Aboard the aircraft carrier in chapter 5, when Ryland learns how widespread the stellar Astrophage infection is, a few particular stars stand out to him. The obvious focal point is our Sun, the dimming of which is the entire reason for the titular Project Hail Mary, given the dire consequences of solar dimming for all life on Earth. The next standout is Tau Ceti, which ultimately becomes the mission's destination (we have discussed some of Tau Ceti's qualities in a recent beanbag).

Another star with a less catchy name is cited as infecting not only the Sun but also three additional nearby stars. WISE 0855−0714 emerges as the apparent “super-spreader” in this scene, so one might wonder if this object has any special properties. While we learn that Astrophage infection is largely a matter of proximity (see 05.258 “every star eventually infects all of its neighbors”), WISE 0855−0714 indeed turns out to be a fascinating object.

For one thing, WISE 0855−0714 is not considered by many to even be a star but rather a sub-brown dwarf. Like a brown dwarf, a sub-brown dwarf is an object that formed through a collapsing gas cloud, much like a star, but never achieved enough mass to sustain nuclear fusion of hydrogen to helium in its core. Sub-brown dwarfs (yes, "dwarfs" as a plural form is correct in astronomy but confuses our Tolkien-appreciating hearts) have planetary masses, leading some to call them planetary-mass brown dwarfs. WISE 0855−0714 is also extraordinarily cold and may contain water ice in its Jupiter-like atmopshere.

The Astrophage infection range is stated in 05.260 to be approximately 8 light-years and WISE 0855−0714 is located 7.2 light-years from our Sun, just close enough for Astrophage to travel the distance. David A. Wheeler has a fun write-up exploring whether it’s plausible for an object as cold as WISE 0855−0714 to support the Astrophage life cycle.