Project Hail Mary is a 2021 science fiction novel by Andy Weir - and a really good one. However, the book doesn't have a map of the relevant parts of space from the book. I think it needs a map... so here's a map.
Warning: Spoilers ahead!
The following map shows our local stellar neighborhood. Every dot represents a rogue planet or a star system (a star system is a star or set of stars that orbit each other, along whatever objects like planets that orbit those stars). For brevity, I'll call each rogue planet or star system a "stellar object". The name of the stellar object is next to it. Near the bottom center of the map, labelled in bold, is our Solar System (a star system centered on our own star, Sol). All distances in this map are measured in light-years (the distance light travels in 1 year in a vacuum). The directed lines show the travel of Astrophage (more about that in a moment).
Click on the map to see just the map (which will will enlarge it); more information follows the map.
One challenge with maps of space is that space is 3-dimensional, but printed maps are 2-dimensional. This map shows, on a 2-D plane, the location of each object on a plane going through the celestial equator (a plane that aligns with the equator of the earth in 2000). In front of every name are square brackets containing the number of light-years the object is above or below this plane. We'll center its height on our own Solar System (so its "height" is 0.0). This map only shows the stellar objects between -5 and +5 light-years of height in the celestial plane, to make the map simpler. Here's an example that might help you understand the map: if we added Polaris (the North Star) to this map, we would add a dot almost on top of the Solar System and write " Polaris" to note that Polaris is 430 light-years in height.
I have made a minor exception with Alpha Centuri, the star system closest to our own. Alpha Centauri is a triple star system, and its star Proxima Centauri so much closer to our Solar System compared to its other stars Alpha Centauri A and B that their locations are visibly different even at the scale of this map. Thus, instead of one dot, the Alpha Centauri stellar system has two adjacent dots, one for Proxima Centauri and the other for the pair Alpha Centauri A and B.
The directed lines show the spread of astrophage per pages 95-96 (all page numbers are per the hardcover 2021 release). Centered on each directed line is the distance of that trip (shown in italics and measured in light-years). Note that the trip distance between each of these stellar objects is less than the 8 light-year maximum transmission range of astrophage. Some are close, such as the 7.8 light-year distance from Epsilon Eridani to Sirius, but they are all less than 8 light-years.
Let's follow the directed lines. It's eventually revealed that Astrophage started at Tau Ceti, in bold near the right center of this map. Astrophage then spread to Epsilon Eridani. It eventually ended up on Rocky's home system of 40 Eridani (per page 204), in bold on the top right of this map. The novel doesn't expressly say it, but by looking at the locations of stars it's almost certain that astrophage spread from Epsilon Eridani directly to Rocky's home system of 40 Eridani. From Epsilon Eridani it also went to Sirius, the brightest star in our sky (it's bright to us in part because Sirius is relatively close to us).
From Sirius astrophage spread to WISE 0855-0714 (full designation WISE J085510.83-071442.5). Finally, from WISE 0855-0714 astrophage spread to at least 4 other star systems: Wolf 359, Lalande 21185, Ross 128, and our own Solar System (page 95). The novel begins after the humans detect its effects in our own Solar System.
That path seems straightward. However, WISE 0855-0714 is where things get weird on this map.
WISE 0855-0714 was only discovered in April 2014 and isn't actually a star. Its mass is so low that it can't fuse any atoms like a main-sequence star or a brown dwarf. It's only 3-10 Jupiter masses (MJ). Normal hydrogen thermonuclear fusion reactions require .08 mass of the Sun - so a main-sequence ("normal") star requires at least around 89 Jupiter masses. Its mass is even below 13 Jupiter masses, the minimum (limiting) mass for thermonuclear fusion of deuterium that is necessary to create a brown dwarf. Its temperature is not like a typical star either: -48 to -13 °C (-55 to 8 °F). Objects like these are often classified as a sub-brown dwarf. It could also be called a rogue planet. It's easy to quibble about categories. However, since it can't fuse any atoms, it's more like a massive rogue planet than a star in the usual sense, so that's what I'll call it here.
But could WISE 0855-0714 support astrophage reproduction? All the other stellar objects on this journey are star systems. It's easy to imagine that they each have a star and at least one planet with the carbon dioxide necessary for astrophage to reproduce. However, WISE 0855-0714 is different! Could astrophage regain enough energy (possibly by reproducing) to travel to other stars? To see if this is justifiable, let's first look at the astrophage lifecycle (primarily from pages 84-85 and 90-91):
So, could astrophage at WISE 0855-0714 eventually end up at other stars? At the very least this requires the astrophage to find a location on WISE 0855-0714 with a temperature over 96.415 °C, so it can get energy. That way, astrophage looking for an energy source (like a star) will have one to find. At first this looks impossible, since WISE 0855-0714's average temperature is -48 to -13 °C. However, averages hide a lot; in 2021 Earth's average temperature was 14.7 °C (58.4 °F) yet the highest and lowest air temperatures on Earth are likely more than 55°C apart (per "Climate Change: Global Temperature" by Rebecca Lindsey and Luann Dahlman, June 28, 2022, Climate.gov). What's more, there are places on Earth (like lava flows) that have locations with far higher exposed temperatures. WISE 0855-0714 is probably a gas giant, so astrophage could simply fall into it until it found pockets where that temperature (energy) is available. In short, it's very plausible that it could find spots above 96.415 °C even though its average isn't that high. What's more, astrophage would be attracted to such sources.
After astrophage fills up on energy, it will need to find a way away from its energy source (its "star") by using a strong magnetic field. Again, this seems plausible. WISE 0855-0714 is likely to have a strong magnetic field. After all, all four of our gas giants have magnetic fields. So once it's filled with energy, it can simply follow the magnetic field that is almost certainly present.
Could astrophage have entire reproduction cycles completely at WISE 0855-0714? That's important, because if it could reproduce on the rogue planet, that would greatly increase the likelihood that it would spread to other worlds (simply because there's more trying).
I think it's plausible that astrophage could reproduce completely at WISE 0855-0714. The key reason is that it's plausible that the rogue planet has carbon dioxide. If that's true,, the rogue planet could fulfill the role of both "star" (energy source) and "planet" (carbon dioxide source). Any astrophage leaving the rogue planet looking for the "other role" has a good chance of rediscovering the rogue planet. The lack of energy on the rogue planet might encourage astrophage looking for energy to go looking elsewhere, as the rogue planet wouldn't "look" as much like a star, resulting in faster spread. It's not clear if lower-energy astrophage could make the trip to another star, given its lack of evergy, but it might. We know that some astrophage (likely the full energy ones) can make the trip.
No matter what, WISE 0855-0714 will have lower available temperatures compared to a star. As a result, it would have taken far longer for astrophage to have gathered energy from this rogue planet instead of its usual diet of stars. I haven't tried to calculate that, but I suspect it would be a long time depending on the temperature available. Perhaps it's best if we don't examine this question too closely :-). Still, humans were able to do it, and it would have sought out energy sources, so I think it's at least plausible.
StarForce: Alpha Centauri is a board game published in 1974 by Simulations Publications Inc. (SPI) that uses a game map based on the real world positions of stars. My page Project Hail Mary and StarForce: Alpha Centauri shows how Project Hail Mary maps to positions on the StarForce game board.
This map is based on data from the Wikipedia List of nearest stars and brown dwarfs. Putting Your Stars in Their Places by Greg Scalise (2003) explains how to convert the usual astronomical coordinate system into the X-Y-Z system used here. In the X-Y-Z system used here, Right Ascension of 0 degrees is directly to the right ('east') of the point (0,0). I created the map using a very common set of data analysis tools (which I already knew how to use), including Python, pandas, numpy, and matplotlib. The code is public and available as open source software if you want to examine or improve it.
I love hard science fiction (where there's an effort to make the plot believable based on our current knowledge of science). Project Hail Mary manages to combine science, a sense of adventure, and a sense of humanity in one awesome book. I appreciate gems like this book!
Feel free to see my home page at https://dwheeler.com.
(C) Copyright 2021-2022 David A. Wheeler. This essay is released under Creative Commons Attribution-ShareAlike version 3.0 or later (CC-BY-SA-3.0+). The map by itself is released under the more-generous Creative Commons Attribution 3.0 or later; please give credit!