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Wolf-Rayet Stars

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Manuel Wolff ; Johannes Zabl ; Jürgen Leschhorn
Leonard Storz

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Group 142


NGC 2359


Crescent Nebula

Example 1: NGC 2359





Factbook: NGC 2359



  wpe28.jpg (10675 Byte)  
Rec:     07h 18m 36s
Dec:    -13° 12' 00''
Constellation:    Canis Major
Distance:    15.000 ly
Diameter:    30 ly


Observing information

     Sky Chart    
NGC 2359, which is according to its shape also known as "Duck Nebula" or "Thor's Helmet", is located in the north-eastern part of CMa. It has a visible size of 9‘ and a real extension of 25‘. Its apparent magnitude is about 9 mag, while its interesting central star, a Wolf Rayet, is 11th magnitude. In our regions (latitude +48°) it could be observed in winter and one should have at least a 130mm telescope. Not too big magnifcations are recommended (only about 70 X). An HII filter will probably help to identify more details.


The chart was created by "Cartes du Ciel" (, a freeware program.


Physical information:

The nebula was discovered in 1785 by William Herschel. It is about 15.000 light years away from earth and its bubble has a diameter of 30 light years. The WR star has lost about 20 solar masses through the years due to its heavy stellar winds, which are interacting with the interstellar medium forming the bubble. Another interesting thing is that it is one of the hottest stars known.


Example 2: WR 104


     Quiz   Factbook: WR 104
    What does this picture show?
Rec:    18h 02' 04"
Dec:    -23 37' 42"
vmag:    14.90
spectral type:    WC 9



  • Embryo
  • Spiral Galaxy
  • WR - OB binary system
  source: [2]

Observing information:

With a hobby telescope WR 104 can only be observed as a point.

Physical information:

WR 104: (A new type of stellar nebula - WR-OR binary system)

The picture above shows the top view on a binary system consisting of the Wolf-Rayet star WR 104 and his companion, an OB star. Though not being a double of the W-R Star, nevertheless it has strong stellar winds of its own. As the winds of the two stars crash, a shock front arises.

By the assumption and the consequences of this theory (a companion and a shock-front), a problem for which astronomers haven't had a solution for years, could be explained.

The problem is that there is much dust in the stellar winds. According to the physical attributes of a Wolf-Rayet star it shouldn't be there. In the direct neighbourhood of the star the intense radiation would make it impossible for the dust to exist. If far enough away from the star, it would be too cold for the dust to condense.

  WR 104 keck graph.gif (43596 Byte)  The binary system and the hot dust spiral

The solution is the above mentioned shock front (see picture above). It causes high density and moderate temperature. By these circumstances dust can be born.

The spiral shape is caused by the superimposition of two movements. On the one hand the collision front moves with the OB star on its orbit (220 days). On the other hand the matter is radically swept outwards as a consequence of the stellar winds.


Example 3: Crescent Nebula




Factbook: NGC 6888

crescent1.jpg (41548 Byte)

Q: What does this picture show?
Rec:    20h 12' 48"
Dec:    +38° 20'
central star:    HD 192163
spectral type:    WN 6


A: NGC 6888, the so called Crescent Nebula. Further information
    in the text.
Telescope: 0.9-meter telescope on Kitt Peak with the NOAO Mosaic CCD camera
source: [3]


Observing information:

     Sky Chart    
NGC 6888 is located south of Gamma Cygni (Sadr). Its popular name is "Crescent Nebula".
Despite being one of the brightest Wolf-Rayet-Nebulas, still a 12" telescope is necessary to recognize details. In the 18' x 13' wide field one can find a faint arc. As a consequence of O III emission, a filter can help. Due to its shape it was erroneous assumed to be a supernova remnant.



The chart was created by "Cartes du Ciel" (, a freeware program.


Physical information:

The central star of NGC 6888 is a WN 6 Wolf-Rayet with a temperature of about 55.000 K . Its catalogue number is HD 192163 or WR 136. The structure of the shell gives evidence for the theory that a W-R star could evolve from a red giant.

In his red giant phase HD 192163 ejects his outer layers as consequence of his increasing size. About 200.000 years later, a very short time for a star, the immense radiation of the exposed inner layers causes heavy stellar winds of gases ( c haracteristics of a Wolf-Rayet Star). They flush with great velocities into the surrounding space. As the speed of this stellar wind is much higher then the one ejected by the former Red-Giant star, a crash is the consequence. The result is a shell and two shockwaves. One moves outwards, the other inwards forming a very hot X -ray emitting bubble. These phenomena could be seen on the image below.




crescent-chandra.jpg (73991 Byte)

Q: What does this picture show?
A: The red colored part shows the shell. Green in the image is the outwards moving shockwave. And finally red is the hot X-Ray emitting inner bubble.
Telescope: Chandra X-Ray telescope

Because of having such a heavy mass the Wolf-Rayet star will end up as supernova. Though having lived only 4.5 million years, it is expected to do so in the next 10.000 years. A really dramatic end.

The crescent nebula co uld be photographed by amateurs.



  NGC6888.jpg (81006 Byte) Q: What does this picture show?
A: It shows NGC 6888 photographed by an amateur

Telescope: Astro-Physic's 130 EDT f/8