# 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.

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’re mapping the night sky, so make the background black. You can do this via Map Properties:

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.

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:

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. You can ignore the asterisms layer for now.

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 using Graduated Symbols.

Note that you have to make the minimum size larger than the maximum.

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 this funny little button:

And choose Vary symbology by attribute.

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.
Here’s how you can set it up:

And here’s what it looks like:

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, which I do not understand well enough to describe to you here, but the gist of it is that 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:

Source: What color are the stars?

And then I built a color scheme in Pro using those values:

Don’t bother making this color scheme yourself, not unless you enjoy tedious tasks. You can find the color scheme ready to go included with the download listed above.

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 apply the field and color scheme.
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:

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.

Press the Back arrow button again to return to the main Symbology page, and press the More button. Uncheck the Show excluded values option.

Now those three ambiguous stars will not draw on the map at all.

Let’s take a look at the final result:

To finish things up, turn on the Asterism layer and give it some faint symbology that doesn’t distract too much from the stars:

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 in the comments below.

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