by John H. Bickel, Ph.D.
Denver Astronomical Society, Member

What is a Wolf-Rayet star?

In 1867, French astronomers Charles Wolf and Georges Rayet using the 40 cm Foucault telescope, at the Paris Observatory [Ref. 1], discovered three stars in the constellation Cygnus (now designated HD191765, HD192103 and HD192641) that displayed broad emission bands instead of the typical absorption lines seen in most stellar spectra. By the early 20th century, the extreme widths of the emission bands had been attributed to Doppler broadening from fast stellar winds, revealing that these stars were shedding stellar mass at extraordinary rates. In 1938, the International Astronomical Union formalized the spectral classification into WN (nitrogen-rich) and WC (carbon–oxygen–rich) types, establishing the framework still used to study these rare, evolved massive stars. WO stars were recognized only after the broader Wolf–Rayet classes (WN and WC) were established after astronomers noticed a handful of Wolf–Rayet spectra dominated by high‑ionization oxygen lines, especially the hallmark O VI¹ feature. These signatures indicated temperatures far beyond typical WC stars, up to ~210,000°K, and hotter than almost any other known stellar photosphere.

¹ O VI or triple ionized oxygen atoms.

Why are Wolf Rayet stars Physically different from main sequence stars?

Wolf–Rayet stars are hot, massive², and highly evolved stars whose spectra are dominated by strong, broad emission lines from helium, nitrogen, carbon, or oxygen. [Ref. 2] These emission lines arise from powerful, fast, radiatively driven stellar winds that strip the star’s outer layers, revealing chemically processed material from deep inside the star. Most Wolf–Rayet stars have evolved beyond the main sequence and are typically core helium‑burning, hydrogen‑depleted objects, with surface temperatures between 30,000 – 210,000°K. These extreme temperatures occur because Wolf–Rayet stars have lost their outer hydrogen layers, exposing compact, highly luminous cores with intense radiation-driven stellar winds.

Wolf–Rayet stars are rare compared to ordinary stars. The current census lists 667 WR stars in the Milky Way, with thousands more likely in nearby galaxies such as the Large Magellanic Cloud and Small Magellanic Cloud. Even so, they represent only a tiny fraction of all massive stars because the Wolf–Rayet phase is short-lived and requires either very high initial mass or strong stripping through winds or binary interaction.

Wolf–Rayet stars differ from main‑sequence stars in both structure and appearance. Main‑sequence stars are hydrogen‑burning objects with relatively thin winds and absorption‑line spectra. In contrast, Wolf–Rayet stars have optically thick winds, broad emission‑line spectra, and surfaces enriched with the products of nuclear burning (N for WN stars, C/O for WC/WO stars). Their extreme luminosity‑to‑mass ratios place them near the Eddington limit, driving mass loss far stronger than anything seen in main‑sequence stars.

²Wolf-Rayet stars average from 9 to more than 110 Solar Masses.

What happens at the end of life of Wolf Rayet stars?

At the end of their lives, most Wolf–Rayet stars explode as core‑collapse supernovae, typical of Type Ib or Ic, depending on how much of their outer layers have been stripped. Wolf–Rayet stars have already lost their hydrogen envelopes, so when their iron cores finally collapse, the resulting explosion lacks hydrogen lines (Type Ib) or both hydrogen and helium (Type Ic). This pathway is well‑supported by stellar‑evolution models and observations of Wolf–Rayet environments, and it reflects the fact that WR stars are in the final, exposed stages of nuclear burning.

The most extreme Wolf–Rayet stars, especially WO stars, with surface temperatures up to ~210,000°K are thought to be immediate progenitors of the most energetic explosions, including broad‑lined Type Ic supernovae and possibly long‑duration gamma‑ray bursts, if rapid rotation is present. Their cores progress through fusion of heavier elements until reaching iron, at which point collapse becomes inevitable. This is consistent with their classification as the hottest and most chemically evolved Wolf–Rayet subtype.

A small minority of Wolf–Rayet stars in binary systems may produce even more exotic outcomes. If the Wolf–Rayet star transfers mass to a compact companion or merges with it, the system can produce X‑ray binaries, stripped‑envelope supernovae, or, in rare cases, black‑hole binaries. But in all standard single‑star evolutionary tracks, a Wolf–Rayet star’s fate is the same: a short, violent life ending in a catastrophic supernova, enriching the surrounding interstellar medium with heavy elements forged in its core.

Spectral Imaging of WR134, WR135, WR136, WR137. WR139, and WR140 in the Constellation Cygnus.

Using a 100mm Skywatcher EVO refractor telescope, mounted on a ZWO AM3 mount, with a ZWO ASI585MM monochrome camera, and SA100 spectral grating, the astronomical spectra of six different Wolf-Rayet stars were captured in August 2026 from a 10,000’ elevation dark site in Grand Mesa area. Stacked 10.0 second raw images at a camera Gain of 450 were collected for offline processing. The low resolution raw monochrome spectra were then processed using the RSpec2 software [Ref. 3] without instrument sensitivity corrections over a range of 3,500 – 10,000Ǻ. To augment the RSpec2 database, chemical compositions in the Infrared spectral region were identified using the NIST Atomic Spectra Data Base [Ref. 4]. The results are shown below.

WR134 (HD191765) is a spectral type WN6-s star captured about 10:30pm on August 17^th. Along with large hydrogen and helium emissions, there are strong nitrogen, carbon, oxygen, silicon, krypton, potassium, and iron emissions. The published temperature for the star is: ~63,100

WR135 (HD193103) is a spectral type WC8 star captured at 9:30pm on August 20^th. The signal to noise ratio, while not as good as captured on WR134, still shows prominent carbon, silicon, oxygen, and iron emissions along with the hydrogen and helium. The published temperature for this star is ~63,000°K.

WR136 (HD192163) is a spectral type WN6(h)-s star captured at 11:00 pm on August 17^th. As a WN-type star it shows prominent emissions of nitrogen, carbon, argon, neon and iron along with hydrogen and helium. The published temperature for this star is ~70,800°K.

WR137 (HD192641) is a spectral type WC7pd star captured at 9:30pm on August 19^th. As a WC-type star it shows prominent nitrogen, carbon, oxygen, silicon, argon, and iron emission lines along with hydrogen and helium. The published temperature for the star is 60,000 ± 5,000°K.

WR139 (HD193576, V444 Cygni) is a spectral type WN5 star captured at 9:20pm on August 19^th. The WN-type star shows very large nitrogen and carbon emission lines along with hydrogen, helium, and argon emissions. The published temperature for the star is ~85,000°K.

Spectral image of WR139 (HD193576, V444 Cygni)

WR140 (HD193793, V1687 Cygni) is a spectral type WC7pd star (similar to WR137) captured at 9:00 on August 19^th. This WC-type star exhibits strong carbon and iron emission lines in addition to the hydrogen and helium lines. The published temperature for the star is ~70,000°K.

Spectral image of WR140 (HD193793, V1687 Cygni)

How DAS Members can look for Wolf-Rayet stars with a home telescope

The constellation Cygnus is a great stellar region for observing Wolf-Rayet stars. Because of its Northerly location, Cygnus is visible throughout most of the year – but between July through October it reaches its highest vertical track – meaning your telescope will be near vertical for observing or imaging. The Wolf-Rayet stars, noted previously, all have apparent magnitudes in the 6 – 8 range, implying they would be readily visible with a home telescope. Wolf-Rayet stars: WR141 and WR142 are also located in Cygnus but have apparent magnitudes of 10.23-10.95 making them a more serious challenge. This is unfortunate: as WR142 is one of the hottest Wolf-Rayet stars at 200,000°K. Table 1 shows the Right Ascension and Declination coordinates and Apparent Magnitudes of the Wolf-Rayet stars in Cygnus.

Table 1

WR Designation	Other Identifiers	Spectral Type	RA J2000 (h m s)	Dec J2000 (° ′ ″)	V Mag
WR 133	HD 190918	WN5+O9I	20 05 57.0	+35 47 18.0	6.71
WR 134	HD 191765	WN6b	20 10 14.0	+36 10 35.0	8.24
WR 135	HD 192103	WC8	20 11 52.0	+36 11 51.0	8.35
WR 136	HD 192163; P Cyg comp	WN6(h)	20 12 06.0	+38 21 18.0	7.48
WR 137	HD 192641	WC7pd+O9	20 13 59.4	+36 46 37.0	7.97
WR 138	HD 193077	WN5+OB?	20 16 43.0	+37 30 24.0	8.13
WR 139	HD 193576; V444 Cyg	WN5+O6	20 19 39.0	+38 44 00.0	8.05
WR 140	HD 193793	WC7pd+O4-5	20 20 28.0	+43 51 16.0	6.85
WR 141	HD 193928	WN5+O5V	20 21 22.0	+37 22 00.0	10.23
WR 142	Sand 5	WO2	20 21 44.4	+37 22 27.0	10.95