When building a solar system, or even just a planet, the star(s) it revolves around is one of the most crucial aspects. The star affects the planet's atmosphere and life-supporting capabilities, temperature, year length, climate, life expectancy, mineral composition, evolutionary trends, and a host of other, smaller aspects. 

I'm not going to give you numbers and maths - it would take far too long to explain, and it's usually not strictly necessary. If you want the maths, I recommend World Building by Stephen L Gillet. At some point I'll dig out and share the tiny program I made to calculate these things for me, so I didn't have to. The important thing to remember when creating stars (and indeed any aspect of worldbuilding) is: everything is interlinked. Changing one thing will ripple changes through the whole design. This is clearly demonstrated by star-building:

A star's temperature and its colour (more correctly termed Spectrum - red, blue, yellow) are inextricably bound - the colour indicates the temperature, and vice versa. Red stars are the coolest, from brown-dwarfs which are only debatably stars up to red giants, then yellow stars (like ours) are the medium range, and blue are the hottest.

The spectrum, coupled with how bright the star appears in the sky (called the Magnitude), determines everything else - its mass, diameter, density, how much energy it radiates, the distance planets must be to have liquid water (and therefore their year length, the star's size in the sky, whether the planet is tidally locked (always shows the same side to the sun, like our moon does to us)), how close they can be before gravitation forces rip them apart. 

The spectrum will also determine the star's life expectancy - as you would expect, hot flames burn through their fuel much faster, cooler ones last much longer. If we'd been rotating around a Blue Giant, we wouldn't have a sun by now; it would have died. This means that if you want to have a blue sun as your central star, your solar system will be very young, and not have long to live; certainly not long enough for intelligent life to spontaneously evolve. The hottest of blue stars survive only half a million years or so - not even enough time for planets to form!

Red stars aren't actually red

This mistake has been made so many times in science fiction: no matter the spectrum, unless your star is a brown dwarf, both it and the light it radiates will appear white. (The definition of a brown dwarf isn't set, as to my knowledge we haven't actually discovered one, but the current definition has it as an almost-star, a jupiter-like planet that had just that bit of extra mass to generate aome fusion, but not regular star fusion. It can't emit the full spectrum, only the red end, including a lot of infrared) This is because a star emits all wavelengths of light (and therefore all colours, thus appearing white). They're termed blue and red because their light temperature shifts them to produce more in the ultraviolet (blue stars) or infrared (red stars) range - but they still produce enough light in the visible spectrum to appear white.

Blue stars aren't friendly to life

The radiation from blue stars is strongly biased toward the ultra-violet end of the spectrum - that's the end that gives us sunburn and skin cancer. Our yellow sun already emits enough to cause serious problems, even though we're protected by our ozone layer, which absorbs most of the UV wavelengths. Not all planets have ozone layers - ours is a quirk of our particular atmosphere mix - so any life on a planet around a blue star will have to have some way of avoiding or counteracting the UV radiation, which breaks down bonds in DNA, or it won't survive.

The bigger it is, the hotter it burns

Mass and luminosity (the total energy the star radiates) are directly linked - stars which have more mass put out more energy; stars which put out more energy must have more mass. When you take the star's temperature into account as well, you'll get the diameter. The calculation's a little complex here, as a higher temperature increases diameter, but higher mass decreases it (from gravitational effects).

Stand back, please

There's a zome around each star termed the 'habitable zone', where the temperature is warm enough to allow water to melt into liquid oceans, but not so hot that it will evaporate into steam. These zones are pretty narrow - take a look at Mars and Venus, both on the outer edges of the zone - and they depend on the star's luminosity - how much energy it's putting out. Stars that are brighter (more luminous) have their habitable zones further out.

This does pose a problem for red stars. A planet can only get so close to another solar body before the gravitational forces between them rip the smaller body apart. This is called Roche's limit, and it's calculated using mostly the mass and radius of the larger body, with the mass of the smaller body providing a slight impact (usually because the size difference is enormous - earth isn't even visible compared to the size of the sun). If a red star has too low a luminosity, its habitable zone may well be inside the Roche's limit. Even if it's not, that close a proximity to the star's gravitational forces are likely to tidally lock the planet - it will only ever face one side to the sun, the other will permanently remain in darkness.

How long until Christmas

A planet's year length is determined by its orbit (distance from the star) and the star's mass. It's not strictly proportional - speed decreases as the length of orbit increases. A planet orbiting a blue sun in its habitable zone will have a much, much longer year (maybe centuries longer) than a planet orbiting a red sun in its habitable zone.

As I said - no maths involved, here. And unless you're writing hard SF, you're not likely to need the numbers. All you really need to keep in mind is the relationships between the aspects:

• Blue stars: hotter, short-lived, massive, more UV radiation, longer and slower orbits
• Yellow stars: happy medium on all fronts
• Red stars: cooler, longer-lived, smaller, more infrared radiation, shorter and faster orbits.

Both red and blue star types have rather serious difficulties if you want them to support life, but these should be seen as prompts for creativity and innovative solutions rather than problems. If you follow the science, you're far more likely to create an innovative, rich and original world than if you just slap any old star above a planet and label it 'done'.

• Add New
• Search
• RSS

Comments (0)

Write comment

Your Contact Details:

Name:

Email: do not notifynotify

Website:

Comment:

Title:

Message:

Security

Please input the anti-spam code that you can read in the image.

Powered by !JoomlaComment 4.0 beta2