Here’s the first section of the world-design system laid out in the Architect of Worlds project.

As a preview: the design sequence begins by walking the user through the parameters of a star system, one or more stars in a gravitationally-bound group that move together through the Galaxy. We begin by determining the mass of the primary star, which we define here as either the only star in the group, or the one that begins its life with the greatest mass. In later steps we will determine whether there are any additional stars in the system, the mass of any companion stars, the age and metallicity of the overall system, the current status of each star, and finally the orbital parameters of the system.

In later sections of the design sequence, the user will be able to place planetary systems around a given star, and design the physical parameters of individual worlds.

Readers may be a little confused as to why we’re beginning by generating the primary star’s mass. Most design sequences like this one (including at least one previous version of Architect of Worlds) start by determining how many stars are in a given system, and then move on to generate the details of each one. It turns out that a star system’s multiplicity is strongly dependent on the primary star’s mass; more massive stars are significantly more likely to appear in pairs or larger groups. That dependence is complex enough to require we take things in this order if we want plausible results.

One more thing I’d like to point out (the final book will be explicit about this): what we’re generating here is the initial mass of a given star. It’s entirely possible that the object will end up with different mass than what we have here, specifically if we find that it has aged past its red-giant phase and is now a stellar remnant. That detail will be addressed in a later step of the sequence.

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Step One: Primary Star Mass

This step determines the initial mass of the primary star in the star system being generated. We will measure the mass of stars in solar masses.

The lowest-mass objects to be generated here are brown dwarfs, substellar objects massive enough to have planetary systems of their own, but not massive enough to sustain hydrogen fusion. Brown dwarfs are not stars, but they are sometimes referred to as such, and for the purposes of setting design they can be treated that way. Brown dwarfs have masses between about 4,000 and 25,000 times that of Earth, or between about 0.15 and 0.08 solar masses.

At 0.08 solar masses and above, objects can sustain hydrogen fusion and are considered stars. Most stars, by far, form with between 0.08 and 2.0 solar masses.

Stars can be extremely massive, up to a theoretical maximum mass of about 150 solar masses, but such gigantic stars are quite rare. Very massive stars also tend to burn through their hydrogen fuel and die very quickly, which means that they rarely get the chance to move far from the open clusters or OB associations where they were formed. Most local neighborhoods of the galaxy will have no such massive stars.

Procedure

Select a mass for the primary star of the star system being generated. To determine a mass at random, begin by rolling d% on the Primary Star Category Table.

Primary Star Category Table
Roll (d%)	Category
01-03	Brown Dwarf
04-82	Low-Mass Star
83-95	Intermediate-Mass Star
96-00	High-Mass Star

Depending on the category the primary star falls into, roll d% on the pertinent columns of the Stellar Mass Table on the next page. The result will be in solar mass units.

Feel free to select a mass for the star that is somewhere between two specific entries on the table. For example, if the result on the table indicates an intermediate-mass star of 0.92 solar masses, it would be appropriate to select an actual value greater than 0.92 but less than 0.94 solar masses. Such a selection will require you to do interpolation of several table entries in later steps.

Selecting for an Earthlike world: Instead of determining the primary star’s mass completely at random, assume it is an intermediate-mass star, and go directly to those columns on the table to determine its mass. Stars in this range are bright enough that they can have Earthlike worlds at a distance sufficient to avoid tide-locking, but are also long-lived enough that complex life is likely to have time to evolve.

Stellar Mass Table
Brown Dwarfs		Low-Mass Stars		Intermediate-Mass Stars		High-Mass Stars
Roll (d%)	Mass	Roll (d%)	Mass	Roll (d%)	Mass	Roll (d%)	Mass
01-10	0.015	01-13	0.08	01-07	0.70	01-06	1.28
11-29	0.02	14-23	0.10	08-13	0.72	07-12	1.31
30-45	0.03	24-34	0.12	14-19	0.74	13-18	1.34
46-60	0.04	35-43	0.15	20-24	0.76	19-23	1.37
61-74	0.05	44-52	0.18	25-29	0.78	24-30	1.40
75-87	0.06	53-59	0.22	30-34	0.80	31-36	1.44
88-00	0.07	60-65	0.26	35-39	0.82	37-43	1.48
		66-70	0.30	40-43	0.84	44-50	1.53
		71-74	0.34	44-47	0.86	51-58	1.58
		75-77	0.38	48-51	0.88	59-65	1.64
		78-80	0.42	52-55	0.90	66-71	1.70
		81-83	0.46	56-59	0.92	72-77	1.76
		84-86	0.50	60-62	0.94	78-84	1.82
		87-89	0.53	63-65	0.96	85-93	1.90
		90-92	0.56	66-68	0.98	94-00	2.00
		93-95	0.59	69-71	1.00
		96-97	0.62	72-74	1.02
		98-99	0.65	75-78	1.04
		00	0.68	79-82	1.07
				83-85	1.10
				85-89	1.13
				90-92	1.16
				93-95	1.19
				96-97	1.22
				98-00	1.25

Examples

Alice is aiming for a star system in which an Earthlike planet will appear, so she ignores the Primary Star Category Table and assumes the primary star will of intermediate mass. She rolls d% for a result of 36 and consults the appropriate columns on the Stellar Mass Table. The primary star’s mass is 0.82 solar masses.

Bob has no preconceived ideas about the nature of the Beta Nine system, and indeed he is designing a setting in which even small red dwarf or brown dwarf stars might be significant. He therefore rolls on the Primary Star Category Table and gets a result of 10 on the d%. The Beta Nine primary is a low-mass star. He rolls on the Stellar Mass Table, consulting the columns for low-mass stars, and gets a result of 48 on the d%. The primary star’s mass is 0.18 solar masses.

Modeling Notes

Astronomers have developed several different empirical rules for the distribution of stellar mass, each of which follows one or more power laws. In other words, the frequency of stars of a given mass seems to be proportional to that mass raised to a given power. The specific distribution we observe is called the initial mass function, and it appears to be consistent no matter where in the Galaxy we take a census of stars.

The Primary Star Category Table and Stellar Mass Table here are derived from an estimate for the initial mass function developed by the astronomer Pavel Kroupa. Citation:

Kroupa, P. (2001). On the variation of the initial mass function. Monthly Notices of the Royal Astronomical Society, volume 322, pp. 231–246.

One of my ongoing projects is a book, with the working title of Architect of Worlds. The goal of this book will be to provide science-fiction fans with the tools they need to design plausible worlds and planetary systems, using any preferred combination of random chance and deliberate selection.

From the draft Introduction for the book:

As a child in the late 1960s and early 1970s, I was fascinated by astronomy. I haunted the local library and read every book they had on the subject. I pestered my parents to plan trips to the planetarium, or to the local college when the astronomy department gave talks for the public. When I was twelve, my father purchased a high-powered telescope for me. That Celestron 8 machine saw a great deal of use over the next decade. Forty years later it is still in my possession, although (alas) I now live in a part of the country where city lights make star-gazing impractical.

I can’t be sure whether my life-long love for science fiction is a cause or an effect of that fascination with the universe around us. I grew up on stories by Asimov, Clarke, and Heinlein, and watched Star Trek re-runs at every opportunity. I was enthralled by stories of men and women going to the places I read about in my astronomy texts, discovering new worlds and meeting the strange people who lived there. I was particularly struck by authors who “showed their work” with respect to the physical environment. Authors like Poul Anderson, Hal Clement, or Larry Niven could make the settings of their stories both plausible and compelling.

At some point I discovered the world of conflict simulation games, sophisticated tabletop games that were designed to emulate various real-world political or military struggles. Most such games focused on historical conflicts, such as the American Civil War or the Second World War. A few, though, ventured into the realm of science fiction.

Almost by accident, my father brought home one such game for me, and it proved quite the revelation. This was the game Starforce, released by Simulations Publications, Incorporated in 1974.

Starforce was a simulation of human expansion into interstellar space, beginning in the twenty-fourth century. Redmond Simonsen, the game’s designer, meticulously worked out the technologies, the social and political conditions, and a complete “future history” for the game. In particular, he did careful research to build a map of space within about 20 light-years of Sol. His map included dozens of stars that could only be found in obscure astronomical catalogs, every one accurately placed.

Be it admitted, Star Trek has never worried very much about the real geography of the galaxy. Some of my favorite literary authors have done a much better job. But the fact that this game map existed – that we knew enough about our galactic neighborhood to make it possible – set my imagination alight. I spent years poring through what sources I could find, making lists of nearby stars, studying everything that was known about them, drawing maps and imagining what worlds might actually be out there.

One tool that came to my hand was another game. My father (again) came across the classic roleplaying game Traveller, and brought home a copy of the core rules for me. That game, published in 1977 by Game Designers’ Workshop, was the first to include a semi-random process for the design of star maps and worlds as settings for play. The world-building rules in the core game were very simplistic, but in the Scouts supplement (1983) a more sophisticated version appeared. This version took into account the properties of a world’s primary star, which made it possible for me to apply the system to the real stars I had been studying.

One result was the first original space-opera universe I ever designed – one which is never likely to be published, since it’s a little too obviously the result of an immature imagination. Another was a growing awareness that the Traveller rules were incomplete. To be sure, the designers produced a remarkable achievement, versions of which are still in use among Traveller players to this day. Still, they had oversimplified some details for ease of use, and the system included a few outright errors.

I set out to learn how to do better. In a sense, I’ve spent most of my adult life in that quest: a study of the universe around us, for the purpose of educating my creative imagination.

Years later, I spent about a decade writing and doing editorial work for the game publisher Steve Jackson Games. Ironically, this was at a time when they held a license to publish materials for the Traveller game and its fictional universe. That gave me the opportunity to design and publish three world-building systems of my own, which appeared in the books GURPS Traveller: First In (1999), GURPS Traveller: Interstellar Wars (2006), and GURPS Space, Fourth Edition (2006).

The last of these was the most comprehensive. It was published after the first discovery of exoplanets, worlds actually known to be circling other stars. It made an honest attempt to take into account some of the things we had already learned about the structure of planetary systems other than our own. Even so, in the years since its publication it has become dated with startling speed. In particular, the launch of the Kepler observatory in 2009 led to the discovery of hundreds of new exoplanets in just a few years. It’s become clear that the model of planetary formation I’ve used in the past was naïve at best, hopelessly wrong at worst.

Fortunately, as of this writing, the astronomical community seems to be converging on a new model, similar to the old but considerably refined. This model accounts for the great variety of exoplanets we’ve discovered, while still explaining most of the known features of our home planetary system. There is still a great deal of work to be done, and we’re likely to be surprised by what we learn in the years to come. Still, it seems possible to build a new set of world-building guidelines for the creative imagination, one which once again takes into account all that we’ve learned about the universe.

This book is intended as a resource for authors, game designers, game referees, readers, and fans of science fiction. It presents an overview of scientific concepts that might be applied to high-level design of a space-based fictional setting: the placement of stars, the arrangement of planetary systems, and the properties of individual worlds. It also presents a set of procedures for such design, allowing the reader to generate regions of space suitable for science fiction stories or games. The results should at least be plausible, given our present understanding of the universe.

More personally, this book is a collection of all the research I’ve done over the past forty years, ever since I was first inspired by those games I enjoyed as a teenager. Over the decades I’ve picked up many world-building tricks to apply in my own game writing and literary work, which I hope will be of use to others.

I’ve actually written two complete sections of Architect of Worlds, laying out how to design stars and their planetary systems. At the moment, I’m reviewing those sections and applying minor tweaks, and also ensuring that I’ve included citations to my research sources where that’s practical. I’m still working on research and development for the third major section, in which one can work out the physical details of a given world and determine what kind of environments it might support. I’m tentatively planning other sections, but those will come later.

Eventually this project is going to be compiled and sold as an e-book. One thing I plan to do with this blog is to publish an interim draft of the guidelines and systems in the book, so my readers can provide me with feedback. Entries in that series of blog entries will be posted with the architect of worlds and worldbuilding tags. Meanwhile, the book is going to be somewhat reminiscent of my work for the GURPS roleplaying game, even though it isn’t being written specifically for that game and I don’t have any reason to believe that Steve Jackson Games would be interested in publishing it. Still, I imagine GURPS fans may take a specific interest, so I’ll be posting those entries with a gurps tag as well.

Once I’ve finished posting a section of the draft here, I’ll also post a PDF of the complete section to the Sharrukin’s Archive static site. Those will eventually come down, as the book approaches readiness for publication, but that won’t be for a while.