How to Stack Star Trails in GIMP

Creating continuous star trail photographs involves combining a sequence of individual long-exposure images into a single composite. This guide demonstrates how to use GIMP, a free and open-source image editor, to import your night sky sequence as separate layers, apply the Lighten Only blend mode to reveal the trajectory of the stars, and export the finished high-resolution star trail image.

1. Import Your Photos as Layers

  1. Launch GIMP.
  2. Go to the top menu and select File > Open as Layers... (or press Ctrl + Alt + O on Windows/Linux, Cmd + Option + O on macOS).
  3. Navigate to the folder containing your sequential star exposures.
  4. Select all the images you want to stack (click the first file, hold Shift, and click the last file).
  5. Click Open. Depending on the number of photos and file sizes, GIMP may take several moments to load each photo onto its own layer.

2. Verify Layer Order and Alignment

For clean star trails, the camera must not have moved during the shooting session.

  • Check the Layers panel (usually on the right side of the workspace) to ensure all images loaded properly.
  • The base layer (the bottom-most layer) will serve as your primary foreground.
  • Keep the images in chronological order to maintain consistency, though the Lighten blend mode mathematically operates independently of layer order.

3. Change Layer Blend Modes to Lighten Only

The Lighten Only blend mode compares pixels between layers and keeps only the brightest values. Since stars move across a dark background, this mode retains the bright star positions from every single frame while keeping the dark sky and static foreground stable.

  1. Select the layer immediately above the bottom base layer.
  2. In the Layers panel, locate the Mode dropdown menu (set to "Normal" by default).
  3. Change the Mode to Lighten Only.
  4. Repeat this process for every layer above the base layer.

Tip: While GIMP does not natively allow multi-layer batch blend mode changes in older versions, modern versions allow you to group layers or you can quickly click each layer and press the shortcut or dropdown to select Lighten Only. Leave the bottom base layer set to Normal.

4. Clean Up and Masking (Optional)

If airplanes, satellites, or passing car lights created unwanted streaks across certain frames:

  1. Identify the specific layer containing the streak by toggling layer visibility (the eye icon).
  2. Add a layer mask to that layer (Layer > Mask > Add Layer Mask... and choose White (full opacity)).
  3. Select a soft black brush and paint over the unwanted light trail to hide it from the stack without removing the entire frame.

5. Flatten and Export the Image

  1. Once all trails are visible and gaps are bridged, navigate to Image > Flatten Image to combine all active layers into one.
  2. Fine-tune your result using Colors > Curves or Colors > Levels to adjust contrast, deepen the dark sky, and brighten the trails.
  3. Save your master edit by going to File > Save As... (.XCF format).
  4. Export the final photograph by going to File > Export As..., selecting your preferred format (such as JPEG or TIFF), and clicking Export.