Wolf-Rayet stars (also known as WR stars) are old, massive stars (around 20 solar masses) that are ejecting their material into the interstellar medium. Unlike the Sun, which loses 10-5 percent of its mass each year due to solar wind, Wolf-Rayet stars lose 10-14 percent of their mass each year. Their surfaces are extremely hot, with temperatures ranging from 25,000 to 50,000 K.
Wolf-Rayet stars have burned through most of their nuclear fuel, forcing them to fuse heavier elements like helium and nitrogen (“WN”) or helium, carbon, and oxygen (“HCO” sequence) (“WC” sequence). These heavy elements are kicked up around the star’s atmosphere, where they absorb the star’s light and heat and are propelled outwards. The atmospheres of Wolf-Rayet stars are so thick that they are comparable to the star itself.
Because of the large amount of superheated material ejected, Wolf-Rayet stars have prominent emission spectra, making them among the easiest stars to observe in nearby galaxies. In the Milky Way Galaxy, 230 Wolf-Rayets have been discovered, with about 100 in the Large Magellanic Cloud and 12 in the Small Magellanic Cloud.
Wolf-Rayets have a short lifespan because they can’t keep ejecting large portions of their mass while still surviving. The star eventually loses the pressure required to extract fusion energy from its contents, and it collapses, forming a collapsar. The core collapses into a black hole, while the mantle material is drawn in at relativistic speeds, releasing massive amounts of gamma rays. The subsequent explosion is thought to be the physical origin of two-second “long gamma bursts.”
Smaller companion stars are found in about 59 percent of Wolf-Rayets, with whom they interact throughout their evolution. When a Wolf-Rayet explodes as a supernova, it can send nearby stars soaring through the interstellar medium at supersonic speeds. Such stars are referred to as “runaway stars.”