World Building 101: Today's Topic: Holy Magnitude!

Space is huge. Really huge. Mind-bogglingly, oh-my-gawd huge. Trying to wrap your brain around just how huge space is is a daunting task. But we'll give it a try.

Why does one need to know just how huge space is? Well, it allows you to do something amazing. Winning the lottery sort of amazing. You can imagine your world just about anyway you want, and if it can theoretically exist, then chances are it does, or did, or will. The numbers are on your side.

Our solar system is comprised of our lonely sun, a single star, and eight decently large planets, four rocky and four slushies, and a multitude of smaller icy planetoids. (Nine? You say? Well, not really, since Pluto hardly qualifies... but I digress).

How many more stars are out there in our galaxy? About 200,000,000,000. That's two hundred billion suns in our local Milky Way galaxy alone. And how many galaxies are there? Well, current estimates put the value around 100 to 300 billion galaxies. Our Milky Way is considered pretty large on the ol' galaxy scale, so we can't use it as an average, but assuming an average of only 1 billion stars per galaxy we end up with 100,000,000,000,000,000,000 stars in the known universe! That is a lot of stars! That is ten raised to the twentieth power, also called a hundred quintillion.

So, assuming for the moment that only one percent of those stars have planets in stable orbits, and assuming only one percent of those have been around long enough for life to form on the planets, and further assume that only one percent of those actually have planets in the right orbital temperate zone, and assume that one percent of those have the right collection of elements to start life, and in the right proportions. How many planets does that give us? That reduces our huge number solar systems by eight powers of ten, leaving 1,000,000,000,000 planets that have all the right stuff for life. One trillion planets. Odds are, one of them fits your profile.

Now, my 'one percent' assumption could be off, it could be off a lot, but even if it's off several magnitudes, we still have plenty of life producing planets to choose from. (This thought experiment is brought to you by something called the Drake Equation.)

What I'm getting at is that a fantasy writer doesn't have to worry about the odds of a habitable planet existing. Chances are, it does. Somewhere out in the vastness that is space, a planet probably exists with that blue sun, three distinct rings, and two moons that you want.

The real problem is distance. Space is huge, as I've said. And most of it is filled with nothing (or dark matter, which is still mostly nothing). Traveling from one solar system to another is a monumental undertaking.

Most stars are light years apart. A light year is about six trillion miles.

How long would it take to travel a light year? Well, Voyager 1, our own unmanned space probe sent out 28 years ago (in the summer of '77 - and still running, I might add!), is now 8,700,000,000 miles away. That's about 0.15% of a light year. At that speed (about 38,000 mph), it will be another 18 thousand years before Voyager 1 is one light year away.

Even assuming you made a craft that travels a thousand times faster than Voyager 1, it would still take you a good 18 years to get one light year. And our nearest neighbor, Alpha Centauri, is four light years away! That's 72 years at 38 million mph. And, of course, once you got there and tried to phone home, you'd have to wait four years for the signal to reach Earth, and another four years for the answer to come back. It's hard to carry on a conversation with an eight-year lag time! Can you hear me now?

And, traveling very fast creates new problems, problems that are weird and spooky, which I'll save for a some future lecture.

All this humongous distance of space is the reason why sci-fi writers invented warp drive. Flying from star system to star system via faster-than-light space travel is de rigueur. It is so much nicer to be sitting around playing holographic chess for a few hours while hurtling through space at super-light speed rather than waiting a whole lifetime just to reach your stellar neighbor.

In summary, space is huge and empty, yet contains billions and billions of stars, some of which probably have a habitable planet orbiting it that can be your fantasy world. But good luck getting there.

World Building 101: Today's Topic: Color Me Blue!

Stars come in various colors, depending upon how hot they are burning. Blue is the hottest, followed by white, then yellow and red.

Can your fantasy world have a blue, white, or red sun, rather than a 'normal' yellow sun? Possibly. I'll talk about each one.

A blue star is burning hydrogen just like our yellow sun, but at a much faster rate. It must be many times larger than our sun in order to have enough gravitational pressure to up the temperature to the blue range. This means it won't last as long (the whole a candle that burns twice as bright lasts half as long thing).

So the question becomes, could a planet orbiting a blue star be around long enough for life to form? It's estimated that a blue giant star has a life of about 10 million to 100 million years. Hardly enough time for a planet to cool and form life. But, spectral maps of star clusters reveals a fair number of 'blue stragglers' -- blue stars that for some reason haven't shifted to the red giant phase. Some could be as old as a billion years. This puts it in range of life forming planets, so, it's theoretically possible. Particularly if you have a blue star that's only just blue, that is, about ten times as massive as our sun, as opposed to hundred or two hundred times more massive.

What would a blue sun look like? Well, it would be a smaller disc, since the temperature is so much higher the planet needs to be father away to have a similar climate. But it would very bright, like the difference between a yellow incandescent and a blue-beam headlight.

White stars come in two flavors: main and dwarf. A white main sequence star is just a star like our own, but a few times larger. It won't live as long as our sun, but should still be good for a few of billion years worth of stable solar energy. So no problem having a world orbiting a main sequence white star.

White dwarves, on the other hand, are old stars that have already gone through the red giant phase. This means that any planet orbiting the star has been engulfed when the star expanded to a super-size red sun. So, there wouldn't be any stable planet orbiting a white dwarf star close enough for the solar energy of the dwarf to keep the world temperate.

Red stars also come in two flavors: Giant and main. A red giant is a blue, white, or yellow star that has used up all of its hydrogen and is now burning helium. Red giants are huge, puffed up versions of the original star. Our own sun will become a moderately large red giant in about 5 billion years. It will expand and engulf the four inner planets (Mercury, Venus, Earth, and Mars). At that point, moons orbiting Jupiter might become life-supporting. But that phase will only last for a few million years at most. So, one could have such a world, but the life would probably need to come from somewhere else.

Main sequence red stars are stars with a mass slightly less than our sun, 80% to 30% of our sun's mass. It burns cooler, and therefore has a red hue. These types of stars are quite common, probably the most common, and are very stable. So a fantasy world orbiting a main sequence red is no problem at all.

What would a red sun look like in the sky? It would be a larger disc than our own sun, and have an amber or orange hue. The infrared spectrum would be stronger than our own yellow sun, and therefore the sunlight would 'feel' hot.

What about smaller than a small red main sequence? Suns that small don't have enough gravitational pressure to cause nuclear fusion, and therefore don't give off light. So-called 'brown' and 'black' stars (not to be confused with a black hole), these stars give off infrared light only. A planet could orbit around such a star, and life could possibly exist, for the infrared heat could keep a planet from being a frozen hunk of rock.

More info on stellar classification can be found here.

World Building 101: Today's Topic: You've Got Twins!

Having a twin solar sunset has to be one of the cooler images in the science fiction/fantasy genre. That and dragons...

A solar binary, a system where two stars orbit each other, is actually quite common in our universe. Our closest neighbor, Alpha Centauri, is a binary (actually, it's a triple, with a third red dwarf star out far from the main pair). Star pairs, and double pairs, occur throughout the galaxy in high numbers.

The question becomes, can a planetary system exist in a binary (or higher) system? Or can only the lone stars, like our sun, manage to keep planets in a stable orbit?

The answer is, sometimes. The trouble is that many of the binary systems have stars that are very close to each other, so close that the two gravity wells are overlapping to a huge degree. This makes it highly unlikely that a planet could develop any sort of stable orbit about either sun. If a planet was circling one star, then each time it came near the other star the tidal effects would be large enough to shear the planet apart, or at least pull it out of orbit.

So, to have a planet orbiting a sun that's part of a binary system necessitates that the two suns are a good distance apart. How far apart? Well, there's a lot of math involved, but once it is all said and done it works out that the second sun must be at least five times the distance from the planet to the primary sun.

For example, Alpha Centauri's two main suns are 11 AU (Astronomical Unit: The distance from the Earth to the Sun, about 93 million miles) apart from each other on their closest approach. Meaning that a planet could be orbiting one of the suns as long as it was less than 2 AU from it. So, the Earth could exist in an orbit about Alpha Centauri A or B.

But, as viewed from the surface, the second sun would look a hundred times smaller and fainter than the primary. This is because that while the Earth is one AU from the primary sun, it would be ten AU from the secondary, and light drops off in brightness at the square of the distance (10 x 10 = 100). This is still much brighter than our moon, even during a full moon, and would lend an eerie orange glow to the landscape. The third sun, Proxima Centauri, would just be a little red dot, as its distance is 13,000 AU from the two pairs, and its less bright to begin with.

So, as a fiction writer, binaries, trinaries, and more are certainly possible, but one needs to take into account that the planet will be circling a primary sun, and all the other suns will be progressively further out, by a factor of at least five.

But, Professor Thule, what if the second sun is a giant star, you ask? That's certainly another possibility. Suns come in many sizes, from the little red dwarf to the mighty blue super-giant. This implies a chance where a planet circles a medium-yellow star like our sun, but then have a second sun that is a giant, and while farther away, is many times bigger, thereby making it appear as if the two suns are equal in the sky. Now, giant stars have giant gravity wells, and our ballpark factor of five becomes a factor of ten or even twenty.

We can try this using Betelgeuse, a red giant some 400 light years from our solar system. It's a huge sun, 270 million miles across, big enough that if was dropped down in our system the Earth would be inside of it!

Assuming, for the moment, that one put our sun in orbit about Betelgeuse, could you get a double sunset? Yes. Betelgeuse is about 13,000 times brighter than our sun. So, if it was a bit more than a hundred AU from Earth, it would look as bright as our sun. So we have a factor of 100, more than enough to insure that the planet doesn't get pulled apart from the tidal effects of the second sun. Note that the Earth would now be receiving twice as much solar energy, and would therefore get much hotter. So, as a fantasy writer, you'd have to back off both suns a bit so that the total energy is about the same as one sun.

The problem, however, is that giants and super-giants have a tendency to be unstable. Meaning they often change in brightness, or go supernova into oblivion. This would be devastating for a planet's inhabitants.

In summary, a world can be part of a double sun system. In most cases, the second sun would be a dimmer, smaller sun to the primary. But, in a special case, both suns could appear to be the same size.