Posted by: movieotaku | April 6, 2007

The Scale and Structure of the Universe

“Space is big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, that’s just peanuts to space.”

— Hitchhiker’s Guide To The Galaxy by Douglas Adams

Just how big is the Universe? Infinite. How do we know? We don’t. Funky, huh? But that doesn’t change the fact that the distances we do know about are mind bogglingly huge. We’re talking about numbers and distances that our primitive brains can’t even comprehend. They are literally unthinkable. We can manipulate the numbers, but that doesn’t mean we really understand them in a deep, intuitive sense. They’re just that big.

So we’ve got a big universe, but how is it organized? Do you know what the order of structures in the Universe? If you do, you’re light-years ahead of most Hollywood sci-fi writers. Lost in Space thought they might be getting near home when they passed Uranus, but apparently still couldn’t find Earth. Battlestar Galactica traveled between galaxies, or did they mean star systems?

Bad astronomy is second on my list of most irritating science blunders. It’s the kind of blunder that is easy to fix. Basics astronomy is simple and knowing it can increase your enjoyment of sci-fi if they do it right. Now let me take you on a tour of the Universe from the our homeworld, Earth, to the reaches of your imagination.

The Journey of a Trillion Miles Begins With One Step

Astronomers throw around numbers like millions and billions casually like they’re 1’s and 2’s to the rest of us, but almost none of us can even remotely grasp just how big those numbers are, and by extension, just how far things are in space. You can probably wrap your mind around something that’s a mile long, because it’s a distance you can easily walk and is on a human scale, but trying to extend that understanding to the distances of space usually fails. Let’s take a walk and see just how big the Universe really is.

The way most of us come to understand a distance is to walk it with our own two feet; somehow that creates a more direct connection with that distance than driving it. So how can we relate to a relatively large distance like 24 miles? Let’s try walking it. You walk about 3 miles-per-hour. It would take you eight hours to walk that distance assuming flat level ground. The average city block is 1/10th of a mile, so that’s 240 city blocks. 240 city blocks is a stupendous distance bigger than most cities. Odds are, you’ve only walked 20 or 30 blocks at a stretch, but can you imagine walking for 8 hours straight? Starting at 9 a.m., you’d spend an entire day walking a distance that would take you half an hour in a car. And that’s just a drop in the bucket.

On the scale of the Earth, 24 miles is an insignificant distance. Los Angeles is over 3000 miles away from New York. Walking that distance would take over a 1000 hours, but 3000 miles is nothing when compared to the distance to the Moon.

The Moon is, on average, 384 401 km (238 855 miles) away from the Earth. You’d have to walk over 128,000 hours non-stop to reach the Moon. That’s walking non-stop for over fourteen years! Imagine if you started walking fourteen years ago and didn’t stop (even to sleep or eat). In those years, there would be three presidential elections. A child will grow from a newborn to a 9th grader by the time you walked to the moon. Fourteen years of your life just to travel to our nearest neighbor in space.

The distance to the Moon is a big distance. At that distance, even the speed of light begins to look slow: it takes light over a second to reach the Moon. So trade in your sneakers for a laser beam, because the distances we’re about to travel will make a walk to the moon seem like a stroll in the garden.

The distance from the Earth to the Sun, which is 1 Astronomical Unit or AU, is 8 light minutes. If the sun were to spontaneously go dark, we wouldn’t find out for 8 minutes. But as you might suspect, that’s still a relatively tiny distance. The distance from one end of the solar system to the other (bounded by the orbit of Pluto) is about 80 AU. It would take over 11 hours for a laser beam to race that distance. I’m not even including the true bounds of the solar system as defined by the heliopause, which is estimated to be between 80 and 130 AU from the Sun, which means the solar system is up to 260 AU across. Light takes a day to cross that distance. If you had to walk that distance from one end of our solar system to the other, it would take 1.5 million years to walk that distance. That’s almost 60,000 generations1 to walk that distance with your piddling little gait. That’s longer than the entire history of civilization.

Now we are at the scale where distances are so large that there is no way for your mind to comprehend them. The larger structures of the Universe like galaxies, local groups and superstructures, will make that 1.5 million year walk seem like a micrometer.

This is where I will leave you for now.

Astrocartography 101: Do You Know Where Your Homeworld is?

Can you find Earth on a map of the Galaxy? Probably not, but I’m sure you can at least tell the difference between a solar system and a galaxy. Too bad I can’t say the same about TV. One correspondent shared with me his favorite science blunder from Lost In Space:

[The] Lost In Space TV series was full of them. Like in theepisode WILD ADVENTURE…the quote “We’re getting closer to Earth, we’ve just passed Uranus and Arcturus”

— Char-El <>

Or there was Star Trek which was unable to tell the difference between a galaxy and a Universe. Or Battlestar Galactica which believed galaxies and star systems were the same thing. The list goes on.

Learning the basics of astronomy is easy, and it will give you an appreciation of the awesome Universe we live in. Knowing these details are not an impediment to good drama; they are a dramatic leaping point to new stories and adventures guaranteed to generate awe and wonder.

Planet Earth: Homeworld

First off, you live on a planet called Earth (in English). There is no official, UN sanctioned universal name for our homeworld (A homeworld is the original world for a species or race). The International Astronomical Union, the organization most people recognize as the official astronomical name-giver, prefers the English name Earth so we will use it.

Solar System: Sol

We’re the third planet in our solar system of nine planets orbiting the star Sol; although the International Astronomical Union seems to prefer people just calling it “The Sun” in their native language. Our mean distance from the Sun is 1 AU.

Galactic Arm: Orion Arm

Our sun is in the Orion Arm of the Milky Way. A minor arm on the outskirts of our galaxy about 26 000 light years from the center.

Galaxy: Milky Way

While other galaxies get cool names, like Andromeda or M-31, ours gets a folksy name. The name came from the Romans (and supposedly the Greeks) who called the band of stars and dust across the night time sky the “milky way”.

Galaxy Group: Local Group

Continuing in the tradition of unique and cool sounding names, our local group of galaxies is called, unsurprisingly, The Local Group.

Supercluster: Virgo Supercluster

The Virgo Group is the nearest galaxy group to our. It is so large it exerts a gravitational influence over its neighboring groups, including our Local Group, thus the supercluster is called the Virgo Supercluster.

Universe: Universe

To the best of our knowledge, there is only one Universe. There maybe others, but until they are proved, this is where we stop.

Bestiary of Galaxies, Nebulae and Other Things That Go Bump In The Night

There are some days if I wonder the science education of Hollywood writers consist of what they read off the back of cereal boxes. Battlestar Galactica used to confuse galaxies and solar systems daily, often switching meanings between episodes. So, to avoid future confusions, here’s a semi-canonical list of the structures of the universe.

You know what the Earth is, right? It’s the planet you are currently on–I hope. It’s a ball shaped rock orbiting in an almost perfect circle around the Sun. The accepted definition of a planet these days is anything that’s not a star. There’s actually a debate about the exact definition of a planet, but here’s a useful definition: a large body of approximately spherical shape with enough mass to have significant gravity. In our Solar System, nine planets travel elliptical paths around the Sun. To the best of our knowledge, other solar systems probably have similar structures.

The Solar System is not just the sun and the planets, but the debris left over from its creation: asteroids and comets. An asteroid is a rock with little to no ice and does not create a cloud around itself as it approaches the Sun. Asteroids can be made of metals (e.g., iron-nickel, magnesium) or carbon and silicates (plain old rock). They usually have orbits randomly oriented around the Sun and are scattered across the Solar System. There’s a large concentration of asteroids in the orbit between Mars and Jupiter, the Asteroid Belt, that may be the remains of a planet that didn’t have a chance to form properly.

A comet is a dirty ball of ice that creates a cloud around itself as it approaches the Sun. Comets usually come from a surrounding cloud of comet nuclei called the Oort cloud that surrounds the solar system, but some formed close to the Sun and simply fell into orbit. Comets contain water, rocks and complex organic molecules. Complex organic molecules? Yes, like methane (natural gas), amino acids (building blocks of protein) and other fun stuff. Comets may have been instrumental in bringing the chemical soup to Earth that enabled the creation of life.

Notice I haven’t mentioned meteors? That’s because an asteroid is a rock in space while a meteor is anything (comets, asteroids, space stations, etc.) plunging through our atmosphere and burning up. If it’s not in our atmosphere, it is not a meteor; if it is in our atmosphere, it is a meteor. Clear?

A solar system usually belongs to a larger group of stars called a galaxy. There are dozens of different types of galaxies–spiral, flat, elliptic, clouds–but all have the common property that they are a distinct group of stars separated from each other by enormous distances. The galaxies themselves also cluster into groups, and the groups into superclusters forming huge structures spanning the Universe.

Now we leave the realm of known and enter speculation. To the best of our knowledge, there is only one Universe: this one. There is nothing outside of the Universe because there is nothing outside of the Universe. There was no time before the Universe, nor is their time after the Universe. Mind bending, isn’t it? Now, some scientists conjecture there may be other Universes “outside” our own, or parallel to ours in other dimensions. There isn’t any real evidence of this yet, but it can be fun to speculate. Do these other Universes have the same laws of physics? Do they have intelligent life?

Journey Beyond the Infinite…

This is just the beginning. There are endless areas you can explore, and you will always learn something new. I don’t know why some writers think knowing real science somehow constrains the imagination. The Universe is a big and interesting place, and knowing it can only provide more fuel for the imagination. Hopefully, armed with this knowledge, you too can be driven nuts when you hear someone confuse a solar system with a galaxy, or you can scream at the TV when the Moon in Space: 1999 can travel interstellar distances in a couple months.

There really is no excuse for these blunders; you can find out all these things in a children’s astronomy book. The only way things can get better is if the audience becomes smarter, so I hope this essay has given you a better understanding of astronomy. Who knows, it might even get you excited in learning more about astronomy, and the more we, the fans, know, the better our entertainment will get.

1Thanks to Sven Silow for pointing out my numerous numerical blunders on this page.



  1. This page needs updating, there are only 8 planets in our solar system.

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