Run StarGen

Introduction

This page provides a small user interface to the StarGen program. Its purpose is three-fold:

• to allow users with no other access the ability to generate systems,
• to demonstrate how StarGen command line options are constructed, and
• to share experimental features with users who are helping me develop them.

This interface does not include all the options and features that StarGen supports, but it does provide what I believe to be the most common, or with which I am seeking assistance. The experimental sections are highlighted in pink in the Control Panel. If you are here just to learn about StarGen, you'll want to use those sections only after you fully understand the rest.

StarGen Control Panel

Catalog:	None Dole SolStation All The Solar System Alpha Centauri A Alpha Centauri B Epsilon Eridani 61 Cygni A Tau Ceti 40 Eridani A 70 Ophiuchi A Sigma Draconis HR 5568 Eta Cassiopeiae A HR 7703 (HJ 5173) A 82 Eridani Delta Pavonis HR 8832 Xi Bootis A HR 753 A BD+04 123 (HR 222) Beta Hydri 107 Piscium Pi3 Orionis A p Eridani A p Eridani B Chara Xi Ursae Majoris Ba 61 Virginis Zeta Tucanae Chi1 Orionis A 41 Arae A BD-05 1123 (HR 1614) A CD-27 14659 (HR 7722) Gamma Leporis A Beta Comae Berenices Kappa Ceti Gamma Pavonis HR 4523 61 Ursae Majoris CD-32 8179 (HR 4458) 12 Ophiuchi BD+63 238 (HR 511) Alpha Mensae Zeta 1 Reticuli Zeta 2 Reticuli 44 Bootis A 47 Ursae Majoris Iota Horologii Epsilon Indi 70 Ophiuchi B 36 Ophiuchi A 36 Ophiuchi B MLO 4 (HR 6426) A BD-05 1844 A All The Solar System Alpha Centauri A Alpha Centauri B Epsilon Eridani Tau Ceti 70 Ophiuchi A Eta Cassiopeiae A Sigma Draconis 36 Ophiuchi A 36 Ophiuchi B HR7703 A Delta Pavonis 82 Eridani Beta Hydri HR8832 HR753 A
Filter:	Output all systems Only systems with habitable worlds Only systems with 2 habitable worlds Only systems with earth-like worlds
Options:	Set random number seed: Specific stellar mass (.2-1.5): Repeat count: Increment each seed by: Use atmosphere model Use highly experimental moon code Use experimental CSV file output Use experimental SVG graphics

The table to the left contains a number of controls that will allow you to specify the number and types of systems that you want to create. As you do so, it will build a StarGen command line at the bottom of the table.

If you press the stargen button to the left of the generated command line options, a CGI script will run and the resultant systems will be displayed in this window. You should be able to use your browser's Back button to return to this page.

Catalog

Your first choice of options is whether you want to use a real star (or set of stars) to build your system(s) around. In the top row, there is a menu offering three Catalog options: None, Dole and SolStation. If you select None, then you can either specify the stellar mass to use, or leave it to StarGen to generate a random mass. On the other hand, if you choose either Dole or SolStation, then a second menu will appear, allowing you to choose stars from the corresponding catalog. This will also disable the ability to specify a stellar mass.

The Dole catalog consists of 15 stars listed by Stephen Dole in his book Habitable Planets for Man, upon which many elements of StarGen are based. These are the nearby stars that he suggested were most likely to have planets that could support human life. You can either choose an individual star from this catalog or generate a system for every star in the catalog.

So, suppose you want to create a setting for a game or science fiction story, or just want to explore the possibilities of life in the nearby universe. You might select Dole and All in the two menus in the top row. This will make -d appear in the text box at the bottom. Pressing the stargen button in the lower left is the same as issuing the command “stargen -d” in a command line environment. Try that now, remembering that you can get back to this page by using the Back button on your browser.

When you do so, this page should be replaced by one similar to the one shown in the picture to the right. The number and type of planets in each system will be different from the picture, but each system in the Dole catalog should be represented. Not only will the planets be different from the ones I generated for the picture, but if you come back to this page and issue the very same command, the results should be different from the results you got the first time. This is fine if you are exploring the possibilities of StarGen and the Dole catalog, but if you are doing this to generate a game world or fictional setting, you will probably want to be able to come back to the same system. That is what the Set random number seed field is for.

Set Random Number Seed

In order to create the systems, StarGen uses a pseudo-random number generator, which is a function that produces a series of numbers that are not obviously related to each other. Since a mathematical formula is used, the sequence is the same each time it is calculated. The function actually takes one number and produces the next one in the sequence. You can specify the first number used for a system. If you don't, the program picks one based on the time of day in seconds.

So, say you want a reproducible set of systems. Select Dole and All again; this time put a number like 42 in the Set random number seed box. Press the stargen button. This should produce a set of systems that you can come back to again and again. It is the equivalent of using the “stargen -d -s42” command.

Stellar Mass

If you have not chosen a Catalog, you can instead enter a Specific Stellar Mass to be used for all the stars you generate. Please note that stellar mass in StarGen is limited to the range from 0.2 to 1.5 solar masses. This is because of the assumptions made years ago by the folks at the Rand Corporation who constructed the original model. They were interested in understanding the possible distribution of habitable planets among the stars. Because of this, they ruled out stars that are too large, which will not have a stable temperature long enough for life to evolve, and stars that are too small and so will have a habitable zone so close to the star that the planet’s rotation will be slowed to a stop.

There is a second limitation regarding mass inherent in the basic StarGen model. The way StarGen creates a solar system is by simulating the formation of planets in an “accretion disc”, a ring of gas and dust thought to surround newly formed stars. The model that the Accrete program (upon which StarGen is based) generates the initial disk used to initially distribute the mass around a star results in a disk that seldom generates large enough planets close enough to very small stars to be within the habitable zone. As a result, I have never seen a habitable planet generated around a star with a mass of less than about 0.63 solar masses.

I have considered raising the 0.2 solar mass limitation, but I am considering changes to StarGen that might allow suitable planets to show up around these smaller stars. The observations of actual extra-solar planets resulted in the surprising discovery that gas giants are often seen far closer to their stars than previous theory or StarGen’s model would allow. One possibility that has been suggested is that these planets formed further out but had their orbits disrupted, resulting in them falling to a lower orbit. If I put logic in to simulate that, suitable earth-sized planets may migrate close enough to these smaller stars. Until then, masses in the 0.2 to 0.625 range are probably not useful.

Atmosphere Model

If you want a little more detail to flesh out the planets in your setting, you could try adding in the atmosphere model. (Technically, the model is still “experimental”, but it has been a feature for years and should not go away, though it may be changed. No need to hesitate to use it.) This changes both the thumbnail display and the full planet descriptions. The thumbnails will indicate whether the atmosphere is breathable or poisonous and so on.

In the main descriptions, it turns this:

Molecular weight retained	5.2 and above	N, O, CH4, NH3, H2O, Ne, N2, CO...

into this:

Molecular weight retained	5.2 and above	N, O, CH4, NH3, H2O, Ne, N2, CO... Nitrogen  87.8%  1602 mb  (ipp: 1547)   Oxygen  11.1%  202 mb  (ipp: 195)   Argon  1.0%  19 mb  (ipp: 18)

The presence of the Inspired Partial Pressure (ipp) figure allows StarGen to make some judgments about the breathability of the atmosphere.

Repeat Count

Now suppose the game or story you want to create a setting for takes place in a particular star system and you have some particular requirement, like a high-gravity world, or a cold one. This is what the Repeat count field is for. You can generate several versions of the same system and then choose the one you need. To see this, choose Dole and instead of All, pick a specific star from the second menu, say Tau Ceti. This will change the options listed in the text box at the bottom to -D4 (4 is Tau Ceti. If you pick another star, the number will be different.)

Hitting the stargen button at this point will generate just a single system. Adding a specific seed will make this reproducible, but if the result isn’t what you’re looking for, you need to keep trying different seeds until you find something better. Try this now: set the seed to 42, and the Repeat count to 10. This time you’ll get ten versions of Tau Ceti.

Filter

The problem with just doing this is that if you’re looking for a particular type of planet, you’ll have to wade through a lot of systems, most without habitable planets. To make this easier, you can select Only systems with habitable worlds from the menu in the second line. This will change the command line options to “stargen -D4 -s42 -n10 -H”, and greatly reduce the number of systems generated. Try it. This should produce only 1 or 2 thumbnails. (Warning, if no systems get through the filter, the resulting page may be blank.) At the bottom of the page will be a table summarizing the results. It will look something like this:

Summary
Earthlike planets	0
Breathable atmospheres	2
Breathable g range	0.78 - 0.81
Terrestrial g range	0.81 - 0.81
Breathable pressure range	303.55 - 365.29
Breathable temp range	-26.7 C - -7.9 C

Now that only habitable planets will be listed, you can afford to generate more systems without filling up the disk and the Thumbnails page with tons of useless systems. So, let’s pump the count up to something like 200. This will result in options of “stargen -D4 -s42 -n200 -H”. Consulting the summary, we can see that a small, cool star like Tau Ceti tends to create smaller, colder worlds. So, if we want larger or warmer planets, we may want to use a different star, say Delta Pavonis:

Tau Ceti Systems -D4 -s42 -n200 -H Summary Earthlike planets 2 Breathable atmospheres 43 Breathable g range 0.74 - 1.19 Terrestrial g range 0.74 - 1.09 Breathable pressure range 220.13 - 3375.09 Breathable temp range -26.7 C - -1.0 C

Delta Pavonis Systems -D11 -s42 -n200 -H Summary Earthlike planets 6 Breathable atmospheres 63 Breathable g range 0.75 - 1.29 Terrestrial g range 0.75 - 1.29 Breathable pressure range 201.39 - 4388.83 Breathable temp range -27.6 C - +5.2 C

Systems with two planets with breathable atmospheres are even more rare, but are dramatically quite interesting. We therefore have an option that is useful in searching for them. The Filter can also be set to Only systems with 2 habitable worlds. If you do so with the Catalog set to Dole and All, the random number seed to 5, the Repeat count to 100, and check Only systems with 2 habitable worlds and optionally Use experimental atmosphere model (this last isn’t required as both habitability filters imply the gas model) the options become “-d -s5 -n100 -g -2“, and the results should look like this:

System 42 -- Thumbnails

Eta Cassiopeiae A
8 Planets (size proportional to Sqrt(Radius)) See Key
#3 Terrestrial: Low-G, Cold, Few clouds, Thin atmosphere (O2, N2 - breathable) #4 Terrestrial: Cold, Few clouds, Thick atmosphere (N2, He, O2 - breathable)

Summary
Earthlike planets	42
Breathable atmospheres	259
Breathable g range	0.73 - 1.27
Terrestrial g range	0.73 - 1.27
Breathable pressure range	193.16 - 4275.93
Breathable temp range	-28.1 C - +8.1 C
Breathable illumination range	0.50 - 1.24
Terrestrial illumination range	0.50 - 1.24

As you can see, this has reduced more than 250 habitable systems to just the one that has at least 2 planets with breathable atmospheres according to the gas model. Notice that the system is part of a binary star system. I’m not entirely certain why, but this tends to be the case. Most systems with 2 habitable planets are in binary pairs. I think this is because the accretion disc is narrower and more even in thickness.