Wolf-Rayet Stars

**Wolf-Rayet stars** are a rare kind of star at the end of it’s life that show broad emission lines of ionised Helium, nitrogen and oxygen. They are known for the very high surface enhancement of heavy elements, depletion of hydrogen and strong stellar winds, all three are related to it's age. Example of Wolf-Rayet stars include R136a1 and VFTS 682.

## Types

A type of Wolf-Rayet stars, known as classic Wolf-Rayet stars, are usually evolved, massive stars that shed it's outer hydrogen layer and are fusing helium and other heavy elements through the immense pressure at their core. There is also a subset of population I Wolf-Rayet stars showing hydrogen lines in their spectra known as WNh stars; they are massive stars that are young and still fusing hydrogen at the core, with helium and nitrogen exposed to the surface by strong mixing and radiation mass-driven loss. A separate group of stars with WR spectra are the central stars of planetary nebulae, known as CSPNe, are post-asymptotic giant branch stars that were similar to the Sun while on the main sequence, but have now ceased fusion and shed their atmospheres to reveal a bare carbon-oxygen core.

## Properties and evolution

Wolf-Rayet stars are apart of the evolution stage of massive stars, in which strong emission lines of helium and nitrogen (WN sequence), carbon (WC sequence) and oxygen (WO sequence) are visible. Their emission lines are so strong they can be identified in nearby galaxies, in the Milky Way, there are about 500 Wolf-Rayet stars discovered in our galaxy. and more are getting discovered in other galaxies. Wolf-Rayet stars are common in starburst galaxies (galaxies that are producing large amounts of stars in a shorter amount of time), especially Wolf-Rayet galaxies.

Most Wolf-Rayet stars are usually classic population I type. Those classic types of Wolf-Rayet stars are a natural stage in the most massive stars (not counting those appearing in planetary nebulas). Appearing after the supergiant stage or the most massive of the main sequence stars. However red supergiants with a somewhat lower mass or will supernova in that stage while more massive ones become hotter as they expel their atmosphere. Some explode while as yellow hypergiants or LBV, many will become Wolf-Rayets. They have lost or burnt almost all of their hydrogen and are now fusing helium in their cores, or heavier elements for a very brief period at the end of their lives. Usually, Wolf-Rayet stars will end their lives violently by a supernova rather than becoming a white dwarf. Thus making every star higher than 9 solar mass to end in a supernova, many of them from the Wolf-Rayet stage. This process usually only last for 1 million years.

## Worldbuilding in Practise

Moirai is a Wolf-Rayet star located in the Hathor Galaxy, named after the incarnation of fate and destiny, Moirai has 37 M☉ and is heading towards the end of it's life. In less than a million years, it will go out in a supernova so bright it will be visible across the galaxy and even to the nearby Ripple Galaxy, the Federation of Gaia are planning on evacuating the local lifeforms when it happens, but before it does, the star is a spectacle for people to point their telescopes at.

## References

1. ↑ https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet\_star#Examples
2. ↑ https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet\_star#Properties
3. ↑ https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet\_star#Evolution