Wolf-Rayet Stars
WR 142
Image from Trolligi
WR 142, a Wolf-Rayet star 5,382 ly from Sol


Wolf-Rayet stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. They are characterized by a spectrum dominated by emission lines of highly ionized elements. These extremely hot (up to ~260,000 K) and very luminous (5E+4 to 1E+7 solar luminosities, with an average of 3E+5) stars are very rare, reflecting the short timescales of the Wolf-Rayet stage - only around 30 of these stars are currently in the Terragen Sphere. Intense stellar winds drive mass loss rates of between 1E-6 and 1E-4 solar masses per year; the latter are many times greater than expected for other hot, O-type or B-type stars, the former of which are the predecessors to these stars.

There are three main types of Wolf-Rayet stars - carbon-rich (WC), nitrogen-rich (WN), and hydrogen-rich (WNh, also rich in nitrogen), the latter two of which are more common, especially in low-metallicity galaxies. The evolution of the star in its Wolf-Rayet stage is dependent on its initial mass. On their own, lower-mass WRs are unable to evolve into the WC class (and so are confined to the WNh class and then the WN class) as they don't have powerful enough stellar wind to eject enough mass to expose the carbon and oxygen fusing in their cores, although binary stripping - where a more massive companion strips the outer layers of the WN star - can reveal the latter's carbon-rich interior and making it a WC star. This is also how some very low mass stars can become small WN stars. More massive stars, on the other hand, can naturally attain the WC class through a lot of mass loss and proceed to get hotter and hotter, finally transitioning to the WO stage (the oxygen-rich stage) where they become extremely hot (over 130,000 K), and after under twenty thousand years, explode as type Ic supernovae.

The composition of WR stars can be quite extraordinary. WNh stars are not too different from regular stars, with a significant percentage of nitrogen. This amount increases as the WNh star loses mass very rapidly (WNh stars are especially prone to extremely strong mass loss, on the order of 1 solar mass every 10,000 years), expelling all of hydrogen and becoming a hydrogen-free WN star.

WN stars are made almost entirely out of helium with ~1% (a sizable amount) of it being made of nitrogen. Then, mass loss carries even more material away and then starts to expose the inner layers of the star. The WN star briefly becomes a WN/WC star, also called a transitional Wolf-Rayet star, another rare subtype of WR. Carbon level rises during this period.

After the star becomes a WC star, the level of helium substantially decreases to only half of the star's mass. The other half is carbon with a small amount of oxygen making up some 1-10% of the star. As the WC star shrinks and heats up, helium level decreases further, while carbon and oxygen levels increase. As the star transitions from WC to WO, helium now makes up between 30 and 10% of the star, while oxygen comprises about 20% of the star, and the rest is taken up by carbon. There is also a significant amount of iron in the star (~0.1%).

Finally, the WO star explodes and scatters all these elements across space.

Utilisation of Wolf-Rayet stars within the Terragen Sphere

Due to their high masses, WR stars are often utilized by archailect-level entities. Most WR stars are in the process of being disassembled by starlifting, the extracted mass either used to build computation nodes or other types of mass-intensive constructions. Since WC stars produce a lot of dust, it is possible to mine this dust and obtain a lot of carbon and oxygen as an alternative, slower resource extraction method.

As Wolf-Rayet stars are extremely rare and mostly limited to young star clusters, they do not generally pose a great threat except to nearby systems. WRs are extremely bright and emit a fair bit of high energy radiation such as UV and X-rays. Therefore, colonizing systems located very close to Wolf-Rayet stars generally require radiation-resistant modifications, such as those utilised by the Radiation Nation and similar tweaks.

Wolf-Rayet stars can become tourist sites because of reasons such as their rarity, and the surrounding nebulosity, which many WRs possess. A lot of precaution is often taken when travelling there for touristic purposes, and for a good reason.

List of Wolf-Rayet Stars within the Terragen Sphere

About half of these stars have been colonised by non-Sephirotic metaempires, particularly the Solipsist Panvirtuality, so information about these systems is limited.

Name Spectral type on discovery Distance from Sol (light years) Constellation Notes
Gamma2 Velorum A / WR 11 WC8 1100 Vela First WR star to be stabilised
WR 94 WN5o 3100 Scorpius stabilised
WR 146 A WC6 3590 Cygnus stabilised (by Solipsist Panvirtuality)
WR 90 WC7 3750 Scorpius stabilised
WR 78 WN7h 4080 Scorpius stabilised
WR 139 A WN5o 4270 Cygnus stabilised (by SP)
WR 79 A WC7 4470 Scorpius stabilised
WR 145 WN7o/CE 4670 Cygnus stabilised (by SP)
WR 120 WN7o 4890 Scutum stabilisation commenced
WR 110 WN5-6b 5150 Sagittarius stabilised
WR 111 (Muora) WC5 5320 Sagittarius This star exploded in 8684 AT, after Terragen explorers arrived
WR 140 A WC7 5350 Cygnus stabilisation commenced (by SP)
WR 142 WO2 5380 Cygnus This star exploded as a Gamma-Ray Burst in 2055 AT, luckily missing the Terragen Sphere completely
WR 105 WN9h 5640 Sagittarius stabilisation commenced
WR 52 WC4 5710 Centaurus Went supernova
WR 134 WN6b 5710 Cygnus stabilisation commenced (by SP)
WR 144 WC4 5710 Cygnus Went supernova
WR 93 A WC7 5740 Scorpius
WR 142-1 WN6o 5770 Cygnus
WR 147 A WN8(h) 5840 Cygnus
WR 113 A WC8 5870 Serpens
WR 142a WC8 5900 Cygnus
WR 159 WN4 5940 Cassiopeia
WR 133 A WN5o 6030 Cygnus
WR 113-2 WC5-6 6070 Scutum
WR 141 A WN5o 6290 Cygnus
WR 136 WN6b(h) 6290 Cygnus
WR 70-5 WC9 6360 Norma
WR 98 WN8o/C7 6390 Scorpius
WR 25 A O2.5If*/WN6 6430 Carina
WR 135 WC8 6460 Cygnus
WR 85 WN6h 6490 Scorpius
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Development Notes
Text by Trolligi
Additional material by The Astronomer
Initially published on 24 July 2021.