Wolf-Rayet Stars

Wolf-Rayet stars are a very rare and diverse breed

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The Discovery

These stars were first discovered by the astronomers Charles Wolf and Georges Rayet in 1867. The stars had a very broad emission spectrum, which is unlike most stars. The emission spectrum of stars is based on the absorption energy of the atoms that make up its outer shell. The positions were also hard to understand, as they were caused by an undiscovered element at that point in time. Edward C. Pickering tried to place it as an unknown form of hydrogen, but it was helium, which was discovered the next year. The widening of the spectrum was caused by doppler broadening. The doppler effect is a change in wavelength of a photon caused by movement. Doppler broadening happens when a group of particles moving in random directions causing doppler shifts in every possible direction. The hotter/faster the particles the wider the spectrum becomes, and Wolf-Rayet stars eject large amounts of extremely energetic particles. [2] [3] [4]

These stars are so rare there have been only 639 discovered (March 2017). Since they have a very low lifespan they act as a marker for recent high-mass star formation in clusters. The rarity and use of these stars mean finding them is a big deal. There are two main methods to searching for these stars, the “narrow band” approach, and the “broad band” approach. The narrow band approach is done by taking a narrow band spectrum of where a Wolf-Rayet star is most active, and then subtracting a broad-band spectrum from that. What is left over should reveal if it matches up with a Wolf-Rayet star. The broad band approach is helpful for stars with strong winds. Energetic winds will release infrared particles and leave behind a spectrum unlike any other kind of star. The broad band method is less certain and more confusing, but it can utilize already existing databases.
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The Evolution

Wolf-Rayet Stars are a step in the evolution of the most massive stars, which are at the very minimum twentyfold the mass of our sun. They start out as main sequence stars; just like our sun. Main sequences stars primarily fuse hydrogen into helium in their cores. Once they run out of hydrogen they turn into a red giant, blue giant, or sometimes directly into a nitrogen class Wolf-Rayet star. These giant stars will turn into a Wolf-Rayet star once they have run out of all hydrogen to fuse. Since they have strong convection they also use the hydrogen at the surface, which leaves high amounts of helium. After this step they begin the carbon-nitrogen-oxygen fusion cycle, which ends up leaving large amounts of nitrogen on the surface of the star. These are nitrogen class Wolf-Rayet stars. Whether the Wolf-Rayet star becomes an oxygen class or carbon class next is most likely dependent on its mass. Those with mass of 20 to 45 times that of our sun become a carbon class Wolf-Rayet star, while those with a mass 45 to 60 times that of our sun become an oxygen class Wolf-Rayet star. [7] [3]

As with every star over nine solar masses, Wolf-Rayet stars will end their lives with a supernova. Wolf-Rayet stars are thought to cause type Ib and type Ic. Both type Ib and Ic are caused by core collapse, like type II supernovae. Unlike type II supernovae, type Ib and Ic supernovae do not have an outer hydrogen shell, and have more heavy elements. Type Ib has an outer helium shell, while type Ic has no helium shell. [8]

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The Characteristics

Since the heavier elements have more atoms per unit of space, they release more energy overall through fusion. This causes the star to produce extremely fast winds anywhere from 3.6 million to 9 million kilometers per hour. This heat also means their radiation is mainly in the ultraviolet portion of the spectrum, while the dust around them radiates mostly in the infrared spectrum. This makes it impossible to see with the naked eye. [7] [5]

The faster a star spins, the longer it tends to spend in main sequence. This is because the spinning mixes the hydrogen into the core, extending the amount of time it spends in fusion. As main sequence stars become Wolf-Rayet stars they lose large amounts of mass to solar wind. As they lose mass they lose angular momentum, which slows down the rotation of these stars. The more metal the slower the star will end up. This means all Wolf-Rayet stars have a far slower rotation than the majority of other stars.
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Wolf-Rayet stars are some of the rarest stars. Their characteristics are unlike most other forms of star and that makes them really interesting to study and find.

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