Stars and Solar Systems

Today we’ll discuss the creation of stars, determining which stars are best suited for worldbuilding life-bearing planets, as well as why stars are more likely to predict life than the planets life lives on.

Hey everyone, my name is Matthew, at least that’s what astronomers have named me, and this post is part of a series where I will be going through a science-adjacent worldbuilding process step-by-step. Last time we created Locus, the rogue planet that we will be using for all of our worldbuilding moving forward. We established all of information about Locus, its origins and its composition.

For today’s discussion, we’re going to be creating the stars that form the centre of the solar system that Locus will be pulled into to settle in. We’ll be looking at how different kinds of stars are classified, how to set up solar system boundaries, and perhaps most importantly as worldbuilders, how to establish a habitable zone.

To start, let’s quickly look at the definition of a star, which is an astronomical object comprising of plasma that is undergoing nuclear fusion and is held together by its own gravity. Stars are birthed from bodies of cosmic gases called nebula, composed mostly of hydrogen. Almost all aspects of a star’s life and evolution are determined by its mass, including its size, luminosity, age, and eventual fate. A star’s mass can be anywhere between 0.1 solar masses, or about 10% the size of our sun, all the way up to 200 solar masses.

So, we can just make a star 200 times as massive as the sun and call it a day, right? Well, if you want your star to have planets that can support life, probably not. The more massive the star is the more material it sucks in from its surrounding environment to form itself, and more importantly the less time it lives for before going supernova. The biggest stars live on the scale of millions of years, not billions, and so considering planets take a while to stabilise, and that life takes a really long time to evolve, these stars aren’t really suitable for worldbuilding a life-bearing system.

In fact, any star that is going to stick around long enough to reasonably support life is going to be 2 solar masses or less, and even that is pushing it. This is the equation used to determine a star’s life span, but there’s a link to a spreadsheet in the description that will do all the maths for you. What we can see here is that for a star to still be alive as long as the earth is right now, it would have to be 1.37 solar masses or less. Even then, a star of that mass would be nearing the end of its life right now, compared to our own 1 solar mass sun that has around 7 to 8 billion years still left in the tank.

What this means for us as worldbuilders is that the more massive the star, the less time life has to evolve within the solar system. This can present some pretty cool worldbuilding scenarios, such as life evolving intelligence just in time to address their star reaching the end of its life, having to scramble to find a way off the planet and out of the solar system before things go supernova.

That sounds a little stressful for me though, so I’m going to worldbuild stars with a longer lifespan. Stars are classified using the Morgan-Keenan or MK system, which organises them based on their temperatures. First, stars are graded using the letters O, B, A, F, G, K, and M, starting from O being the hottest to M being the coolest, and then each letter class is further divided with a 0 to 9 numerical system, with 0 being hottest and 9 being coolest. For example, B8, B9, A0, and A1 are sequentially listed from hottest to coolest.

We can immediately rule out O, B, and A class stars, which are the hottest and most massive. These three classes are also exceptionally rare within the universe, making up a combined total of less than 1% of all stars, due to requiring so much matter to form and living very short life spans due to being so massive. O class stars specifically make up around 0.00003% of stars, to give you an idea of just how uncommon they are. You might find these stars towards the centre of galaxies, where there’s more matter floating around to birth stars from, though most likely this area will be outside of the Galactic Habitable Zone we discussed when we worldbuilt the galaxy.

F class stars sit between 1.04 and 1.4 solar masses and are the first reasonable star to worldbuild for habitable solar systems. They are yellow-white in colour, and those on the smaller side can live up to 9 billion years. They are a little rarer though, making up only 3% of stars across the universe. G class stars, like our own yellow sun, make up around 7.5% of stars in the universe, weighing in between 0.8 and 1.04 solar masses, with lifespans up to 17 billion years, making it possible for G class and lower stars formed at the beginning of the universe to still be around today. K class stars are orangey-yellow, make up around 12% of stars in the universe, fitting between 0.45 and 0.8 solar masses, with incredible lifespans up to 73 billion years.

Finally, we have M class stars, more commonly known as red dwarves or red giants, two names which I’m sure won’t cause any confusion at all, which are overwhelmingly the most common stars in the universe, making up around 76% of all stars. They can be as small as 0.1 solar masses, however there are instances of M class stars right down to 0.08 solar masses, though these stellar bodies blur the line between planet and star, and it is around this range is where some brown dwarves exist.

A Brown dwarf simply being a stellar object that is more massive than the most massive gas giant planets, though less massive than the least massive stars.

All of these types of stars, from class O right through to M are all considered ‘main sequence’ stars, meaning that they are undergoing fusion of hydrogen into helium. There’s a lot involved between the main sequence of a star and the end of its life, but for worldbuiling, that period is usually extremely violent on a stellar scale, and life is unlikely to survive. At the end of a star’s life however, it will either become a white dwarf, a neutron star, or a black hole if it is sufficiently massive.

White dwarves are the remnant cores of stars with masses from around 0.1 up to 10 solar masses, while neutron stars are the remnant cores of those stars that were between 10 and 25 solar masses. Anything above that is likely to form a black hole, which we’ve covered in greater detail in our episode on galaxybuilding. Both white dwarves and neutron stars are not undergoing fusion, with the light and heat they emit coming from residual thermal energy. It is theoretically possible for a habitable planet to exist around one of these two types of stars, though there are some problems you’d need to address if you wanted to worldbuild a situation like this. Due to their change from their previous form, white dwarves don’t have planets in their habitable zones, and so you’d need to put it there with some other external force. And neutron stars pulse extreme amounts of deadly radiation, meaning that a planet would need an enormous atmosphere to not be stripped away by the output of the extremely dangerous star. Once again though, these constraints aren’t quite what I’m looking for, for the world we’ll make across this series.

That doesn’t mean though that we can’t have an interesting solar system. Let’s say that the solar system that Locus is picked up by has two stars that form a very close binary system, one much bigger than the other. A binary system refers to a solar system with two stars that have a stable orbit around each other, sharing a gravitational bond. Importantly, they are both together going to determine how much light and heat is output into the solar system, as well as where the habitable zone lies. Let’s make one that is slightly bigger than our real-life sun, with a mass of 1.133, and another that is smaller, with a mass of 0.376, making them class F and class M stars respectively.

Let’s call the big one ‘Flavus’ and the small one ‘Rufus’, Latin for yellow and red respectively, based on their colours. Just by determining the star’s mass, we are able to use some equations to determine lots of values comparative to our real-life sun, such as their radius, circumference, surface area, volume, density, luminosity, diameter, surface temperature measured in Kelvin, their solar class, and how many billions of years the star will survive before it runs out of energy. Don’t worry, the spreadsheet in the description will do all of the math for you.

Now you might be asking, why is any of that relevant? Well, it’s not only going to determine how the stars orbit each other, but also where our planet is going to be able to safely orbit as well. The distance the planet orbits changes things like how long it’s year is, and how warm the planet is, which plays a huge factor in whether or not the planet can support life as we know it.

To figure this out, we need to first determine the orbit the stars have around each other. Let’s set them at an average distance of 0.175 AU apart, with 1AU being the distance earth is to our real-life sun. You’ll find that almost all calculations of distance within a solar system are done using AU. With these numbers we can work out the distance each star is from a point called the barycentre, the point between the stars that is their gravitational centre. Everything in the solar system, including the two stars, will orbit this point. We also need to give them an eccentricity; that is, how elliptical their orbit is, with 0 being a perfect circle, and 1 forcing the movement to no longer be a circle. Let’s give them an eccentricity of 0.413. From here, we can work out a huge number of factors all measured in AU, such as the maximum and minimum distance of each star from the barycentre, the maximum and minimum distance of each star from each other (which should never, ever, be less than 0.1AU), the inner and outer points of the gravitational dead zone for the stars, inside which objects will have unstable orbits due to the gravitational interaction between the stars, the inner and outer points of the solar system that planets can orbit in, the crucially important habitable zone, which is where we’ll need to settle Locus down into, as well as the frostline or the point where compounds such as water, carbon dioxide and methane will condense into solid forms due to lack of heat. We’re also able to determine the time it takes for the stars to orbit each other in earth years, as well as in earth days.  

That’s… a lot of information, but for the most part as long as your stars don’t come within 0.1 AU of each other, and you aren’t putting your planets too close to the stars, then everything should be okay.

If you’re worldbulding a system with just one star, then any time an equation calls for the two stars values to be added together, you just need to use the one star’s values. If you’re using a system with more than two stars, yep, you guessed it, just add the extra stars into the equations values. Obviously the habitable zone is, but we’ll talk more on that next time. For now, let’s focus on the stars’ orbital period. Considering the two stars are orbiting so close to each other, and that one is about twice as large as the other, to someone viewing the stars from the distance of the habitable zone they’re likely to just look like one oval shaped star. Maybe at the point of their rotation where they’re viewed side by side Rufus might be visible separately to Flavus but given the much greater luminosity of Flavus and the distance of the viewer, without special equipment it’s probable that on most days Rufus would not be able to be seen separately at all. Despite being half Flavus’ size, Rufus is only about 1.5% as luminous. The possible exception to Rufus’ invisibility would be on the day that Rufus is directly eclipsing Flavus. Visually, the star wouldn’t change size, as Flavus would still be visible from behind Rufus. However, Rufus gives off significantly different wavelengths of light to Flavus. Whilst this isn’t going to drastically change the colour of the star or even the atmosphere of a planet within the solar system, it would be enough of a comparative difference that creatures with colour receptors are likely to notice. For the period that Rufus is in front of Flavus, the sky on a planet with an atmosphere would appear redder. Definitely not ‘red’, but reddER. This will probably have some interesting religious and calendar implications down the line.

So, to recap, we’ve created two stars that orbit each other in a binary system, one class F star similar to our sun called ‘Flavus’, and one class M red dwarf called ‘Rufus’. We’ve worked out a tonne of important information for how the solar system itself will be structured, with a spreadsheet that is going to do the heavy lifting for our maths.

Join me next time when we’ll finalise everything else within the solar system, placing Locus into orbit and building the other planets that will be its new neighbours. You can find all the information for this video and other resources for worldbuilding in general over at worldbuildingcorner.com, and if you enjoyed this video don’t forget to like and subscribe to follow the world-building journey. And until next time… stay awesome!