Stellar System Generator, Part 1

Copyright RMF Runyan © 2011

Edited by Aaron Smalley for The Guild Companion

"Do not enslave yourself to the dice! This system is intentionally designed so you may generate as little or as much data as desired."

Editor's Note: The following is the forward and first chapter of the documentation provided by the author. This as well as the following four installments (five in all, over the next several months) are intended to provide background information as to the science and reasoning behind the application that RMF Runyan has put together. Once we publish the last installment (tentatively planned for December of 2011), we will also provide the entire documentation in the form of a pdf, which will include all of these article installments as well as a table of contents. In the meantime we are also publishing the "SSG Users Guide" put togehter by RMF Runyan which includes a link to download the application that he is developing. He is continuing to work on the application to improve it and create more functionality within it, and as such he is looking for comments and suggestions from users, which can be posted on the appropriate forums at We will release each updated and improved version of the application through the end of the series of articles in December of this year.


This generator is actually based on a system I wrote way back in 1983 before the Hubble was launched. When I found a print out of it in some old file boxes, I was shocked it had even survived. You ought to see how yellowed the paper was.

I have searched the Web for other stellar system generators. However, none were very accurate in the way a stellar system was generated. In fact, there was only one generator I found that was actually well thought out and used some actual astrophysics. Cosmos II, written by Mark Peoples, is the best one I have found. For most people, Cosmos II would be good enough, even if it is written specifically for the Alternity RPG system. My current intention is to get the SSG updated as previously written.

Even with the first generator I wrote, I was striving for as best accuracy as I could achieve with my limited understanding. Time has moved on as has discoveries in astronomy. In 1983, there were no extra-solar planets discovered; only theorized. Since then, there have been over 400 planets discovered orbiting other stars (see the Exoplanets Explorer , although I have not taken the time to figure out how it works). The Doppler Spectroscopy method is the most prolific planet detection method to date. Stellar system formation is one of the most viral topics in modern astronomy, evolving rapidly in several different directions. It will be some time before any definitive theory is created. Just 15 years ago, we were discovering our first definitive proof of extra-solar planets. In the next 15 to 20 years, we may be able to actually get pictures of those planets. (With today's unstable financial markets and lack of interest in astronomy and space in general, it will probably be more like 50+).

Although this revised edition of SSG may not be entirely accurate, I did strive to be as accurate as possible. Many of the equations are simplified from some of the truly complex calculus. Even the most inaccurate equation has an error margin of only 0.037%. I feel that is accurate enough.

Although I have been an amateur astronomer and amateur astrophysicist for practically my whole life, do not just accept my word on everything in this document. After all, all my knowledge comes from self-studies, with no formal education into astronomy. If you feel something is wrong, then research it yourself. However, please, inform me of my inaccuracy. Simply visit this Real Role Playing topic (requires sign in/registration to post).


This work is dedicated to my favorite science fiction authors: Isaac Asimov, Arthur C. Clarke, Robert L. Forward, Robert A. Heinlein, and Larry Niven. The best in their business.

It is also dedicated to me mom, to whom I owe a debt that can never be paid in full.


Many thanks to NASA , Jet Propulsion Laboratory , Space Telescope Science Institute , and HubbleSite Organization , of which I have borrowed much information and many of the images in this document. Also, I would like to acknowledge Mark Peoples and his Cosmos II for giving me the inspiration and desire to re-write my SSG. And Wikipedia, of which I make several references for articles. And thanks to Ray Larabie for the Metal Lord font.

Scientific Notation

Throughout this guideline, I use scientific notation using this format: 1.98892e30; where the "e(numeral)" denotes the exponent in which 10 is raised then multiplied by the decimal portion preceding the "e". In other words, the above example would read: 1.98892 1030.


The application was originally written as four different applications, which have since been combined into a single tabed application. This application was built using Visual Basic inside Visual Studio 10. You will need to download the latest VB6 Runtime Library at: . I know Windows 7 machines already have this Runtime Library. I think Windows Vista has it; at least mine did before installing VS10. Earlier machines I am fairly certain do not have it unless you have installed it.

Copyright and Usage Licenses

Stellar System Generator by RMF Runyan is Copyright 2011 by Concept Visions, LLC and licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. All other rights reserved.

The application: SSG-Calculator.exe is Copyright 2011 by Concept Visions, LLC and licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License. All other rights reserved.

This work is licensed under the Creative Commons Attribution-NonCommercial 3.0 Unported License. To view a copy of this license, visit or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA.

And a Final Note

Do not enslave yourself to the dice! In all of the steps with random tables, you may choose your result if desired. Just use some logical reasoning. Although this document is based on current astronomical knowledge, it does offer some randomness in a campaign setting. You do not need to follow this document in the order it is presented. You may skip around as desired. If there is data you do not want, skip it. You do not need to generate it. This system is intentionally designed so you may generate as little or as much data as desired.

This author believes we are unique in this universe. However, this author also believes it to be complete arrogance to assume there is no other life out there in the universe. After all, our galaxy is believed to have somewhere around 300 billion stars. As of 2008, it is estimated that there are over 100 billion galaxies in our universe. That means a possible total of 30 sextillion stars (30,000,000,000,000,000,000,000 (30 trillion billion)). Surely at least one of them has similar conditions to the ones here on Earth.

Besides, no matter how low the probability that any given galaxy will have intelligent life in it, the universe must have at least one intelligent species by definition; otherwise, the question would not arise.

"I'm sure the universe is full of intelligent life. It's just been too intelligent to come here [Earth]." - Arthur C. Clarke

Chapter 1: Galaxies

Thanks to the Hubble Telescope, we know definitively that galaxies come in many various shapes and sizes. Many of the objects we thought were planetary nebula, when seen from Earth based telescopes, have now been proven to be galaxies and interacting galaxies. In fact, it is almost impossible to view an image made with the Hubble space telescope without also seeing numerous galaxies in the background.

Most galaxies come in four basic types: spiral, barred-spiral, elliptical, and irregular. There are also globular clusters and lenticular types and the rarer ring type. Galaxies also come in three size classes: dwarf, galaxy, and giant. Virtually all galaxies fall into the Hubble Classification Scheme created by Edwin Hubble in first half of the 20th century. The original was dubbed the "tuning fork" diagram, but it has since been updated and revised. The below images show the original and updated versions.

Original Hubble Classification Scheme

Revised Hubble Classification Scheme

Original Sized Image (4200pxw 3600pxh) @SINGS

Spiral and Barred-Spiral Galaxies

This is the common form of galaxy. It is also the shape we tend to think of when we think "galaxy." They have a round, spheroidal core surrounded by the classic "pinwheel" with at least two arms, but most often have more. The core can actually resemble an elliptical galaxy and contains the metal-poor stars found in elliptical galaxies. Thus, the core stars can have a distinct yellowish to whitish tint, where the arms will have a bluish-white tint due to the younger stars that can be found there due to the rich star-breeding grounds of large dust and gas clouds like the Orion Nebula. The arms can be loosely or tightly packed, intact or patchy, closely or loosely wound. Barred-spirals will tend to have arms that are more loosely wound than in a spiral galaxy. Although it may appear to be so, the space between galaxy arms is far from empty and can have healthy star-breeding grounds. In fact, our stellar system is situated on the inner side of the Orion Spur. The image below is a best guess layout of our Milky Way Galaxy and shows the position of our stellar system.

Milky Way Showing Our Sun Position

The images below show an assortment of spiral and barred-spiral galaxies.

NGC 2841: A "grand design" spiral.

NGC 1300: A "classic" barred-spiral.

NGC 5584: A loosely wound barred-spiral.

The "Sunny Side Up" Galaxy: An older spiral.

Ring Galaxies

Ring galaxies were perhaps spiral or barred-spiral before colliding with another galaxy. These are the rarest of all galaxies. So far, we have only found 43 ring galaxies out of the hundreds of millions of galaxies discovered. This works out to about 1 in 13,000,000 galaxies being a ring galaxy. This type of galaxy has a structure similar to the spiral and barred-spiral galaxy; however, instead of the classic "pinwheel" structure, the ring galaxy has an elliptical core surrounded by a ring of stars. Some traces of spiral structuring may be seen between the ring and the core. Although ring galaxies have a smaller habitability zone, they may still have stellar systems that can be habitable.

Hoag's Object: Considered the most beautiful of all ring galaxies. Notice the other ring galaxy in the far background between the ring and core near top center.

Cartwheel Galaxy: The bluish galaxy on the right is the one considered to have passed through the original spiral galaxy. The yellowish one is in the far background.

AM0644-741: An off center ring galaxy.

Arp 148: This is perhaps what the Cartwheel Galaxy looked like just after collision. The central streak will continue going while the remainder will become a ring galaxy.

Irregular Galaxies

Irregular galaxies are just that, rough assemblages of stars with little or no regular structure. The Large and Small Magellanic Galaxies are irregular companions of our Milky Way Galaxy. In fact, they may be remnants of collisions or close encounters with other galaxies. Irregulars tend to be too small to generate star-breeding grounds, but on occasion, they can. The Tarantula Nebula in the Large Magellanic Galaxy is a star-breeding area. Because some irregular galaxies can have star-breeding areas, there is a chance for stellar systems possessing terrestrial planets.

"Bowtie" Galaxy: This one is the result of a collision/merge of at least two galaxies.

NGC 1427-A: This one looks like it is trying to become a spiral or barred-spiral.

Polar Ring Galaxy: Notice the elliptical core is horizontal and the spiral ring is vertical.

Warped Galaxy: Although it is also a lenticular with a spiral band, this one qualifies as an irregular.

Open Clusters

Although not true galaxies, some consider open clusters to be dwarf galaxies inside other galaxies. Open clusters are actually regions within galaxies where new star formation is currently occurring or has recently occurred. They tend to have only a few tens to a few hundred stars and are rarely larger than 15 to 20 parsecs (48.925 to 65.234 light years) and rarely have any definitive structure. Open clusters are dominated by the young Population I blue-white O and B class stars and are also associated with emission and reflection nebulae (q.v.). Open clusters make poor areas for habitable terrestrial planets due to the immense radiation pumped out by the O and B stars. However, asteroid, planetesimal, and planetoid mining might be profitable, albeit dangerous.

NGC 290

NGC 3293

The Pleiades


Elliptical Galaxies

Also referred to as "dead" galaxies, this type of galaxy has very little gas and dust for star-breeding and are predominantly old, metallicity poor stars. Elliptical galaxies have the widest range of sizes ranging from a couple thousand light years to truly immense monsters such as M87 in the Virgo Cluster which is over 3000 the size of our Milky Way (>30,000,000 ly across). Monsters like M87 tend to sit in the centroid region of galactic superclusters. Some elliptical galaxies may possess a dusty disc which could indicate a near dying region of star-breeding. However, there will be very little new star-breeding.

M87: The largest know galaxy. Notice the jet from the black hole on the right side.

An elliptical with some spiral banding.

NGC 5846 & 5847: It is assumed the larger one (5846) will consume the smaller.

Close-up view of the M87 jet. Photo was made using X-ray spectrum instead of visible light.

Lenticular Galaxies

As with the elliptical galaxies, lenticulars are considered to be "dead" galaxies. They can have a spheroidal and/or barred core, but exhibit very little else in common with the spiral and barred-spiral, except for overall shape. These galaxies have used up all of their interstellar hydrogen and helium and, thus, they will have an orangish-yellow to orangish-red glow. Lenticulars also have very little in the way of star-breeding areas since they have very little gas and dust. Virtually all of the stars in a lenticular galaxy are very old, most being twice as old as our star or older.

Centauri A

NGC 2787

The "Lost" Galaxy: Although I searched, I could not find out why this one is called the "lost" galaxy. Maybe someone saw it once, and then lost where it was located?

Globular Clusters

These are very similar to open clusters, except in size and age. Globular clusters tend to be fuzzy balls of stars with an orangish-yellow to orangish-red glow. Like a lenticular galaxy, the stars tend to be old stars, most being 2 or older than our sun. The stars tend to be more closely packed in the center than with open clusters. The core, which is usually one to two parsecs in size, can contain as many as one to three thousand stars. It has never been verified that any globular cluster has a black hole as most other galaxies have. Mostly there will only be Jovian planets and Pluto-like ice balls within a globular cluster.

Globular Cluster 47 Tucanae from SALT

NGC 4163

Virgo Cluster Galaxy NGC 4458

Virgo Cluster Galaxy VCC 1993

Interacting Galaxies

This is at least two galaxies, or more, that are about to collide or have just collided. Interacting galaxies tend to be hot breeding grounds for new stars, revitalizing perhaps two dead galaxies. Some of these can produce spectacular layouts.

Antennae Galaxies: This name originates from it being called the Antennae Nebula due to the fact that it appeared to be a two horned nebula before the Hubble Telescope.

Seyfert's Sextet

The "Rose"

Arp 194: a.k.a. The Question Mark

NGC 5257

Arp 147 scores a perfect 10


One cannot discuss a galaxy's habitability without discussing stellar evolution. I am not going to go into explicit detail about stellar evolution because it is a subject that could literally fill a 500 page book. For further details than those discussed below, do a search on the Web for stellar evolution and star evolution. Just make sure to visit the sites that are on science organizations or universities. All others will more than likely be personal websites of persons who may or may not know what they are talking about. For a primer, visit this Stellar Evolution page at Wikipedia. It also provides two excellent links to the Department of Astronomy at the University of Maryland and Department of Astronomy at Ohio State University. Here is another nice article about stellar evolution written by Dr. Evil Ganymede (he has a doctorate in Planetary Science ).

I break down stars into three types called Population I, Population II, and Population III stars. My Population type classification is not the same as the one used in true astronomy. I have simplified them for my Stellar System Generator. For the SSG, Population I stars are the Newborn and Young stars, Population II stars are the Mature stars, and Population III stars are the Old and Remnant stars.

In star-breeding areas, the first type of star to form is O and B class stars. These are referred to as Population I stars. These stars tend to be very massive and burn with very high temperatures. This causes these types of stars to live for only about 10 to 20 million years. O class stars have the shortest life, perhaps as short as 5 million years. B class stars are in the middle with a lifespan of 10 million to 1 billion years. A class stars can be classified as Population I, but they are longer lived stars, perhaps living for about 1 to 4 billion years. A class stars are on the borderline, being either Population I or Population II. At the end of their lives, these stars become very unstable and eventually destroy themselves and any planets in a supernova.

It is the remnants of these supernovae which will later create Population II stars which tend to be the A, F, G, and K stars. These are the best stars for a planetary system that can support life similar to that here on Earth. These stars live for about 4 to 15 billion years as Main Sequence stars. Towards the end of their lives, they will go through a red giant stage either burning or consuming the inner planets before they then nova, creating a planetary nebula and leaving behind a White Dwarf which will eventually cool down to a black dwarf in another estimated 15 to 25 billion years. Since the universe is only about 13.750.25 billion years old, there are no White Dwarfs which have cooled to black dwarfs.

Population III stars are very old stars that are the M and D class stars. These stars are near the end of their lives, having burned all or almost all of their hydrogen and helium. These stars are usually not warm or luminous enough to be conducive for supporting life except for the hardiest life, or the planet orbits very close to the star. However, some M class stars may have a life bearing planet.

Lenticular and globular cluster galaxies will be predominantly Population III stars. Thus, they will not be very habitable. Although elliptical galaxies may have more Population II stars, they are also mostly comprised of the older Population III stars.

Open clusters and some regions of interacting and ring galaxies will also not be very habitable due to the large number Population I stars. Since these stars are younger, they tend to pump huge amounts of radiation compared to Population II and III stars. It is due to this radiation that these regions are not very habitable.

The best galaxies for habitability are the spiral, barred-spiral, and ring galaxies. This is due to larger number of Population II stars within their boundaries.

Any galaxy can have life. Any galaxy can be habitable. Just remember that and you will not be wrong. Even a long dead globular cluster could have a G2V star with an Earth-like planet.

On a further note, the core of a galaxy will not be very hospitable for life due the proximity of the stars. A cubic parsec in the core can contain upwards of several thousand stars. This proximity from so many stars, although they may be old stars, will irradiate the region beyond the ability for normal life to exist. Also, this close proximity will not allow for any planets, most having been ejected from the galaxy core, but most likely consumed by the stars.