How Big is Our Galaxy? Answer: It’s Bigger Than That

Posted: June 3, 2016 in science, science fiction, space, Writing
Tags: , , ,

You say: “It’s _____________.” To which I say, “No, it’s bigger than that.” Let me explain. I’m working on the setting for a new series of novels, and one key decision is how Luis Calçada_under the milky waymuch of the Milky Way Galaxy do I take on? How much space could humanity colonize or even explore in a century or two or three (assuming we could travel faster than the speed of light)? If there are intelligent aliens out there (in my stories, this is a given), how much space would they have occupied if they’ve had spacecraft technology for a thousand years or a hundred  thousand or a million or more? Most science fiction books, television shows and movies only address a small Exoplanet with rings_Image courtesy of Ron Millernumber of locations, partly due to the limited budgets and screen time, or perhaps due to the effort involved in world-building. Yet the real galaxy is overflowing with locations.

Our galaxy is a disc roughly 100,000-180,000 light years across and around 2000-12,000 light years thick (excluding the Halo and the central bulge). Astronomers currently estimate it contains 100-400 billion stars and 100 billion planets, possibly many more than that. We occupy the Orion Arm, a minor arm (probably), and it alone is 3500 Hubble-Views-Globular-Cluster-IC-4499light years wide and perhaps 10,000 light years long. For comparison, using estimates from An Atlas of the Universe, the sphere of space going out a mere 20 light years from Earth contains around 109 stars (117 including brown dwarfs). Go out 50 light years, and the number of stars jumps to about 2000. Expand out to 250 light years and you have (by my calculations) around 250,000 stars. Expanding one more time to 5000 light years gives you 600 million stars. So far, we’ve identified almost 2300 exoplanets going out to a mere 8500 light years, yet our planet-detection technology is still in its infancy. That number will almost certainly rise. Conclusion: our galaxy is DENSE with stars and planets. It’s difficult to talk about occupying space at all given such an amazing concentration of potential places we could be. Even if we 600px-Nearby_Stars_(14ly_Radius).svgcan get to a certain distance, what does that mean, if we’ve put thousands or millions of unexplored star systems between us and home? When that journey eventually happens (I’m optimistic), will human expansion occur by leapfrogging swathes of unexplored space, or as a slower consolidated front of outward migration? Either way, what will that imply for governance, communication, military power, and cultural evolution in general?

I also find it hard to get a grasp on unique locations in the Milky Way, its geography. It’s not like a continent, where there are easily recognizable features such as mountain ranges, lakes or rivers. Yes, there are plenty of interesting Hubble_Sees_a_Horsehead_of_a_Different_Colorfeatures: nebulae (giant clouds of gas and dust) and star clusters, for example. But they are generally described as seen from Earth. In the context of a human, space-faring future (or an alien perspective), they wouldn’t look the same from other viewing points (the Horsehead Nebula wouldn’t resemble a horse head from a different angle). They can’t really be seen in the normal sense of the word at all (by a person viewing them without any optical assistance), given these structures are generally tens or 100’s of light years across and sometimes not even viewable with visible light. Visualization is a problem, at least for me. It’s true there are the spiral arms, nebulae and clusters, but they are structures of vast scales. If you want to look 100,000 starsmore finely at individual stars, there are maps out there (An Atlas of the Universe, Wikipedia, etc.) and even some nice 3-D visualizations (3DgalaxyMap, 100,000 Stars, etc.), but I find it hard to get a sense of location looking at star maps. Space, and the stars within it, seem more like a scatter of dust motes caught in a sunbeam. Turn it whichever way, but for a non-astronomer like me, it’s still just a volume of particles suspended in space.

If you know of better tools for visualizing space, please let me know. And if any aliens out there are reading this, I’d like to hear from you, too. Help us out here. How do you navigate through all that space?



Image credits (from top):

Luis Calçada: under the milky way

Exoplanet with rings_Image courtesy of Ron Miller


Nearby Stars (14ly Radius).svg


Screencap from 100,00 Stars


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