W Stars: Definition

Wolf-Rayet stars are hot (25-50,000+ degrees K), massive stars (20+ solar mass) with a high rate of mass loss. Strong, broad emission lines (with equivalent widths up to 1000Å!) arise from the winds of material being blown off the stars.

W Stars: Description

First identified by French Astronomers Charles Wolf and Georges Rayet in 1867, Wolf-Rayet, or W-R stars are exceptionally large stars that are quite rare. Current surveys of our own Milky Way galaxy estimate that there are 100 billion to 400 billion stars in the galaxy, however, we have found fewer than 300 Wolf-Rayet stars.

A Wolf-Rayet star differes from the average star like our sun in several ways: they are very massive, and so they burn very hot; since they burn so very hot, they will burn through their fuel supply in only several million years finally, because they are burning so hot and fast, they have a tremendously strong stellar wind.

The nuclear fusion process that fuels a star, releases tremendous amounts of outward pressure on the gasses that make up the star. Most of this pressure is overcome by the star’s gravity; however, some of the star’s outer atmosphere is propelled in to the solar system resulting in a constant outflow of charged particles we call the Solar Wind. For a huge W tape star its solar wind is a raging hurricane by comparison to that of the Sun! In almost explosive bursts, this stronger solar wind not only hurls simple ions into space but also heavier elements such as Carbon, Oxygen, and Nitrogen. According to NASA, in a year a typical Wolf-Rayet star can expel as much mass as the Earth weighs.

So the typical Wolf-Rayet star has a large cloud of molecular gas surrounding it, which in and of itself is interesting but not the end of the story. All of the radiation the star emits slams into the gas resulting in several phenomena we can easily observe with typical amateur telescopes.

First, the gas becomes ionized and emits strongly along several emission lines that we can easily separate from other wavelengths of light being radiated by the star. Most Wolf-Rayet star nebulas are strong Oxygen III, or OIII emitters. That is the specific wavelength of light emitted when the ionized oxygen returns to its lower energy state. Most amateur astronomers keep an OIII filter in their eyepiece case to help separate the glow of wispy nebulas from background stars or more frequently, nearby light pollution.

Secondly, the radiation exerts a physical pressure on the gas and pushes it away from the star. This results in a nebula that resembles an expanding bubble, which is essentially what it is: an expanding bubble of gas in space.

When a Wolf-Rayet star dies, it often undergoes a supernova detonation. Some of these supernovae are sufficiently energetic to be classified as hypernova. And some of these hypernovae are accompanied by a gamma-ray burst as the core undergoes a gravitational collapse into the most awesome object in the Universe, a black hole: an object whose gravitational pull is so powerful nothing, not even light, can escape.

Wolf-Rayets stars are divided into 3 classes based on their spectra, the WN stars (nitrogen dominant, some carbon), WC stars (carbon dominant, no nitrogen), and the rare WO stars with C/O < 1.

The WN stars optical spectra show emission lines from H, NIII (4640Å), NIV, NV, HeI, HeII, and from CIV at 5808Å. In the UV, there are strong emission features from NII, NIII, NIV, NV, CIII, CIV, HeII, OIV, OV, and SiV.

The WC stars optical spectra show emission lines from H, CII, CIII (5696Å), CIV (5805Å), OV (5592Å), HeI, and HeII. No nitrogen lines are seen in the WC stars. In the UV, there are strong emission features from CII, CIII, CIV, OIV, OV, SiIV, HeII, FeIII, FeIV, and FeV.

Characteristics by Type of W Star

WN Type

Nitrogen Line Criteria

Other Criteria

WN 9

NIII present; NIV weak or absent

lower Balmer series; HeI

WN 8

NIII >> NIV

NIII 4640Å < < HeI ; 4686Å>

WN 7

NIII > NIV

NIII 4640Å < <>

WN 6

NIII = NIV; NV present but weak

WN 5

NIII = NIV = NV

WN 4.5

NIV > NV; NIII weak or absent

WN 4

NIV = NV; NIII weak or absent

WN 3

NIV << < or weak NIII>

WN 2

NV weak or absent

Strong HeII


WC Type

Carbon Line Criteria

Carbon/Oxygen Criteria

Other Criteria

WC 9

CIII > CIV

OV weak or absent

CII present

WC 8.5

CIII > CIV

OV weak or absent

CII not present

WC 8

CIII = CIV

OV weak or absent

WC 7

CIII << CIV

CIII >> OV

WC 6

CIII << <CIV

CIII > OV

WC 5

CIII << <CIV

CIII < < OV

WC 4

CIII <<< <CIV

OV moderate

W Stars: Variability/Peculiarity