![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
Star mapping, part 1 of 4: the basics
Sep. 28th, 2010 06:26 pmSince outer space is three-dimensional and paper maps are two-dimensional, no star map that you can put in a book (or on a Web page) is going to be completely accurate. How can you make a flat map that still gives the user a reasonable idea of where the stars are in relation to each other?
First, pick the stars you want to put on the map. This probably seems like a straightforward step; after all, by the time you get around to making a star map, you’ve probably already got a story idea or setting in mind. However, as we shall see, there can be wrinkles. For now, though, let’s say you know what stars you want to use. For the purposes of this exercise, we want a map of the Solarian Federacy, which (I just decided) consists of planets in the following systems: Sol, Epsilon Indi, Sigma Draconis, Eta Cassiopeiae, Gliese 667, and p Eridani. (It also includes a bunch of stars with no habitable planets, which we will keep off the map to avoid overcrowding.)
Next, get the stars’ galactic XYZ coordinates (look here if you don’t know what XYZ coordinates are). Sources for these coordinates include:
The HYG Database: this is a database that can be imported into Microsoft Excel and other spreadsheet programs. It incorporates data from three star catalogues and includes all bright stars as well as all stars within 50 parsecs (160 light years) from Sol. It’s available at http://astronexus.com/node/34. It’s very large, and the same site has several stripped-down versions if you don’t need a database that includes 120,000 entries!
The Internet Stellar Database: this is an online database that you use by looking up individual stars. It lists most stars within 75 light years, although it does omit a lot of obscure red dwarfs. It’s at http://www.stellar-database.com/.
The HabHYG Database: the HYG Database cross-referenced with another database created by Margaret Turnbull and Jill Tarter. HabHYG, created by Winchell Chung, is particularly useful if you’re specifically looking for information on stars that are good bets to have habitable planets. It’s at http://www.projectrho.com/smap06.html.
Because you’re just doing a map for your own quick reference (and eventually, I hope, your readers’ as they look at the front of your book), you can round down the coordinates to the nearest one-tenth of a light year. Don’t worry -- no astrogator will die because your numbers aren’t entirely accurate! Besides, there’s still a certain margin of error even for the distances of the nearest stars. Here are the coordinates for our chosen stars:
Sol: 0, 0, 0
Epsilon Indi: -7.2, -3.2, -8.9
Sigma Draconis: -3.5, 17.1, 7
Eta Cassiopeiae: -10.5, 16.2, -1.6
Gliese 667: 23, -3.1, 0.3
p Eridani: 4.3, -12.6, -23
Third, decide on the orientation of the map. As you might expect, on most maps of this kind, the X (horizontal) axis represents galactic-X and the Y (vertical) axis, galactic-Y. The X axis is customarily aligned so that the galactic core is to the right of the map and the galactic edge is to the left. (In other words, the higher a star’s galactic-X number, the closer it is to the core.) The Y axis is aligned so that the direction in which the galaxy is spinning is up on the map and the reverse is down. And the Z axis (which does not show directly on the map) is aligned so that the viewer appears to be looking at the map from the galactic north; the farther away from you a point is on the Z axis, the further it lies in the direction of galactic south. We’ll follow these conventions in creating our sample map.
Fourth, plot the stars’ locations on the X and Y axes using a sheet of graph paper.
Finally, label each star with its name and its location along the Z axis, to give anyone reading the map a quick idea of how far “north” and “south” they are.
Congratulations, you’ve got a star map!
To spare you having to read my writing, I’ve actually done the map using Microsoft Excel and then made a copy in PDF. Using Excel has a few wrinkles of its own, which I’ll get to in a couple of days.
As you can see, this map is reasonably accurate. Epsilon Indi appears closest to Sol, which in fact it is. Sigma Draconis and Eta Cassiopeiae are farther out, and Gliese 667 is farther still. The only joker in the deck is p Eridani, which is farther from Sol than Gliese 667 but appears closer on this map. Can you see why? Yes, it’s because p Eridani is much farther to the galactic south than any of the other stars pictured; it’s the most distant from the viewer who is looking “down” at these stars. Still, as long as you include a note on the map about depth of field, it’s a pretty good approximation of reality.
Next: what to do when the map's approximation of reality is way off!
First, pick the stars you want to put on the map. This probably seems like a straightforward step; after all, by the time you get around to making a star map, you’ve probably already got a story idea or setting in mind. However, as we shall see, there can be wrinkles. For now, though, let’s say you know what stars you want to use. For the purposes of this exercise, we want a map of the Solarian Federacy, which (I just decided) consists of planets in the following systems: Sol, Epsilon Indi, Sigma Draconis, Eta Cassiopeiae, Gliese 667, and p Eridani. (It also includes a bunch of stars with no habitable planets, which we will keep off the map to avoid overcrowding.)
Next, get the stars’ galactic XYZ coordinates (look here if you don’t know what XYZ coordinates are). Sources for these coordinates include:
The HYG Database: this is a database that can be imported into Microsoft Excel and other spreadsheet programs. It incorporates data from three star catalogues and includes all bright stars as well as all stars within 50 parsecs (160 light years) from Sol. It’s available at http://astronexus.com/node/34. It’s very large, and the same site has several stripped-down versions if you don’t need a database that includes 120,000 entries!
The Internet Stellar Database: this is an online database that you use by looking up individual stars. It lists most stars within 75 light years, although it does omit a lot of obscure red dwarfs. It’s at http://www.stellar-database.com/.
The HabHYG Database: the HYG Database cross-referenced with another database created by Margaret Turnbull and Jill Tarter. HabHYG, created by Winchell Chung, is particularly useful if you’re specifically looking for information on stars that are good bets to have habitable planets. It’s at http://www.projectrho.com/smap06.html.
Because you’re just doing a map for your own quick reference (and eventually, I hope, your readers’ as they look at the front of your book), you can round down the coordinates to the nearest one-tenth of a light year. Don’t worry -- no astrogator will die because your numbers aren’t entirely accurate! Besides, there’s still a certain margin of error even for the distances of the nearest stars. Here are the coordinates for our chosen stars:
Sol: 0, 0, 0
Epsilon Indi: -7.2, -3.2, -8.9
Sigma Draconis: -3.5, 17.1, 7
Eta Cassiopeiae: -10.5, 16.2, -1.6
Gliese 667: 23, -3.1, 0.3
p Eridani: 4.3, -12.6, -23
Third, decide on the orientation of the map. As you might expect, on most maps of this kind, the X (horizontal) axis represents galactic-X and the Y (vertical) axis, galactic-Y. The X axis is customarily aligned so that the galactic core is to the right of the map and the galactic edge is to the left. (In other words, the higher a star’s galactic-X number, the closer it is to the core.) The Y axis is aligned so that the direction in which the galaxy is spinning is up on the map and the reverse is down. And the Z axis (which does not show directly on the map) is aligned so that the viewer appears to be looking at the map from the galactic north; the farther away from you a point is on the Z axis, the further it lies in the direction of galactic south. We’ll follow these conventions in creating our sample map.
Fourth, plot the stars’ locations on the X and Y axes using a sheet of graph paper.
Finally, label each star with its name and its location along the Z axis, to give anyone reading the map a quick idea of how far “north” and “south” they are.
Congratulations, you’ve got a star map!
To spare you having to read my writing, I’ve actually done the map using Microsoft Excel and then made a copy in PDF. Using Excel has a few wrinkles of its own, which I’ll get to in a couple of days.
As you can see, this map is reasonably accurate. Epsilon Indi appears closest to Sol, which in fact it is. Sigma Draconis and Eta Cassiopeiae are farther out, and Gliese 667 is farther still. The only joker in the deck is p Eridani, which is farther from Sol than Gliese 667 but appears closer on this map. Can you see why? Yes, it’s because p Eridani is much farther to the galactic south than any of the other stars pictured; it’s the most distant from the viewer who is looking “down” at these stars. Still, as long as you include a note on the map about depth of field, it’s a pretty good approximation of reality.
Next: what to do when the map's approximation of reality is way off!