Wolf–Rayet star | Wikipedia audio article

Wolf–Rayet star | Wikipedia audio article


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 210,000 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. The naked-eye stars Gamma Velorum and Theta
Muscae, as well as the most massive known star, R136a1 in 30 Doradus, are all Wolf–Rayet
stars.==Observation history==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 these 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, Carlyle Smith Beals identified emission lines of carbon, oxygen and nitrogen
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.==Classification==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 541.1 nm 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. 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.
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. Detailed modern studies of Wolf Rayet stars
can identify additional spectral features, indicated by suffixes to the main spectral
classification: h for hydrogen emission;
ha for hydrogen emission and absorption; w for weak lines;
s for strong lines; b for broad strong lines;
d for dust (occasionally vd, pd, or ed for variable, periodic, or episodic dust).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.==Slash stars==
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: f for N iii and He ii emission
f* for N and He emission with N iv stronger than N iii
f+ for emission in Si iv in addition to N and He
parentheses indicating He ii absorption lines instead of emission, e.g. (f)
double parentheses indicating strong He ii absorption and N iii emission diluted, e.g.
((f+))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:
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.==Nomenclature==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==
Properties==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.===Metallicity===
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.===Rotation===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.===Binaries===
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.==Nebulae==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.==Wolf–Rayet galaxies==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.==Evolution==
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.===History===
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.===Current models===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.
Key: O: O-type main-sequence star
Of: evolved O-type showing N and He emission BSG: blue supergiant
RSG: red supergiant YHG: yellow hypergiant
LBV: luminous blue variable WNh: WN plus hydrogen lines
WNL: “late” WN-class Wolf–Rayet star (about WN6 to WN9)
WNE: “early” WN-class Wolf–Rayet star (about WN2 to WN6)
WC: WC-class Wolf–Rayet star WO: WO-class Wolf–Rayet starWolf–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.===Supernovae===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.==Examples==
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

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