Often, when I world build, I’ll start with an idea for a planet, and then build a solar system around it. The idea may stem from a random scene of a group of characters. From that seed, I build a solar system.
What are the first two aspects to consider for such a project?
The Sun and your planet.
Our sun is a source of energy, of light, of warmth, and security. I use the term security in that the planet would not be able to function without the gravity of the sun keeping it in its orbit. If the sun’s gravity wasn’t there, the planet would spin off into space, becoming a giant cold stone that could never sustain life.
How do you pick a sun? First think about your planet. Do you want it to be fairly close to the sun, so that the sun takes up a fairly large portion of the sky? If so, then you’ll want a smaller star that isn’t too hot. If the star is too hot, then your planet would end up like Mercury, where it’s too hot or too cold, depending on what side is facing the sun, for life to be exist or evolve. However, if a star is too small, the planet could be locked in an orbit where one face always faces the sun. This would lead to conditions making higher life forms nearly impossible.
The tidal habitable zone is a term often used to describe the area in space around a star that allows liquid water to exist. All stars have this zone, and although you could calculate it out with physics, it’s often easier to just look it up in databases to help you choose your star with minimum computational effort. If you are seeking to create lifeforms that use a liquid different from water, there is still a specific habitable zone suitable for that particular liquid to be able to form. There is various speculation on what liquids are suitable for the creation of organic lifeforms, and here is some references to read if you’d wish to delve more into this concept: Limits of Organic life in Planetary Systems, Alternative Biochemistry (sift through the sources in the article for some great scientific articles. The Wiki entry itself is fairly decent, but the sources are far superior in regards to accurate information), and Planetary Habitable Zones defined by alien biochemistry.
If you wish to focus on a planet that can sustain water, then the tidal habitable zone is best. Most earth lifeforms need water to survive, so this is best if you wish to have human-like cultures. If you are going for a more alien culture, then you will have to think about their biochemistry, and examine the three links I provided to help you see what elements are best for the formation of that alien chemistry. The habitable zone will change depending on what type of biochemistry you are considering. This is due to the fact that certain types of molecules can only obtain a solid, gas, and/or liquid state at a certain distance from the sun. For example, water has a narrow temperature zone, where it can exist as a liquid. If a planet is outside of that temperature zone, the sun’s output covers the planet with either too much radiation and heat – if the planet is too close to the sun – or too little radiation and heat – if the planet is too far away from the sun.
This is why astronomers came up with the habitable zones of stars in order to find suitable planets for life. You can use their findings and theories to help build your own world.
For a water-based world, here is a diagram that shows where the habitable zones are for various types of stars:
Tidal Habitable Zones, where the habitable zone is in green. The red area signifies too hot, and blue too cold. (The orbits in this diagram signify the orbits for Mercury, Venus, Earth, and Mars. This is to help see the actual distance of each of the zones.)
For stars smaller than the sun: 
Notice how with a larger star, the zone shifts outward, and for a smaller star it shifts inward. So keep this in mind when you sit down to pick a star for your solar system.
Once you have an understanding of the habitable zones, you can pick a star from the star spectral classification:
Star’s Spectral Classification
An image provided by the Jamie Takahashi at DeviantArt, which shows the classifications quite accurately: 
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This classification is a tool astronomers use to classify stars by their temperatures.
Once you have the spectral class, there is a range of sizes and masses allowed within each spectral mass, which will give you a more accurate idea of the habitable zone for that star. This link is an useful tutorial that explains the connections between a star’s temperature, it’s luminosity, and its mass: Hertzsprung-Russell Diagram. The equation: 
is simple to compute, where the luminosity of the Earth’s sun is obtainable via Nasa and the luminosity of your sun for your world is obtained by just taking your spectral class and finding it on the Hertzsprung-Russell Diagram:
On the Hertzsprung-Russell Diagram, you’ll notice that there is a main sequence line of stars, an area for super giants, a smaller area for giants, and an area for white dwarfs. This diagram is the life of a star. When a star ignites fusion inside its core, it enters the main sequence, where it lives out most of its life. This is where Earth’s sun is now. However, once it uses up is fuel of hydrogen and helium, a star begins to consume other layers of its insides, which causes it to expand due to the forces within it. It now enters either giant stage or super giant stage depending on the original mass of the star. This stage may last a few million to a billion years at most, which is short compared to two to eight billion years a star may exist in the main sequence area of the graph. Now the death of a star depends again on its mass. Less massive stars will become white dwarfs, which is the fate of Earth’s sun; a white dwarf is about the size of Earth. Some of the more massive stars will become neutron stars, which is a star that is the size of a large city like Manhattan in New York. The most massive stars will explode in a violent supernova and become black holes. This is something else to keep in mind if your solar system, and its life on it spans the course of billions of years. The giant/supergiant stages and the death stages of a star’s life can destroy a planet entirely or destroy its ability to hold life, so if your cultures span several billion years, you’d have to take that into consideration. However, since that’s a pretty epic scope, most stories and novels will take place while the star is in one stage, allowing for you to neglect these epic stellar events. It’s just a fun side note to keep in mind in this discussion on stars.
For example, I chose spectral class F for Elivera’s sun. A quick class at the HR diagram, shows that if its a main sequence star (like Earth’s sun), then the luminosity is approximately ten times the luminosity of Earth’s Sun. I can put the ten on the left side in place of the luminosities, solve the equation for M, since we know the mass of Earth’s sun, and calculate the answer. Keep in mind, this is just an estimate, and useful mostly for planning purposes. The mass of the star allows you to calculate the gravity exerted on the planet itself, which could influence tides, shape of planet, shape of planet’s magnetic field, and so on and so forth.
Once you have these parameters, you can then place your planet at any orbit that fits within the habitable zone of your newly created star. You can estimate the zone via the above diagrams based on the approximate mass of your star. This will give you a planet that can hold liquid water, and the size of the orbit itself will give you the size of your years.
Now for the planet itself, you have to determine the tilt of its axis. For Earth, the axis tilt is around 23.2 degrees offset from perpendicular to Earth’s orbital plane. This allows for Earth to experience fairly significant seasons at its middle latitudes. Its equatorial and polar regions would experience only two seasons due to the equator receiving most of the sunlight throughout the year, and the polar regions receiving the least amount of sunlight throughout the year.
This is the first part in a series about the creation of a solar system. I am detailing a way to build this without extensive math. In my case, I don’t use the simplified math here, but instead use more accurate physics equations, but for a typical writer, such accuracy is not needed for world-building, and the approximations here are good enough for such purposes.
Let me know if you have any questions!
Reshaping Reality 
I must admit i have never considered giving any of my worlds a different kind of star. An oversight? Maybe, but perhaps it’s because i generally don’t write Sci-fi on a large scale. I must say i like Spectral Class O – The idea of a Blue sun is very cool.
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It’s definitely a lot of fun since how the sun looks in the atmosphere will differ based on its spectral class. Class O is the hottest stars out there, so your planet would be a bit farther out than Earth is to our sun in order to be habitable. So the sun may seem smaller in the sky, but it definitely won’t look yellow or orange to the people on the planet. I wonder what sorts of myths could evolve from that?
I tend to cater to the F through O classes myself to be honest.
I’m preparing a part two that focuses on the planet itself, which should be interesting hopefully. (I’m trying to write this without much math, which is a bit hard at times since I come at it from a more mathematical view.)
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