In 1867, Two Astronomers Charles Wolf and Georges Rayet found Three stars in Cygnus Constellation : HD 191765 (WR 134), HD 192103(WR 135) and HD 192641(WR 137). these stars had not only Broad Emission Bands but they were Continuous Type Spcetrum. While Most of stars only show absorption Lines in their spectra, its element was able to absorb light at specific Frequencies thus they were very Weird at that time and nobody knew the nature of this kind of emission bands and it Remained Mystery for several decades.

Later, Scientists Discovered that many of the Wolf Rayet (WR stars) were the central stars of some planetary nebulae. Similar to a galaxy and its central Black Hole.

The width of Emission lines were caused by Doppler Broadening (when atoms and molecules have their Different velocities then Their Corresponding spectral lines will be Broad Due to doppler effect,
That's called Doppler Broadening)

Thus their Surrounding Gas was moving at 300 - 2400 Km/s along line of sight. that's why they had Broad and unusual Emission Lines in their spectra. it's meaning was They were Continually Ejecting their gas in space at High Rate, Producing a Nebula like structure.

You know most stars form and use Hydrogen and Helium gas clouds and Use their hydrogen as fuel,
But in case of WR stars, They have Ionised Helium and Highly Ionised Nitrogen and Carbon. they have Less hydrogen and Create Very Strong Stellar Winds. Their General Temperature is in between
20,000 K - 210, 000 K, means Hotter than any kind of stars and Early time of their discovery they were Known as W - Type stars in their classification.

In addition to Helium, Researchers have found lines Carbon, Nitrogen and Oxygen in their their spectra.

Some of them Produce Nebulous Structure Through their winds. it is thought these Stars are Extreme O Type Star but they have lost their outer layers through Extremely powerful Stellar winds.



A nebula like Object Created by Wolf - Rayet Star

The Wolf Rayet Stars are Sit on the beyond of upper limit of Morgan Keenan System (MK) System. You can Say They are more Advance than O - Type stars.

But here a question Grows in mind We know There is Brown Dwarfs are beyond of lower limits, more below than M stars and WR stars above the O - Stars then Why there is only O to M groups. Well, These are Common stars and less rare than any other star like objects Moreover They are real Stars who have their common properties that's why in MK system consists only O to M - type stars.

Wolf rayet Stars Features Higher Temperature and much more, So They are different than O class and other Group has been made for them.

There are Three main types of WR Stars with several Different Sub classes.
Each of these major groups information is written in their appropriate Sections
Here let's talk how they Classified

in general, a Wolf rayet star's Type is based on Which element Lines is prominent in its spectra Unlike MK System where Colour, Temperature and Size matter.

The 3 main types of Category is shown Below :

This class members have Ionised Nitrogen lines besides Helium.
This Category is divided in following Sub Groups - :

WN2, WN3, WN4, WN5, WN6, WN7, WN8, WN8, WN9, WN10, WN11, WN12.

Where,

WN5 - WN8 based on temperature and WN2.5 and WN4.5 are obsolete classes, All These class are Specified with Width of their Spectral Lines. WN1 and Both intermediate Classes were Initially Presented but As the Discovery gone further Cancelled for some Reason.

These WR Stars have Ionised Carbon Lines in their spectra.
Sub groups are - :


WC4, WC5, WC6, WC7, WC8, WC9, WC10, WC11

Where,
WC6 to WC8 is based on temperature, Although WC1 - WC3 was used in some cases but not now.

in this type WR stars have Dominant Ionised Oxygen Lines in spectra.
Divided as follows

WO1, WO2, WO3, WO4

Like other WR Classes They are Also based on width of emission Lines.