Constellation Map Generator

What this generator does

Scatters stars with a minimum separation between them and far more faint ones than bright ones, then picks out groups that sit near one another and joins each group into a tree grown from its brightest star. Where the joining lines cross one another the grouping is rejected and another is tried, and a figure that cannot be built at all is named as not drawn rather than printed empty.

How to use this tool

1. Choose how many stars fill the sky, and how many constellations to pick out of them.
2. Choose how many stars each figure is drawn from — bigger figures need a bigger sky to fit them all.
3. Draw the sky, then turn the joining lines and the names on or off to make a chart to learn from or one to be tested on.
4. Print it: the screen shows a dark sky and the page prints the other way round.

Understanding the controls

How many stars

The sky holds what will fit at the stated separation. Ask for more than that and it holds fewer rather than placing two stars on one dot.

How many constellations

Figures are drawn in turn from stars nothing else has claimed, so asking for more than the sky can feed leaves the last ones undrawn, and named as undrawn.

Stars in each figure

Three to nine. The lines are the shortest way to join the group, which is very nearly the way an eye joins them.

Join the figures with lines / Name the figures

Both off gives a plain sky; names off gives a sheet to be tested on. Neither changes the chart, only what is drawn of it.

Repeat word

The word this sky was built from. Anyone opening the same link sees the same stars in the same places.

Common use cases

• A night sky for an invented world, with figures a character could navigate by
• A printed star chart for a classroom, with the names hidden to be filled in
• A prop map for a tabletop campaign or a piece of fiction
• Practice at reading a chart, where the same word brings back the same sky
• A quiet decorative print, with the lines turned off

How this generator works

The check is separate from the thing that makes the chart. It walks the finished result rather than the intention behind it: every pair of stars measured against the separation, every figure's lines counted against its stars, every figure walked to see that nothing is stranded, every pair of a figure's lines tested for a crossing, and every star tested for a second owner. A shared endpoint is not a crossing — every joint in a figure is a shared endpoint, and counting those would reject every constellation there is.

Randomness and fairness

The sky is built from a repeat word drawn from your browser’s secure generator unless you type one of your own, so a link brings back the same stars. That makes the sequence behind it deterministic and not cryptographic.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

• The sky is invented. It is not the real sky from anywhere, at any date, and no star here corresponds to a real one.
• The figures are named in the register real constellations are named in, but the names are made up and mean nothing.
• The chart is flat. A real sky is a sphere, and no flat chart of one is undistorted.
• Magnitudes are four bands drawn for legibility on paper, not a physical brightness scale.

Privacy and your data

The chart is built on your device and nothing about it is transmitted or stored. The repeat word is a short label this page made or you typed, and it is the only thing that appears in the address; it is never measured.