# What sets of stellar models are freely available for reference when worldbuilding?

Often, when I'm building a world, I want to start out by determining some of its key properties. Maybe I'm trying to calculate a habitable zone, or figure out how long a year would be on a particular planet, or determine what the sky would look like. To tackle any of these, I need to know a bit about the star - or stars - it's orbiting.

Some of the time that's easy. As someone brought up last week, for some stars we can do some approximations, and a couple minutes and a back-of-the-envelope calculation will get me the answer I need. But there are other cases where it's not so easy. Maybe I want my planet to be orbiting an aging red giant, or maybe a massive star in the early universe which would have been, in terms of metallicity, nothing like the stars we know today. The analytical approximations star tto break down a bit in these cases.

One workaround I like to do is find pre-tabulated grids of stellar models. Stellar astrophysicists spend a good deal of time simulating evolutionary tracks and generating populations of stars, and they do numerically - much, much, much more accurately than I could ever do. They often create grids of results, which list the properties of a particular range of stars. For instance, one group might be studying red dwarfs, and they might generate grids of M1V dwarfs, M1.5V dwarfs, M2V dwarfs, etc. This means I can be lazy and just look at a particular set of models, if they're posted online. I can take my pick of metallicities, rotation, masses, ages, etc. If a star of the mass and age I want isn't listed, I can crudely interpolate between grid points. It's much better than me crunching numbers to get results that will be off by a factor of 2 or 3.

I know of a couple sets of models I like to use, and I'm going to write an answer listing them, but I'd also like to find out about other grids, as a question. What sets of grids of stellar models are freely available? To narrow things down a bit, here are my requirements - I'd like as many to be fulfilled as possible:

• They should include main sequence stars of masses $$0.08M_{\odot}.
• They should contain some models of solar metallicity and some models of the metallicities of Population III stars.
• They should also have evolutionary tracks for stars in the above mass range through the asymptotic giant branch phase.
• Simulated spectra would be amazing, if possible.
• They should also be free to access - I can get some with my academic credentials, but ideally these should be available to everyone.

Taking into account rotation would be nice, but it certainly isn't required. I'd also appreciate grids with high-mass ($$\geq10M_{\odot}$$), low-metallicity Population III stars, but that's rather unrelated to the stars I've mentioned above, which won't end their lives as supernovae.

• Is Intellectual Property Rights important in your criteria? 'Freely available' is subject to interpretation. Many such charts are behind paywalls, and strictly NOT available to be copied, but since charts and tables can not be subject to copyright, are jealously guarded. Commented May 18, 2020 at 20:18
• In the case you're referring to, the information is not free to access for everyone, and therefore not freely available, so I would prefer to not get answers involving those. That said, I'd be surprised if many grids fall into that category, given that they're made to be used, and many papers on them, even if paywalled, would likely be posted in preprint form on arXiv. Commented May 19, 2020 at 0:31
• Are you old enough to remember 'CRC Tables'?They were mostly all public domain, and available from other sources for free, but in the pre-Internet time, undergrad engineering, physics and science students were required to purchase the book, and it was not cheap. At the same time, pre-computer photocopying was more expensive, even though the book could not be copyright protected, and the research time it saved was worth the cost. Commented May 19, 2020 at 2:14
• @JustinThymetheSecond I'm honestly not sure what point you're making, nor in what way it's relevant to the question here. Commented May 19, 2020 at 14:15
• The fact that 50 years ago the information you are requesting could not be disseminated as easily as it can be now, and certainly not 'freely' available. That 'freely available' is a concept that is radically different today than when I was a young-un. That 'worldbuilding' and science fiction has changed since the days of early Asimov and Heinlein. It's just a lateral observation that was triggered by your request. A modern day 'CRC Tables' updated edition would have the information you requested, I am certain, summarized in one very thick book, but it has been made obsolete by the Internet. Commented May 19, 2020 at 14:30

As promised, here's my answer, covering the grids I sometimes use.

## The Geneva Grids

These are extraordinarily comprehensive - they actually meet all of my requirements and then many more. Lejeune & Schaerer 2001 go into some of the nitty-gritty details, but the models have been updated even more in the last two decades. Here are some of the features:

• They cover the range of $$0.8M_{\odot}, and metallicities from $$Z=0.001$$ to $$Z=0.1$$, although the metallicity steps aren't too fine.
• They do include pre-main sequence evolution in a couple of the grids - not usually something I worry about, but helpful in niche cases.
• They have some high-mass, extremely low-metallicity models which are actually really useful to me right now.
• Some grids include rotation or mass-loss.
• Yes, there are indeed some spectra, a feature I'd forgotten about!

These are usually my go-to simply because of how thorough they are.

## MESA Isochrones and Stellar Tracks

MESA is a well-known stellar evolutionary code, and a group at Harvard (MIST) has put together a set of isochrones and evolutionary tracks. I'm less familiar with these, but they have a wider mass range than the Geneva models (going up to $$\sim300M_{\odot}$$, and apparently down to brown dwarfs, below the hydrogen-burning limit). I've found them slightly harder to query and find, but I chalk that up largely to just a lack of experience.

Features:

• A wide mass range, as I noted above.
• Good metallicity coverage.
• An online interface to interpolate between models, which seems extremely helpful to the worldbuilder on the go.

I think the interpolator stands out the most for me. It's definitely a handy tool.

Recently, I was introduced to a service called MESA-Web, which is used as an educational tool for stellar modeling. Effectively, MESA is run on a someone else's cluster. It does take some time for the code to be run; the code itself will run for a maximum of four hours, but the queue isn't short, so prepare for possible wait times of about a day. It's designed for educational purposes, and etiquette is to only request 1-2 runs at a time, but it can be handy if you want to see how a given star evolves - at least in the early stages of its life; my $$M=0.1M_{\odot}$$ red dwarf only evolved through $$\sim$$1 million years in four hours. I'd recommend MESA-Web if you want to get a taste for what MESA can do.

## Dartmouth Stellar Evolution Database

I've only started playing around with this recently, so I can't say much about it, but it looks to have a decent set of isochrones and covers a wide range of photometric systems (Gaia included), which might be an advantage over Geneva and MIST. There are also some Fortran codes allowing interpolation in luminosity and metallicity.