Although only 227 Wolf-Rayet stars have been identified to-date, they are a delight to both observers and photographers, for they produce
stunning bubble nebulae around luminous and massive stars nearing the end of their evolutionary cycle. First noted by French astronomers
Charles Wolf and Georges Rayet in the mid 1860's, these stars are characterized with unorthodox spectra with broad emission lines strong in
ionized helium, carbon, oxygen and nitrogen (these stars are further subdivided into WN and WC stars depending on a stronger nitrogen or
carbon emission line respectively).
These very luminous stars are also very short-lived. During the normal phase of their life cycle, the interstellar medium disperses stellar
material, thus forming an interstellar bubble. When the star has then finally reached the end of its life cycle, a strong stellar wind now
helps in the rapid depletion of stellar mass and which accumulates in the form of an (emission) nebula residing into the previously formed
interstellar bubble with the WR star behind this sequence of events being often quite visible (particularly in photos).
Wolf-Rayet stars are characterized with cores whose mass is only 10 to 20 solar masses and temperatures which vary between 25,000 and 60,000
degrees Kelvin. The
Seventh Catalogue of Galactic Wolf-Rayet Stars boasts a short but complete list of 227 entries as of 2001. Approximately 100 of these
stars are found in the Large Magellenic Cloud; they rarely occur in clusters and there is a suspicion that they may be precursors to
supernovas. These bubble clouds can span large portions of the sky (20-30 arc minutes); they can have a high surface brightness; and many are
within easy reach of modest amateur telescopes.
More recently, studies have uncovered approximately 50 planetary nebulae to possess central stars which have characteristic Wolf-Rayet
spectra. For further details, see Pena et al here,
Medina et al here and especially
Gorny et al here.
Note: For two excellent articles, the interested reader is referred to Astronomy Magazine
(Feb/2006: 68-71, Jul/2008: 34-39) as well as the online article
here. For a list of the twenty
brightest Wolf-Rayet stars, click
here. An alternate online list of the
complete catalog is available here and
here.
Finally, for a list of Wolf-Rayet stars visible from the northern hemisphere, click
here and/or
here.
Wolf-Rayet Nebulae | |||||
---|---|---|---|---|---|
WR 155 in Cas |
|||||
WR 128 in Sge |
WR 133 in Cyg |
Crescent Nebula WR 136 in Cyg |
WR 153 in Cep |
||
Bubble Nebula BD+602522 in Cas |
Campbell's Hydrogen Star BD+303639 in Cyg |