Wolf-Rayet stars are very rare, with only around 230 discovered so far in the Milky Way, and about 100 more in the Large Magellanic Cloud.  They start life with around 20 to 25 solar masses or more, so live for short periods of a few million years before turning into red supergiants.  If they retain sufficient mass, they can become Wolf-Rayet stars. 

They are extremely hot, up to 50,000K or more, and this leads to very high luminosity in the range of hundreds of thousands to millions of times that of the Sun.  In this stage, the star has a very short life span of perhaps a million years.  Wolf-Rayet stars can, typically, loose approximately one solar mass every 100,000 to a million years

in their solar winds, which can reach speeds of more than 2,000 km/second.  Thus, in a three million year life time, for example, 3 to 30 times the mass of the sun could be lost; even more with some particularly massive stars.  Most of the solar wind is the surface hydrogen, so more of the lower layers become exposed which is why, depending on the type, we see so much helium, nitrogen, carbon and oxygen in their spectra. 

Outside of the Milky Way, there are a number of galaxies, like Haro15, that are classified as Wolf-Rayet Galaxies due to the high abundance of these star types.  They tend to be small irregular dwarf galaxies, incredibly bright, blue in color, and are usually referred to generically as Starburst Galaxies.  Many astronomers believe that they have been disrupted resulting in the formation of huge numbers of new massive stars. 

Their are several Wolf-Rayet sub-types reflecting the emission lines in their spectra, as shown in the table:

WR Wolf-Rayet Stars

Type
Description
WNL
These are the youngest of the wolf-rayet stars, so still have substantial amounts of hydrogen.  While they are the coolest, they tend to be the largest and most massive, having yet to loose substantial amounts of mass to their stellar winds.  Emission lines show abundant nitrogen with some carbon present.
WNE Further evolved from the WNL type, these have lost their hydrogen shell, and have a helium surface and again show abundant nitrogen and carbon.  They are smaller, hotter and brighter than WNL types. 
WC
The surface contains up to 40% carbon with substantial amounts of oxygen prominent.  Hydrogen and nitrogen both absent, with strong emission lines of carbon, oxygen and helium.  These are the most evolved of the wolf-rayet stars and are the hottest, though they tend to be the least massive and least luminous type. 
WO Very similar to the type WC, but with very substantial amounts of oxygen. 
Only discovered in July of 2010, R136a1 is a blue hypergiant Wolf-Rayet star, and while not particularly large at about 35.4 solar diameters, it is the heaviest known star at approximately 265 stellar masses.  It started its life about one million years ago at around 320 solar masses, and has lost over 50 solar masses in that time.  It is also the most luminous star known, and is 8,700,000 times as luminous as the Sun, with a surface temperature of 53,000K compared to the Sun at 5,778K.  It is likely that the star will explode in a hypernova.  Some astronomers have suggested that it could sucumb to a Pair-instability supernova if it looses more mass, but the amounts of heavy elements in its make up make this unlikely.  R136a1 is not in the Milky Way, but is a member of the R136 star cluster that lies near the center of the 30 Doradus complex (popularly known as the Tarantula Nebula), in the nearby Large Magellanic Cloud galaxy.  There are several other Wolf-Rayet stars in this cluster.  The image to the left shows the R136 cluster in detail, with many of the bright stars visible.  It covers an area approximately 100 light-years across.  To the right there is an image of the entire Tarantula Nebula with a zoom into the R136 cluster, and then into some of its stars.  The Tarantula Nebula is nearly 50 kpc, or about 160,000 light-years, away and the whole nebula is about 500 light-years across. 
R136a1
NGC 3603-A1 is a binary star system located in the NGC 3603 star cluster in the Carina spiral arm of the Milky Way about 20,000 light years from Earth.  The primary star weighs about 116 solar masses, and its companion about 89.  Both are massive WN type Wolf-Rayet stars that orbit each other every 3.77 days.  To the right is a Hubble image of the cluster.  Credit: NASA, ESA, Hubble Heritage Team
NGC 3603-A1
NGC 2359 is an emission nebula surrounding a Wolf-Rayet Star, WR6.  It is about 30 light-years across, and 15,000 light-years away.  The nebula, also known at Thor's Helmet, results from the interaction between the solar wind from the central star and the surrounding interstellar medium.  Although this looks similar to a planetary nebula produced by a star like the Sun on its way to becoming a white dwarf, the mechanism is entirely different. 
NGC 2359 - Thor's Helmet
Astronomy & Cosmology

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Stars - Stellar Classes

WILLIAM & DEBORAH HILLYARD
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