WORLD BUILDING

Notes on:

Astronomical features

Planetary composition, formation and features

Geological processes

Atmospheric/Hydrospheric processes

Biology, and conditions for life

This section strictly sticks to the geographical features of this world, I don't think I can suggest anything sensible on designing cultures and ideologies, or thinking up the various technologies, though I am rather proud of the ones I have displayed on this site, possible or not.

This has all ended up on one very elongated page, I am going to tell you that I thought it would be useful to get all the information on one page, invaluable when you are hunting through your histories, and nothing to do with the fact that I just keep banging on.

Astronomical (^TOP^)

Calculations

Stars: 

Luminosity, 

Surface temperature, 

Peak wavelength, 

classing,

Body information

Surface gravity,

Surface temperature (approximation),

Tides,

Barycentre,

Orbital information

Period,

Eccentricity,

Complete basics:

Volume of a sphere,

Mass of spheres and densities,

Notes

Stars

Multiple star systems

Calculations:

Stars

Mass is the most important thing really you need to consider, mass determines the luminosity, surface temperature, and indirectly things like peak wavelength.

 

Luminosity = (Mass [in sols])^3.5

This works quite well for main sequence stars from classes F to M, though if you look at the Hertzsprung-Russel diagram you get a lot of variation, but this simple method is quite good for 'habitable' stars. (back)

 

Surface Temperature ['K] = (32.175*X*X)+(585.74*X)+5810

Where X = ln Luminosity [sols]

This is the rough approximation I use to get surface temperature form luminosity, there is a better equation out there I know, but when I was building the program I used this instead, I just tried to find a good relationship between luminosity and surface temperature using some standard known stars, the correlation is within about 20'K for f to M class stars obeying my first rule. (back)

 

Peak wavelength (m) = 2.9E-3 / surface temp ['K]

Simple Wiens law equation, this will tell you the peak wavelength emitted by the star, cool stars emit towards the infra-red, brighter ones nearer ultraviolet, you can also use other parts of the peak curve to tell you the 'colour' of the star. (back)

 

Classing stars

Roughly...

7000'C - 6000'C    F class

6000'C - 5000'C    G class

5000'C - 4000'C    K class

4000'C - 3000'C    M class

(back)

 

Body information

Surface gravity

g = ((G*M)/(R^2))

Where 

G is the gravitational constant 6.673E-11

M is mass in kg

R is radius in metres

and g the acceleration due to gravity at radius, in metres per second per second (m/s/s). Earth has a value ~ 9.81 m/s/s (back)

 

Surface temperature ['K] = ( (1 / (Orbital radius [km] * 8E-14) ^ 0.5 ) * (luminosity [sols]) ^ 0.5 )

This is a very rough and ready approximation, but should be accurate to within about 10'K, of course this equation does not count for atmospheres, or slowly rotating planet heat side differences etc. it is best used as an approximate guide. It is basically derived from the inverse square relationship between insolation and distance, you can refine the constant to give better results (back)

 

Tides

This simple equation provides fairly accurate results from tides generated by any body on any other body, it is simply an extension of the forces between two bodies

Tide Height [m] = ( ( m*(R^4)) / (M*(R^3) ) )

where m is the remote mass in kg

where R is the distance between the two bodies in metres

where M is the central mass in kg (back)

 

Barycentre calculation

The barycentre is the centre of mass between two bodies.

s = ( ( m * R ) / ( M + m ) )

s is the distance of the barycentre from the central mass [m]

m is the satellite mass [kg]

M is the central mass [kg]

R is the radius between the two bodies (from their centres) [m] (back)

 

Orbital Information

Period

Period Time [s] =((4*(pi^2)*(R^3))/(G*M));

G = constant (6.673E-11)

R = orbital radius (m)

M = central mass (kg)

This formula assumes for circular orbits, but as Kepler tells us that the angular velocity remains constant, and so provide quite accurate results (less than 0.1% error). (back)

 

Eccentricity

Using the eccentricity system that runs between 0 and 1, where 0 is a perfect circular orbit.

A = R*(1+E)
P = R*(1-E)

where

E = eccentricity, unit-less number

A = Apoapsis distance (most distant)

P = Periapsis distance (closest approach)

The actual distance units are irrelevant as long as the original units are maintained, metres, km, etc.

Most planets have low eccentricities generally below 0.1, Earth's own eccentricity is about 0.016. Comets can have eccentricities of up to 0.3. (back)

 

Complete Basics:

Diameter is of course twice the radius.

Volume of a sphere (can be assumed for planets stars etc.)

Volume = (4/3)*Pi*radius^3 (back)

 

To work out mass of such sphere you need to know the density if you are using g/cm^3 then  

Mass = Volume * Density *1000

Which will give you a mass in kg, as long as volume is in metres cubed, if you want metric tonnes, you can disregard the *1000 as long as volume is still in metres cubed. (back)

 

Notes

Stars

Multiple star systems

These type of systems with more than one sun are unlikely to have a great many planets around them, nor are they readily able to form due to the complicated gravitational fields caused by the large masses of the stars in the system.

But stars orbiting each other at great distances, in excess of a few thousand million km may be able to have relatively normal independent planetary systems. Stars closer than this cause far too much chaos for the development of ‘normal’ planetary systems, there will be few planets, if any.

It may be possible for planets to follow a ‘figure of eight’ between two parent stars, but this kind of orbit is precarious state, with the result of the planet eventually coming to reside around one star, (in a very eccentric orbit perhaps), or the destruction of the planet by falling inwards or being catapulted out of the system. (back)

 

Geological (^TOP^)

Atmospheric/Hydrospheric (^TOP^)

Atmospheric

Oxygen ratios

Methane content

Methane degradation

Hydrospheric

A note on pressure

A note on temperature

A useful table

Oxygen ratios:

Try to keep the percentage of oxygen beneath 21% at Earth like pressures, if above serious problems with runaway fires etc, which are not good for life bearing worlds. you can edge up this percentage a bit if you accept that the foliage and organic things are relatively damp. Pure oxygen atmospheres are not good at all, as it would be very hard to extinguish fires, and even fairly moist things like human flesh will burn readily. (back)

Methane content:

Earth like atmospheres containing methane above ~5% are explosive, so atmospheres containing large amounts of methane and oxygen are highly unlikely to be stable, think of the ignition in the event of a lightning strike, and the cascade combustion. Though it is quite acceptable to have smaller amounts of methane, as this does not preclude life, rather it is more indicative of it, though the more added the more combustion of fires is aided. (back)

Methane degradation

On Earth at least, methane is liable to be broken down by the action of UV light, this effect also appears true even for Titan, which is so many multiples the distance of terra from the sun. Methane breaks down via a series of free radical reactions into water and carbon dioxide, some of the free radicals generated by UV may also take part in side reactions, complicating atmospheric chemistry.

On Earth methane has about a 20yr lifespan before its conversion into carbon dioxide, a gas which is stable over geological time periods, if it is not absorbed by plants or formed into carbonates in water.  (back)

Basics on hydrospheres

A note on pressure

Ensure you have enough atmospheric pressure to allow liquids to exist, in vacuum there is no liquid water, likewise it is also important to note that under too much pressure liquids become supercritical fluids. For water these limits are lower than 6 millibars, or higher than 218 atmospheres, different liquids have different limits. (back)

A note on temperature

Ensure that temperatures are suitable for liquids. There is a point, the critical point, which determines the highest possible temperature that a certain liquid can form, these can be looked up in a table of critical temperatures, these like critical pressures change with different substances. (back)

Nice table

This table lists some useful values, well I think so. (sorry about drifting degrees of accuracy, I am not sure where I collated this data from, and my notes contained all different values)

(back)