Mapping

Map the stars with size, transparency, and color

The night sky mostly looks like a million tiny white pinpricks in the dark. But there is in fact a lot of variation there. Some stars are brighter than others. And if you look closer (try binoculars!) you can see that the stars have different colors. You can use ArcGIS Pro to depict these subtle stellar differences.
stars of the southern hemisphere - finished map

This tutorial delves into the Vary symbology by attribute option in ArcGIS Pro. You can download my project with the data needed to follow along.

Begin by preparing a map. You can use the Starting point map in the provided project.
You’re mapping the night sky, so make the background black. You can do this via Map Properties:
Map Properties window open to the General tab with Background color set to black
Also on Map Properties, you can set a Coordinate System. Do you live in the northern hemisphere? If so I recommend North Pole Stereographic as a projection for this map. Otherwise, you can use its twin South Pole Stereographic. Better yet, you could make two maps and thus cover the entire celestial sphere.

If you want to know more about using earth’s coordinate systems to map the night sky, check out my other tutorial.

The projections I recommended above are only really intended for showing half of the earth at once. Anything beyond the equator is simply too distorted:
map of the world using the polar stereographic projection showing distortion in the northern hemisphere
That’s what the hemisphere layers are for in the provided data. Pick whichever one you need and symbolize it with a black fill color and no outline. Now your map only shows the reasonably undistorted half:
map of the world using the polar stereographic projection with the northern hemisphere masked out
Of course, you are not mapping the earth! But the celestial sphere behaves similarly to the terrestrial one, and grossly distorted constellations are just as annoying and useless as grossly distorted continents.

Place the mask layer above the other ones. Remove the basemap and turn on the Stars layer. You can ignore asterisms for now.
Contents pane showing Northern Hemisphere and Stars as only visible layers

SYMBOLIZE BY SIZE
Stars are typically symbolized based on their magnitude value. This is a measure of how bright or faint they appear from earth. A low magnitude value means a brighter star. We can easily accomplish this in ArcGIS Pro by setting Symbology to Graduated Symbols. Choose Visual Magnitude for the Field and a white circle for the symbol Template.
Symbology pane for stars set to graduated symbols with sizes ranging from 6 to 1 points
Note that you have to make the minimum size larger than the maximum. You can also adjust the class break values to round numbers if you want.
map of the stars using graduated symbols
That looks pretty good. You could stop here. But then you would be missing the entire point of this tutorial.

SYMBOLIZE BY TRANSPARENCY
On the Symbology Pane, click the green Vary symbology by attribute button:
Vary symbology by attribute on the Symbology pane
What is this all about?
Whenever you symbolize your data with anything more than a single symbol, you are using what cartographers call Visual Variables. Check out this nice chart that explains them all.

You’re already using the size visual variable to map the stars based on their magnitude. On the Vary symbology by attribute page you can add more visual variables to your layer. Usually you would do this because your data contains two different attributes and you want to represent both of them at once using different means. But in this situation, you’re going to do something a little different, and use the same attribute (magnitude) for two visual variables, in order to emphasize it.

The variables are size (already applied using graduated colors) and transparency. By adding transparency you can create a more naturalistic image of the night sky. It’s a trick to add some depth, so that the faint stars recede even more into the background.
Set the Transparency Field to Visual Magnitude an the Range between 90% and 0%.
Symbology pane with Transparency settings for the Stars layer
And here’s what it looks like:
map of the stars using both graduated symbols and transparency by attribute

SYMBOLIZE BY COLOR
But you can go farther than this.
Normally I do not recommend using more than two visual variables at a time. It can make for some very confusing maps! But since you are using size and transparency to depict the same attribute, I think that you are ok to add color to the mix.

The star layer has a field called Color Index. More specifically, this is the B-V Color Index. Hotter stars appear more blue in the sky, while cooler stars appear more orange. It’s actually much more complicated than that, but despite this, a number of people have valiantly attempted to assign specific colors to the stars. I borrowed some hex values from the work done by Mitchell Charity:
chart relating astronomy color index values to hex values
Source: What color are the stars?

And then I built a color scheme in Pro using those values:
picture of the color scheme editor in ArcGIS Pro
Don’t bother making this color scheme yourself, not unless you enjoy tedious tasks. The color scheme is included in the projected package that you already downloaded.

But if you ever do find yourself making a color scheme with 49 color stops, I recommend using the Evenly distrubute color stops option. If you look at the table above, you can see that the B-V values are evenly distributed, with a change of 0.05 between each row. Their corresponding color stops need to be just as evenly distributed if they are to match properly.

Back to Vary symbology by attribute. Expand the Color section and choose Color Index for the Field. Open the color scheme menu and choose StarColorIndex(-0.4-2)
The numbers shown on the histogram are based on the min and max Color Index values found in the data. But the stops in the color scheme are meant to be associated with very specific B-V values, and they won’t line up if the min and max are off. You need to edit these values on the histogram before the color ramp will align correctly with the data.
Just double click on the numbers to change them:
picture of the color histogram in ArcGIS Pro
Note that these values aren’t allowed to overlap one another, so they are picky about which order you edit them in. Any stars that fall outside of the -0.4 to 2 range will either draw with the first blue color or the last orange one.

There’s also a few Null values in the data. Three stars have no color index value at all, so they will be ignored by the color scheme, and draw with the fallback color. This is the white color defined in Graduated Symbols. You might be fine with this, but if you’re not, you can exclude them from the map.
Navigate to the Advanced symbol options page:
Advanced symbol options on the Symbology pane

And add a Data exclusion clause of Where Color Index is null.
Data exclusion clause on the Symbology pane: Where Color Index is null
Return to the Primary Symbology page.

Primary Symbology tab on the Symbology pane

and click the More button. Uncheck the Show excluded values option.
Uncheck Show excluded values in the More menu of the Primary Symbology pane
Now those three ambiguous stars will not draw on the map at all.

Let’s take a look at the final result:
map of the stars drawn with size, transparency and color
To finish things up, turn on the Asterism layer and give it some faint symbology that doesn’t distract too much from the stars. (Hint: look in the Symbology Gallery for a pre-cooked symbol.)
Finished map with stars and asterisms
Adding extra visual variables via the options in Vary symbology by attribute can make for some powerful storytelling with your data. If you use them wisely you can add more information to your map without needing to use any text. In this example, the changes are relatively subtle, but the result is a map that is easier to read – the brighter stars stand out more distinctly than before, helping you to find your bearings in the sky more quickly.

Try it yourself! Do you have some data that is just crying to be symbolized in two ways at once? Map it and share what you made.

About the author

Heather is a cartographer and artist who mixes both practices to express and understand landscapes. She works as a product engineer at Esri, where she writes and edits lessons for the Learn ArcGIS website. View more of her work at www.heathergabrielsmith.ca

Next Article

A classical visualization of earthquakes using ArcGIS API for JavaScript

Read this article