HUBBLE SPACE TELESCOPE PHOTO OF WR 124 IN THE NEBULA M1-67
Wolf-Rayet Stars (often referred to as WR stars) are evolved, MASSIVE stars (OVER 20 Solar Masses), which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s. While our own Sun loses approximately 10−14 solar masses every year, Wolf-Rayet stars typically lose 10−5 solar masses a year which is a Billion times more mass loss than our own sun loses.
Wolf-Rayet stars are very hot, with surface temperatures in the range of 25,000 K to 50,000 K. It is believed that the star in the galaxy NGC 2770 that exploded into a supernova on January 9, 2008 — SN 2008D, the first supernova ever observed in the act of exploding — was a Wolf-Rayet star.
In 1867, astronomers using the 40 cm Foucault telescope at the Paris Observatory, discovered three stars in the constellation Cygnus (now designated HD191765, HD192103 and HD192641), that displayed broad emission bands on an otherwise continuous spectrum. The astronomers' names were Charles Wolf and Georges Rayet, and thus this category of stars became named Wolf-Rayet (WR) stars. Most stars display absorption bands in the spectrum, as a result of overlaying elements absorbing light energy at specific frequencies. The number of stars with emission lines is quite low, 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-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium; a gas that was discovered in 1868.
By 1929, the width of the emission bands was being attributed to the Doppler effect, 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.
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.
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) or helium, carbon, and oxygen ("WC" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies. About 230 Wolf-Rayets are known in our own Milky Way Galaxy, about 100 are known in the Large Magellanic Cloud, while only 12 have been identified in the Small Magellanic Cloud.
Several astronomers, among which Rublev (1965)  and Conti (1976) originally proposed that the WR stars as a class are descended from massive O-stars in which the strong stellar winds characteristic of extremely luminous stars have ejected the unprocessed outer H-rich layers. 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 & WO stars). Most of these stars are believed finally to progress to become supernovae of Type Ib or Type Ic. 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 stars that comprise the bulk of the WR class.
Evolution models of WR stars.
For stars of ~40-75MS
For stars of 25-40MS
It is possible for a Wolf-Rayet star to progress to a "collapsar" stage in its death throes: This is when the core of the star collapses to form a black hole, pulling in the surrounding material. This is thought to be the precursor of a long gamma-ray burst.
The best known (and most visible) example of a Wolf-Rayet star is Gamma 2 Velorum (γ² Vel), which is a bright star visible to those located south of 40 degrees northern latitude. One of the members of the star system (Gamma Velorum is actually at least six stars) is a Wolf-Rayet star. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of Southern Skies".
(1 solar mass = the mass of Earth's Sun). Some of them are Wolf-Rayet Stars (WR)
Stellar mass is the most important attribute of a star. Combined with chemical compositions, mass determines a star’s luminosity, its physical size, and its ultimate fate. Due to their mass, most of the stars below will eventually go Supernova or Hypernova, Black Holes and form .
The masses listed in the table are inferred from theory, using difficult measurements of the stars’ temperatures and absolute brightnesses. All the listed masses are uncertain: both the theory and the measurements are pushing the limits of current knowledge and technology. Either measurement or theory, or both, could be incorrect. An example is VV Cephei, which, depending on which property of the star is examined, could be between 25 to 40, or 100 solar masses.
Massive stars are rare; astronomers must look very far from the Earth to find one. All the listed stars are many thousands of light years away, and that alone makes measurements difficult. In addition to being far away, it seems that most stars of such extreme mass are surrounded by clouds of outflowing gas; the surrounding gas obscures the already difficult-to-obtain measurements of the stars’ temperatures and brightnesses, and greatly complicates the issue of measuring their internal chemical compositions.
In addition, the clouds of gas obscure observations of whether the star is just one supermassive star, or instead a multiple star system. A number of the stars below may indeed consist of two or more companions in close orbit, each star being large in themselves, but not necessary massive; alternatively the system may still have one (or more) massive star, with much smaller companions.
Hence many of the masses listed below are contested, and being the subject of current research, are constantly being revised.
Amongst the most reliable listed masses are A1 and WR20a+b, which were obtained from orbital measurements. A1 and WR20a+b are both members of binary star systems (two stars orbiting around each other), and it is possible to measure in both cases the individual masses of the two stars by studying their orbital motion, via Kepler's laws of planetary motion. This involves measuring their radial velocities and also their light curves, as A1 and WR20a+b are both eclipsing binaries.
A number of the stars may have started out with even greater masses than those currently estimated, but due to the huge amount of gas they outflow, and sub-supernova and supernova imposter explosion events, have lost many tens of solar masses of material.
Also there are a number of supernovae and hypernovae remnants whose pre-cursor stars' masses can be estimated based on pre-super/hypernova observations, the energy of the super/hypernova, and the type of super/hypernova event. These stars (if they had not exploded) would have easily made appearances in this list (however they are not shown below).
Known stars with an estimated mass of 20 or greater solar masses:
Main article: Black hole
Black holes are the end point evolution of massive stars. Technically they are not stars, as they no longer generate nuclear fusion in their cores.
Main article: Eddington luminosity
Astronomers have long theorized that as a protostar grows to a size beyond 120 solar masses, something drastic must happen. Although the limit can be stretched for very early Population III stars, if any stars existed above 120 solar mass, they would challenge current theories of stellar evolution.
The limit on mass arises because stars of greater mass have a higher rate of core energy generation, which is higher far out of proportion to their greater mass. For a sufficiently massive star, the outward pressure of radiant energy generated by nuclear fusion in the star’s core exceeds the inward pull of its own gravity. This is called the Eddington limit. Beyond this limit, a star ought to push itself apart, or at least shed enough mass to reduce its internal energy generation to a lower, maintainable rate. In theory, a more massive star could not hold itself together, because of the mass loss resulting from the outflow of stellar material.
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