each WN sub-class were later quantified, and the ratio between the 541.1 nm He II and 587.5m, He I lines was introduced as the primary indicator of the ionisation level and hence of the spectral sub-class. The need for WN1 disappeared and both Brey 1 and Brey 66 are now classified as WN3b. The somewhat obscure WN2.5 and WN4.5 classes were dropped.
|Spectral Type||Original criteria||Updated criteria||Other features|
|WN2||N V weak or absent||N V and N IV absent||Strong He II, no He I|
|WN2.5||N V present, N IV absent||Obsolete class|
|WN3||N IV ≪ N V, N III weak or absent||He II/He I > 10, He II/C IV > 5||Peculiar profiles, unpredictable N V strength|
|WN4||N IV ≈ N V, N III weak or absent||4 < He II/He I < 10, N V/N III > 2||C IV present|
|WN4.5||N IV > N V, N III weak or absent||Obsolete class|
|WN5||N III ≈ N IV ≈ N V||1.25 < He II/He I < 8, 0.5 < N V/N III < 2||N IV or C IV > He I|
|WN6||N III ≈ N IV, N V weak||1.25 < He II/He I < 8, 0.2 < N V/N III < 0.5||C IV ≈ He I|
|WN7||N III > N IV||0.65 < He II/He I < 1.25||Weak P-Cyg profile He I, He II > N III, C IV > He I|
|WN8||N III ≫ N IV||He II/He I < 0.65||Strong P-Cyg profile He I, He II ≈ N III, C IV weak|
|WN9||N III > N II, N IV absent||N III > N II, N IV absent||P-Cyg profile He I|
|WN10||N III ≈ N II||N III ≈ N II||H Balmer, P-Cyg profile He I|
|WN11||N III weak or absent, N II present||N III ≈ He II, N III weak or absent,||H Balmer, P-Cyg profile He I, Fe III present|
The WC spectral sequence was expanded to include WC4 - WC11, although some older papers have also used WC1 - WC3. The primary emission lines used to distinguish the WC sub-types are C II 426.7 nm, C III at 569.6 nm, C III/IV465.0 nm, C IV at 580.1-581.2 nm, and the O V (and O III) blend at 557.2-559.8 nm. The sequence was extended to include WC10 and WC11, and the subclass criteria were quantified based primarily on the relative strengths of carbon lines to rely on ionisation factors even if there were abundance variations between carbon and oxygen.
|Spectral type||Original criteria||Quantitative criteria||Other features|
|WC4||C IV strong, C II weak, O V moderate||C IV/C III > 32||O V/C III > 2.5||O VI weak or absent|
|WC5||C III ≪ C IV, C III < O V||12.5 < C IV/C III < 32||0.4 < C III/O V < 3||O VI weak or absent|
|WC6||C III ≪ C IV, C III > O V||4 < C IV/C III < 12.5||1 < C III/O V < 5||O VI weak or absent|
|WC7||C III < C IV, C III ≫ O V||1.25 < C IV/C III < 4||C III/O V > 1.25||O VI weak or absent|
|WC8||C III > C IV, C II absent, O V weak or absent||0.5 < C IV/C III < 1.25||C IV/C II > 10||He II/He I > 1.25|
|WC9||C III > C IV, C II present, O V weak or absent||0.2 < C IV/C III < 0.5||0.6 < C IV/C II < 10||0.15 < He II/He I < 1.25|
|WC10||0.06 < C IV/C III < 0.15||0.03 < C IV/C II < 0.6||He II/He I < 0.15|
|WC11||C IV/C III < 0.06||C IV/C II < 0.03||He II absent|
For WO-type stars the main lines used are C IV at 580.1 nm, O IV at 340.0 nm, O V (and O III) blend at 557.2-559.8 nm, O VI at 381.1-383.4 nm, O VII at 567.0 nm, and O VIII at 606.8 nm. The sequence was expanded to include WO5 and quantified based the relative strengths of the O VI/C IV and O VI/O V lines. A later scheme, designed for consistency across classical WR stars and CSPNe, returned to the WO1 to WO4 sequence and adjusted the divisions.
|Spectral type||Original criteria||Quantitative criteria||Other features|
|WO1||O VII ≥ O V, O VIII present||O VI/O V > 12.5||O VI/C IV > 1.5||O VII ≥ O V|
|WO2||O VII < O V, C IV < O VI||4 < O VI/O V < 12.5||O VI/C IV > 1.5||O VII ≤ O V|
|WO3||O VII weak or absent, C IV ≈ O VI||1.8 < O VI/O V < 4||0.1 < O VI/C IV < 1.5||O VII ≪ O V|
|WO4||C IV ≫ O VI||0.5 < O VI/O V < 1.8||0.03 < O VI/C IV < 0.1||O VII ≪ O V|
Detailed modern studies of Wolf Rayet stars can identify additional spectral features, indicated by suffixes to the main spectral classification:
The classification of Wolf Rayet spectra is complicated by the frequent association of the stars with dense nebulosity, dust clouds, or binary companions. A suffix of "+OB" is used to indicate the presence of absorption lines in the spectrum likely to be associated with a more normal companion star, or "+abs" for absorption lines with an unknown origin.
The hotter WR spectral sub-classes are described as early and the cooler ones as late, consistent with other spectral types. WNE and WCE refer to early type spectra while WNL and WCL refer to late type spectra, with the dividing line approximately at sub-class six or seven. There is no such thing as a late WO-type star. There is a strong tendency for WNE stars to be hydrogen-poor while the spectra of WNL stars frequently include hydrogen lines.
Spectral types for the central stars of planetary nebulae are qualified by surrounding them with square brackets (e.g. [WC4]). They are almost all of the WC sequence with the known [WO] stars representing the hot extension of the carbon sequence. There are also a small number of [WN] and [WC/WN] types, only discovered quite recently.    Their formation mechanism is as yet unclear.
Temperatures of the planetary nebula central stars tend to the extremes when compared to population I WR stars, so [WC2] and [WC3] are common and the sequence has been extended to [WC12]. The [WC11] and [WC12] types have distinctive spectra with narrow emission lines and no He II and C IV lines.
Certain supernovae observed before their peak brightness show WR spectra. This is due to the nature of the supernova at this point: a rapidly expanding helium-rich ejecta similar to an extreme Wolf Rayet wind. The WR spectral features only last a matter of hours, the high ionisation features fading by maximum to leave only weak neutral hydrogen and helium emission, before being replaced with a traditional supernova spectrum. It has been proposed to label these spectral types with an "X", for example XWN5(h). Similarly, classical novae develop spectra consisting of broad emission bands similar to a Wolf Rayet star. This is caused by the same physical mechanism: rapid expansion of dense gases around an extremely hot central source.
The separation of Wolf Rayet stars from spectral class O stars of a similar temperature depends on the existence of strong emission lines of ionised helium, nitrogen, carbon, and oxygen, but there are a number of stars with intermediate or confusing spectral features. For example, high luminosity O stars can develop helium and nitrogen in their spectra with some emission lines, while some WR stars have hydrogen lines, weak emission, and even absorption components. These stars have been given spectral types such as O3 If∗/WN6 and are referred to as slash stars.
Class O supergiants can develop emission lines of helium and nitrogen, or emission components to some absorption lines. These are indicated by spectral peculiarity suffix codes specific to this type of star:
These codes may also be combined with more general spectral type qualifiers such as p or a. Common combinations include OIafpe and OIf*, and Ofpe. In the 1970s it was recognised that there was a continuum of spectra from pure absorption class O to unambiguous WR types, and it was unclear whether some intermediate stars should be given a spectral type such as O8Iafpe or WN8-a. The slash notation was proposed to deal with these situations and the star Sk−67°22 was assigned the spectral type O3If*/WN6-A. The criteria for distinguishing OIf*, OIf*/WN, and WN stars have been refined for consistency. Slash star classifications are used when the Hβ line has a P Cygni profile; this is an absorption line in O supergiants and an emission line in WN stars. Criteria for the following slash star spectral types are given, using the nitrogen emission lines at 463.4-464.1 nm, 405.8 nm, and 460.3-462.0 nm, together with a standard star for each type:
|Spectral type||Standard star||Criteria|
|O2If*/WN5||Melnick 35||N iv ≫ N iii, N v ≥ N iii|
|O2.5If*/WN6||WR 25||N iv > N iii, N v < N iii|
|O3.5If*/WN7||Melnick 51||N iv < N iii, N v ≪ N iii|
Another set of slash star spectral types is in use for Ofpe/WN stars. These stars have O supergiant spectra plus nitrogen and helium emission, and P Cygni profiles. Alternatively they can be considered to be WN stars with unusually low ionisation levels and hydrogen. The slash notation for these stars was controversial and an alternative was to extend the WR nitrogen sequence to WN10 and WN11 Other authors preferred to use the WNha notation, for example WN9ha for WR 108. A recent recommendation is to use an O spectral type such as O8Iaf if the 447.1 nm He i line is in absorption and a WR class of WN9h or WN9ha if the line has a P Cygni profile. However, the Ofpe/WN slash notation as well as WN10 and WN11 classifications continue to be widely used.
A third group of stars with spectra containing features of both O class stars and WR stars has been identified. Nine stars in the Large Magellanic Cloud have spectra that contain both WN3 and O3V features, but do not appear to be binaries. Many of the WR stars in the Small Magellanic Cloud also have very early WN spectra plus high excitation absorption features. It has been suggested that these could be a missing link leading to classical WN stars or the result of tidal stripping by a low-mass companion.
The first three Wolf Rayet stars to be identified, coincidentally all with hot O companions, had already been numbered in the HD catalogue. These stars and others were referred to as Wolf–Rayet stars from their initial discovery but specific naming conventions for them would not be created until 1962 in the "fourth" catalogue of galactic Wolf Rayet stars. The first three catalogues were not specifically lists of Wolf Rayet stars and they used only existing nomenclature. The fourth catalogue numbered the Wolf Rayet stars sequentially in order of right ascension. The fifth catalogue used the same numbers prefixed with MR after the author of the fourth catalogue, plus an additional sequence of numbers prefixed with LS for new discoveries. Neither of these numbering schemes is in common use.
The sixth Catalogue of Galactic Wolf Rayet stars was the first to actually bear that name, as well as to describe the previous five catalogues by that name. It also introduced the WR numbers widely used ever since for galactic WR stars. These are again a numerical sequence from WR 1 to WR 158 in order of right ascension. The seventh catalogue and its annex use the same numbering scheme and insert new stars into the sequence using lower case letter suffixes, for example WR 102ka for one of the numerous WR stars discovered in the galactic centre. Modern high volume identification surveys use their own numbering schemes for the large numbers of new discoveries. An IAU working group has accepted recommendations to expand the numbering system from the Catalogue of Galactic Wolf Rayet stars so that additional discoveries are given the closest existing WR number plus a numeric suffix in order of discovery. This applies to all discoveries since the 2006 annex, although some of these have already been named under the previous nomenclature; thus WR 42e is now numbered WR 42-1.
Wolf Rayet stars in external galaxies are numbered using different schemes. In the Large Magellanic Cloud, the most widespread and complete nomenclature for WR stars is from the fourth Catalogue of Population I Wolf Rayet stars in the Large Magellanic Cloud, prefixed by BAT-99, for example BAT-99 105. Many of these stars are also referred to by their third catalogue number, for example Brey 77. As of 2018, 154 WR stars are catalogued in the LMC, mostly WN but including about twenty-three WCs as well as three of the extremely rare WO class. Many of these stars are often referred to by their RMC (Radcliffe observatory Magellanic Cloud) numbers, frequently abbreviated to just R, for example R136a1.
In the Small Magellanic Cloud SMC WR numbers are used, usually referred to as AB numbers, for example AB7. There are only twelve known WR stars in the SMC, a very low number thought to be due to the low metallicity of that galaxy
Wolf–Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad emission lines of helium and nitrogen ("WN" sequence), carbon ("WC" sequence), and oxygen ("WO" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies. About 500 Wolf–Rayets are catalogued in our own Milky Way Galaxy. This number has changed dramatically during the last few years as the result of photometric and spectroscopic surveys in the near-infrared dedicated to discovering this kind of object in the Galactic plane. It is expected that there are fewer than 1,000 WR stars in the rest of the Local Group galaxies, with around 166 known in the Magellanic Clouds, 206 in M33, and 154 in M31. Outside the local group, whole galaxy surveys have found thousands more WR stars and candidates. For example, over a thousand WR stars have been detected in M101, from magnitude 21 to 25. WR stars are expected to be particularly common in starburst galaxies and especially Wolf–Rayet galaxies.
The characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar photosphere, which produces a flood of UV radiation that causes fluorescence in the line-forming wind region. This ejection process uncovers in succession, first the nitrogen-rich products of CNO cycle burning of hydrogen (WN stars), and later the carbon-rich layer due to He burning (WC and WO-type stars).
It can be seen that the WNh stars are completely different objects from the WN stars without hydrogen. Despite the similar spectra, they are much more massive, much larger, and some of the most luminous stars known. They have been detected as early as WN5h in the Magellanic clouds. The nitrogen seen in the spectrum of WNh stars is still the product of CNO cycle fusion in the core, but it appears at the surface of the most massive stars due to rotational and convectional mixing while still in the core hydrogen burning phase, rather than after the outer envelope is lost during core helium fusion.
Some Wolf–Rayet stars of the carbon sequence ("WC"), especially those belonging to the latest types, are noticeable due to their production of dust. Usually this takes place on those belonging to binary systems as a product of the collision of the stellar winds forming the pair, as is the case of the famous binary WR 104; however this process occurs on single ones too.
A few (roughly 10%) of the central stars of planetary nebulae are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; i.e. they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they are much older objects descended from evolved low-mass stars and are closely related to white dwarfs, rather than to the very young, very massive population I stars that comprise the bulk of the WR class. These are now generally excluded from the class denoted as Wolf–Rayet stars, or referred to as Wolf–Rayet-type stars.
The numbers and properties of Wolf Rayet stars vary with the chemical composition of their progenitor stars. A primary driver of this difference is the rate of mass loss at different levels of metallicity. Higher metallicity leads to high mass loss, which affects the evolution of massive stars and also the properties of Wolf Rayet stars. Higher levels of mass loss cause stars to lose their outer layers before an iron core develops and collapses, so that the more massive red supergiants evolve back to hotter temperatures before exploding as a supernova, and the most massive stars never become red supergiants. In the Wolf Rayet stage, higher mass loss leads to stronger depletion of the layers outside the convective core, lower hydrogen surface abundances and more rapid stripping of helium to produce a WC spectrum.
These trends can be observed in the various galaxies of the local group, where metallicity varies from near-solar levels in the Milky Way, somewhat lower in M31, lower still in the Large Magellanic Cloud, and much lower in the Small Magellanic Cloud. Strong metallicity variations are seen across individual galaxies, with M33 and the Milky Way showing higher metallicities closer to the centre, and M31 showing higher metallicity in the disk than in the halo. Thus the SMC is seen to have few WR stars compared to its stellar formation rate and no WC stars at all (one star has a WO spectral type), the Milky Way has roughly equal numbers of WN and WC stars and a large total number of WR stars, and the other main galaxies have somewhat fewer WR stars and more WN than WC types. LMC, and especially SMC, Wolf Rayets have weaker emission and a tendency to higher atmospheric hydrogen fractions. SMC WR stars almost universally show some hydrogen and even absorption lines even at the earliest spectral types, due to weaker winds not entirely masking the photosphere.
The maximum mass of a main-sequence star that can evolve through a red supergiant phase and back to a WNL star is calculated to be around 20 M☉ in the Milky Way, 32 M☉ in the LMC, and over 50 M☉ in the SMC. The more evolved WNE and WC stages are only reached by stars with an initial mass over 25 M☉ at near-solar metallicity, over 60 M☉ in the LMC. Normal single star evolution is not expected to produce any WNE or WC stars at SMC metallicity.
Mass loss is influenced by a star's rotation rate, especially strongly at low metallicity. Fast rotation contributes to mixing of core fusion products through the rest of the star, enhancing surface abundances of heavy elements, and driving mass loss. Rotation causes stars to remain on the main sequence longer than non-rotating stars, evolve more quickly away from the red supergiant phase, or even evolve directly from the main sequence to hotter temperatures for very high masses, high metallicity or very rapid rotation.
Stellar mass loss produces a loss of angular momentum and this quickly brakes the rotation of massive stars. Very massive stars at near-solar metallicity should be braked almost to a standstill while still on the main sequence, while at SMC metallicity they can continue to rotate rapidly even at the highest observed masses. Rapid rotation of massive stars may account for the unexpected properties and numbers of SMC WR stars, for example their relatively high temperatures and luminosities.
Massive stars in binary systems can develop into Wolf Rayet stars due to stripping by a companion rather than inherent mass loss due to a stellar wind. This process is relatively insensitive to the metallicity or rotation of the individual stars and is expected to produce a consistent set of WR stars across all the local group galaxies. As a result, the fraction of WR stars produced through the binary channel, and therefore the number of WR stars observed to be in binaries, should be higher in low metallicity environments. Calculations suggest that the binary fraction of WR stars observed in the SMC should be as high as 98%, although less than half are actually observed to have a massive companion. The binary fraction in the Milky Way is around 20%, in line with theoretical calculations.
A significant proportion of WR stars are surrounded by nebulosity associated directly with the star, not just the normal background nebulosity associated with any massive star forming region, and not a planetary nebula formed by a post-AGB star. The nebulosity presents a variety of forms and classification has been difficult. Many were originally catalogued as planetary nebulae and sometimes only a careful multi-wavelength study can distinguish a planetary nebula around a low mass post-AGB star from a similarly shaped nebula around a more massive core helium-burning star.
A Wolf–Rayet galaxy is a type of starburst galaxy where a sufficient number of WR stars exist that their characteristic emission line spectra become visible in the overall spectrum of the galaxy. Specifically a broad emission feature due to the 468.6 nm He ii and nearby spectral lines is the defining characteristic of a Wolf–Rayet galaxy. The relatively short lifetime of WR stars means that the starbursts in such galaxies must have lasted less than a million years and occurred within the last few million years, or else the WR emission would be swamped by large numbers of other luminous stars.
Theories about how WR stars form, develop, and die have been slow to form compared to the explanation of less extreme stellar evolution. They are rare, distant, and often obscured, and even into the 21st century many aspects of their lives are unclear.
Although Wolf–Rayet stars have been clearly identified as an unusual and distinctive class of stars since the 19th century, the nature of these stars was uncertain until towards the end of the 20th century. Before the 1960's, even the classification of WR stars was highly uncertain, and their nature and evolution was essentially unknown. The very similar appearance of the central stars of planetary nebulae (CSPNe) and the much more luminous classical WR stars contributed to the uncertainty.
By about 1960, the distinction between CSPNe and massive luminous classical WR stars was more clear. Studies showed that they were small dense stars surrounded by extensive circumstellar material, but not yet clear whether the material was expelled from the star or contracting onto it. The unusual abundances of nitrogen, carbon, and oxygen, as well as the lack of hydrogen, were recognised, but the reasons remained obscure. It was recognised that WR stars were very young and very rare, but it was still open to debate whether they were evolving towards or away from the main sequence.
By the 1980's, WR stars were accepted as the descendants of massive OB stars, although their exact evolutionary state in relation to the main sequence and other evolved massive stars was still unknown. Theories that the preponderance of WR stars in massive binaries and their lack of hydrogen could be due to gravitational stripped had been largely ignore or abandoned. WR stars were being proposed as possible progenitors of supernovae, and particularly the newly-discovered type Ib supernovae, lacking hydrogen but apparently associated with young massive stars.
By the start of the 21st century, WR stars were largely accepted as massive stars that had exhausted their core hydrogen, left the main sequence, and expelled most of their atmospheres, leaving behind a small hot core of helium and heavier fusion products.
Most WR stars, the classical population I type, are now understood as being a natural stage in the evolution of the most massive stars (not counting the less common planetary nebula central stars), either after a period as a red supergiant, after a period as a blue supergiant, or directly from the most massive main-sequence stars. Only the lower mass red supergiants are expected to explode as a supernova at that stage, while more massive red supergiants progress back to hotter temperatures as they expel their atmospheres. Some explode while at the yellow hypergiant or LBV stage, but many become Wolf Rayet stars. They have lost or burnt almost all of their hydrogen and are now fusing helium in their cores, or heavier elements for a very brief period at the end of their lives.
Massive main-sequence stars create a very hot core which fuses hydrogen very rapidly via the CNO process and results in strong convection throughout the whole star. This causes mixing of helium to the surface, a process that is enhanced by rotation, possibly by differential rotation where the core is spun up to a faster rotation than the surface. Such stars also show nitrogen enhancement at the surface at a very young age, caused by changes in the proportions of carbon and nitrogen due to the CNO cycle. The enhancement of heavy elements in the atmosphere, as well as increases in luminosity, create strong stellar winds which are the source of the emission line spectra. These stars develop an Of spectrum, Of* if they are sufficiently hot, which develops into a WNh spectrum as the stellar winds increase further. This explains the high mass and luminosity of the WNh stars, which are still burning hydrogen at the core and have lost little of their initial mass. These will eventually expand into blue supergiants (LBVs?) as hydrogen at the core becomes depleted, or if mixing is efficient enough (e.g. through rapid rotation) they may progress directly to WN stars without hydrogen.
WR stars are likely to end their lives violently rather than fade away to a white dwarf. Thus every star with an initial mass more than about 9 times the Sun would inevitably result in a supernova explosion, many of them from the WR stage.
A simple progression of WR stars from low to hot temperatures, resulting finally in WO-type stars, is not supported by observation. WO-type stars are extremely rare and all the known examples are more luminous and more massive than the relatively common WC stars. Alternative theories suggest either that the WO-type stars are only formed from the most massive main-sequence stars, and/or that they form an extremely short-lived end stage of just a few thousand years before exploding, with the WC phase corresponding to the core helium burning phase and the WO phase to nuclear burning stages beyond. It is still unclear whether the WO spectrum is purely the result of ionisation effects at very high temperature, reflects an actual chemical abundance difference, or if both effects occur to varying degrees.
|Initial Mass (M☉)||Evolutionary Sequence||Supernova Type|
|60+||O → Of → WNh ↔ LBV →[WNL]||IIn|
|45–60||O → WNh → LBV/WNE? → WO||Ib/c|
|20–45||O → RSG → WNE → WC||Ib|
|15–20||O → RSG ↔ (YHG) ↔ BSG (blue loops)||II-L (or IIb)|
|8–15||B → RSG||II-P|
Wolf–Rayet stars form from massive stars, although the evolved population I stars have lost half or more of their initial masses by the time they show a WR appearance. For example, γ2 Velorum A currently has a mass around 9 times the Sun, but began with a mass at least 40 times the Sun. High-mass stars are very rare, both because they form less often and because they have short lives. This means that Wolf–Rayet stars themselves are extremely rare because they only form from the most massive main-sequence stars and because they are a relatively short-lived phase in the lives of those stars. This also explains why type Ibc supernovae are less common than type II, since they result from higher-mass stars.
WNh stars, spectroscopically similar but actually a much less evolved star which has only just started to expel its atmosphere, are an exception and still retain much of their initial mass. The most massive stars currently known are all WNh stars rather than O-type main-sequence stars, an expected situation because such stars show helium and nitrogen at the surface only a few thousand years after they form, possibly before they become visible through the surrounding gas cloud. An alternative explanation is that these stars are so massive that they could not form as normal main-sequence stars, instead being the result of mergers of less extreme stars.
The difficulties of modelling the observed numbers and types of Wolf Rayet stars through single star evolution have led to theories that they form through binary interactions which could accelerate loss of the outer layers of a star through mass exchange. WR 122 is a potential example that has a flat disk of gas encircling the star, almost 2 trillion miles wide, and may have a companion star that stripped its outer envelope.
It is widely suspected that many type Ib and type Ic supernova progenitors are WR stars, although no conclusive identification has been made of such a progenitor.
Type Ib supernovae lack hydrogen lines in their spectra. The more common type Ic supernova lack both hydrogen and helium lines in their spectra. The expected progenitors for such supernova are massive stars that respectively lack hydrogen in their outer layers, or lack both hydrogen and helium. WR stars are just such objects. All WR stars lack hydrogen and in some WR stars, most notably the WO group, helium is also strongly depleted. WR stars are expected to experience core collapse when they have generated an iron core, and resulting supernova explosions would be of type Ib or Ic. In some cases it is possible that direct collapse of the core to a black hole would not produce a visible explosion.
WR stars are very luminous due to their high temperatures but not visually bright, especially the hottest examples that are expected to make up most supernova progenitors. Theory suggests that the progenitors of type Ibc supernovae observed to date would not be bright enough to be detected, although they place constraints on the properties of those progenitors. A possible progenitor star which has disappeared at the location of supernova iPTF13bvn may be a single WR star, although other analyses favour a less massive binary system with a stripped star or helium giant. The only other possible WR supernova progenitor is for SN 2017ein, and again it is uncertain whether the progenitor is a single massive WR star or binary system.
By far the most visible example of a Wolf–Rayet star is γ2 Velorum (WR 11), which is a bright naked eye star for those located south of 40 degrees northern latitude, although most of the light comes from an O7.5 giant companion. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of the Southern Skies". The only other Wolf–Rayet star brighter than magnitude 6 is θ Muscae (WR 48), a triple star with two O class companions. Both are WC stars. The "ex" WR star WR 79a (HR 6272) is brighter than magnitude 6 but is now considered to be a peculiar O8 supergiant with strong emission. The next brightest at magnitude 6.4 is WR 22, a massive binary with a WN7h primary.
The most massive and most luminous star currently known, R136a1, is also a Wolf–Rayet star of the WNh type that is still fusing hydrogen in its core. This type of star, which includes many of the most luminous and most massive stars, is very young and usually found only in the centre of the densest star clusters. Occasionally a runaway WNh star such as VFTS 682 is found outside such clusters, probably having been ejected from a multiple system or by interaction with other stars.
Only a minority of planetary nebulae have WR type central stars, but a considerable number of well-known planetary nebulae do have them.
|Planetary nebula||Central star type|
|NGC 5189 (Spiral Planetary Nebula)||[WO1]|
|NGC 6369 (Little Ghost Nebula)||[WO3]|
|MyCn18 (Hourglass Nebula)||[WC]-PG1159|
EZ Canis Majoris (abbreviated to EZ CMa, also designated as WR 6) is binary system in the constellation of Canis Major. The primary is a Wolf-Rayet star and it is one of the ten brightest Wolf-Rayet stars, brighter than apparent magnitude 7.HD 151932
HD 151932, also known as WR 78, is a Wolf-Rayet star located in the constellation Scorpius, close to the galactic plane. Its distance is around 1,300 parsecs (4,200 lightyears) away from the Earth. Despite being a blue-colored Wolf-Rayet star, it is extremely reddened by interstellar extinction, so its apparent magnitude is brighter for longer-wavelength passbands. HD 151932 lies about 22′ west of the open cluster NGC 6231, the center of the OB association Scorpius OB1; it is not clear whether it is a part of the association or not. With an apparent magnitude of about 6.5, it is one of the few Wolf-Rayet stars that can be seen with the naked eye (although it can only be seen with the naked eye under excellent viewing conditions).
Like most extremely massive stars, HD 151932 is losing mass via its stellar wind. The total rate of mass loss is 5×10−5 M☉/yr. The multiplicity (i.e., whether the star is a single star or a binary star system) of HD 151932 has not been studied very much. A periodic shift in the spectrum with a period of 3.3 days (implying it is a spectroscopic binary) has been noticed, but it may be spurious; the star appears to be a single star but may be orbiting face-on and/or with a lower-mass companion.The spectrum of HD 151932 is unusual: part of the He I absorption lines are known to be shifted towards the violet side of the electromagnetic spectrum – this has been interpreted as an expanding stellar shell. Related to this is the fact that the Si IV line varies irregularly in radial velocity, the nature of which is largely unknown. X-rays have been detected from this star, along with several other Wolf-Rayet stars such as WR 24 and WR 136.List of stars in Dorado
This is the list of notable stars in the constellation Dorado, sorted by decreasing brightness.Melnick 34
BAT99-116 (commonly called Melnick 34 or Mk34) is a massive luminous Wolf–Rayet star near R136 in the 30 Doradus complex (also known as the Tarantula Nebula) in the Large Magellanic Cloud.NGC 3199
NGC 3199 is a emission nebula in the constellation Carina. The object was discovered in 1826 by the Scottish astronomer James Dunlop. It is a bow shock around the central star, WR 18, a Wolf–Rayet star.R136b
R136b is a Wolf-Rayet star in the R136 cluster in the Large Magellanic Cloud. It is one of the most massive and most luminous stars known. It is found in the dense R136 open cluster at the centre of NGC 2070 in the Tarantula Nebula.
R136b has a transitional spectral type between an O class supergiant and a Wolf-Rayet star, with a mix of absorption and emission lines. Although it shows enhanced helium and nitrogen at its surface, it is still a very young star, still burning hydrogen in its core via the CNO cycle, and still effectively a main sequence object.WR 1
WR 1 is a Wolf-Rayet star located around 6,000 light years away from Earth in the constellation of Cassiopeia. It is only a third larger than the sun, but due to a temperature over 100,000 K it is 250,000 times as luminous as the sun.
Although WR 1 has been recognised as a Wolf-Rayet star since the 19th century, the WR 1 designation does not indicate that it was the first to be discovered. Ordered by right ascension, WR 1 is the first star in the Sixth Catalogue of galactic Wolf-Rayet stars.WR 1 is a member of the nitrogen sequence of WR stars and has a spectrum with HeII lines much stronger than HeI lines, and NV emission more than twice the strength of NIII, leading to the assignment of a WN4 spectral type. The spectrum has particularly wide HeII, leading to the equivalent classifications of WN4-b (for broad) or WN4-s (for strong). The spectrum also includes CIV and NIV, but no hydrogen lines at all.WR 1 is slightly variable and given the variable star designation V863 Cassiopeiae. The total amplitude of the variations is only 0.09 magnitudes at visual wavelengths. The variations are well-defined with a period of 16.9 days, but the light curve is not sinusoidal and its shape may vary. The variations have been ascribed to a dense asymmetric stellar wind and co-rotating interacting regions in ejected material.It has been suggested that the variability and an infrared excess could be due to a cool companion, but WR 1 is now considered to be a single star. The WN-b subclass of Wolf-Rayet star are generally thought to be all single, in contrast with the WN-A subclass which have narrow emission on a stronger continuum and are thought to be binary systems with a more conventional hot luminous star.WR 1 is a possible member of the Cassiopeia OB7 association at a distance of around 1,800 pc, although its Gaia parallax suggests it is more distant. Interstellar extinction is calculated to be 2.1 magnitudes, and at 1,820 pc the bolometric luminosity would be 250,000 L☉. A temperature of 112,200 K is derived from fitting the spectrum, giving a radius of 2.26 R☉.WR 102ka
WR 102ka, also known as the Peony star, is a Wolf–Rayet star that is one of several candidates for the most luminous-known star in the Milky Way.WR 104
WR 104 is a triple star system located about 7,500 light-years (2,300 pc) from Earth. The primary star is a Wolf-Rayet star, abbreviated as WR, with a B0.5 main sequence star in close orbit and another more distant fainter companion.
The WR star is surrounded by a distinctive spiral Wolf–Rayet nebula, often referred to as a pinwheel nebula. The rotational axis of the binary system, and likely of the two closest stars, is directed approximately towards Earth. Within the next few hundred thousand years, the Wolf-Rayet star is predicted to probably become a core-collapse-supernova with a small chance of producing a long duration gamma-ray burst.
The possibility of a supernova explosion from WR 104 having destructive consequence for life on Earth stirred interest in the mass media and several popular science articles have been issued in the press since 2008. Some articles decide to reject the catastrophic scenario, while others leave it as an open question. Scientists currently believe the odds of WR 104 posing a risk to be small.WR 12
WR 12 (V378 Velorum) is a spectroscopic binary in the constellation Vela. It is an eclipsing binary consisting of a Wolf-Rayet star and a luminous companion of unknown spectral type. The primary is one of the most luminous stars known.
The spectrum of WR 12 is dominated by the broad emission lines of the primary Wolf-Rayet star. The lowest ionisation nitrogen emission lines are strongest, with NV lines being very weak. The HeI lines are stronger than the HeII lines. Unusually the hydrogen emission is stronger still, leading to a WN8h spectral class. CIV emission is almost undetectable. It has been suggested that the companion is a class O star, but its lines cannot be seen in the spectrum.The system produces eclipses which dim the brightness of the star by 0.12 magnitudes every 24 days. The stars are detached and so it is classified as an Algol-type eclipsing binary. The inclination of the orbit has been estimated at 78.8° but a full orbital solution is not available because the secondary cannot be directly detected.WR 124
WR 124 is a Wolf–Rayet star in the constellation of Sagitta surrounded by a ring nebula of expelled material known as M1-67. It is one of the fastest runaway stars in the galaxy with a radial velocity around 200 km/s. It was discovered by Paul W. Merrill in 1938, identified as a high velocity Wolf–Rayet star. It is listed in the General Catalogue of Variable Stars as QR Sagittae with a range of 0.08 magnitudes.WR 136
WR 136 is a Wolf–Rayet star located in the constellation Cygnus. It is in the center of the Crescent Nebula. Its age is estimated to be around 4.7 million years and it is nearing the end of its life. Within a few hundred thousand years, it is expected to explode as a supernova.According to recent estimations, WR 136 is 600,000 times brighter than the Sun, 21 times more massive, and 5.1 times larger. Its surface temperature is around 70,000 kelvins.WR 136 blew off a shell of material when it became a red supergiant around 120,000–240,000 years ago with a mass of around 5 M☉ and this is still expanding at 80 km/s. Currently, its fast stellar wind, ejected from the star at around 3.8 million mph (1,700 km/s), is catching up to the material ejected from the star and shaping it into a shell. Ultraviolet rays emitted from WR 136's hot surface cause the shell to glow.There is some evidence WR 136 may be a binary star. Its companion would be a low-mass star of spectral classification K or M that would complete an orbit around the Wolf-Rayet star each 5.13 days, being the progenitor of a low-mass X-ray binary system.WR 156
WR 156 is a young massive and luminous Wolf–Rayet star in the constellation of Cepheus. Although it shows a WR spectrum, it is thought to be a young star still fusing hydrogen in its core.WR 2
WR 2 is a Wolf-Rayet star located around 8,000 light years away from Earth in the constellation of Cassiopeia. It is smaller than the sun, but due to a temperature over 140,000 K it is 282,000 times as luminous as the sun.
WR 2 is considered to be a member of the nitrogen sequence of WR stars, but completely lacks lines of NIII, NIV, NV, and HeI. Its spectrum is dominated by broad rounded emission lines of HeII, leading to the classification of WN2-b (for broad). It is now given the spectral type of WN2-w (for weak), due to the relative strength of the continuum and lack of extremely intense emission lines. It is the only galactic WN2 star known. Weak-lined Wolf-Rayet stars often have hot luminous companions which dilute the emission. WR 2 does have a companion, but it is much fainter than the primary and not thought to be the cause of the weak-lined spectrum.WR 2 is the smallest and hottest WN star known in the galaxy. Its unusual rounded emission lines are thought to be due to extremely fast rotation, although the exact rotation rate is not known. Estimates range from 500 km/s to approximately the breakup rate for the star of 1,900 km/s. The high temperature also leads to a very fast stellar wind of 1,800 km/s, although the overall rate of mass loss is one of the lowest for any Wolf-Rayet star. The combination of a massive Wolf-Rayet star and rapid rotation is likely to result in a gamma-ray burst when the star explodes as a supernova.X-rays have been detected from WR 2 although they may not be due to colliding winds as is common for massive stars.WR 25
WR 25 (HD 93162) is a binary star system in the turbulent star forming region Carina Nebula, about 6,800 light-years from Earth. It contains a Wolf-Rayet star and a hot luminous companion, and is a member of the Trumpler 16 cluster.WR 42e
WR 42e (2MASS J11144550-115001) is a Wolf-Rayet star in the massive H II region NGC 3603 in the constellation of the Carina. It is around 25,000 light years or 7,600 parsec from the Sun. WR 42e is one of the most massive and most luminous stars known.
WR 42e was first catalogued in 2004 as a member of NGC 3603, numbered 954. It was noted as having x-ray and Hα emission. A detailed study published in 2012 showed that the faint red star was actually a highly obscured (6.4 magnitudes in the visual) hot blue Wolf Rayet star and gave it the name WR 42e. Subsequent changes to the naming conventions for new galactic Wolf Rayet stars mean it is also called WR 42-1.WR 42e is located 2.7 arcmin west-northwest of the massive open cluster HD 97950 at the heart of NGC 3603, corresponding to 6 parsecs at the distance of NGC 3603. This is outside the compact core of the cluster where similar massive luminous stars are found. It is speculated that WR 42e was ejected in an unusual three-body encounter possibly involving the merger of two of the stars and the ejection of both the resulting objects.The spectrum of WR 42e shows many characteristics of an OIf* star, such as hydrogen Balmer series absorption lines and emission lines of ionised nitrogen and helium. The relative strengths of the nitrogen emission lines and the lack of absorption in the 468.4 m, helium line indicate a spectral class of O3 If*. However, the Hβ line shows a distinct emission wing. A P Cygni profile for this line is a defining character of the OIf*/WN class and so WR 42e is assigned the type O3If*/WN6.WR 86
WR 86 is a visual binary in the constellation Scorpius consisting of a Wolf-Rayet star and a β Cephei variable. It lies 2° west of NGC 6357 on the edge of the Great Rift in the Milky Way in the tail of the Scorpion.
WR 86 is a binary with two components of equal visual brightness 0.3" apart. One has the emission-line spectrum of a WC7 Wolf-Rayet star, while the other is a B0 giant. The blue giant varies slightly in brightness every 3.5 hours. The WR star may also be slightly variable.The pulsations of the B-type giant are characteristic of a β Cephei variable. Analysis of its pulsations and comparison to the expected properties of a WC7 star suggest that both stars could have evolved without mass exchange. The WR and B stars would have had initial masses of 40 M☉ and 20 M☉ respectively four million years ago.Westerlund 1
Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young super star cluster in the Milky Way galaxy, about 3.5–5 kpc away from Earth. It is one of the most massive young star clusters in the Milky Way, and was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster.The cluster contains a large number of rare, evolved, high-mass stars, including: 6 yellow hypergiants, 4 red supergiants including Westerlund 1-26, one of the largest known stars, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB[e] star which has been proposed to be the remnant of a recent stellar merger. In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.2-455216, a slow rotating neutron star that must have formed from a high-mass progenitor star. Westerlund 1 is believed to have formed in a single burst of star formation, implying the constituent stars have similar ages and compositions.
Aside from hosting some of the most massive and least-understood stars in our galaxy, Westerlund 1 is useful as a relatively nearby, easy to observe super star cluster that can help astronomers determine what occurs within extragalactic super star clusters.Wolf–Rayet nebula
A Wolf–Rayet nebula is a nebula which surrounds a Wolf–Rayet star.
This classification requires detailed study of each nebula and more recent attempts have been made to allow quick classification of nebulae based purely on their appearance. WR nebulae frequently are ring-shaped in appearance, possibly spherical. Others are irregular, either disrupted shells or formed from clumpy ejection.Examples of this type of nebula include NGC 6888, NGC 2359, and NGC 3199. Some WR nebulae have a prominent spiral structure, for example the WR 104, formerly securing them with the categorization of pinwheel nebulae.
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