Method 2: Blending a Layer Stack
If you don't want to use the Statistics application, you can place your star trail exposures in a Photoshop layer stack, select all the layers, and change the blending mode of each layer to blend the layers and create star trails. If you use the Lighten blending, the results of stacking your images will be identical to what you would get from using the Maximum stack mode in the Statistics application.
If you start with Adobe Bridge, you can load your star trail exposures by choosing Tools > Photoshop > Load Files into Photoshop Layers (see Figure 8).
Figure 8 You can load RAW files directly into Photoshop layers from Adobe Bridge.
Once the star trail files are loaded in a Photoshop layer stack (see Figure 9), you can multi-select the layers in the Layers panel; for example, by clicking on the top layer, holding down the Shift key, and then clicking on the bottom layer.
Figure 9 Loaded Photoshop layers.
Next, with the layers selected, use the Blending Mode drop-down list to choose the desired blending mode. I normally use Lighten to make the brightest possible star trails against the dark night sky (see Figure 10).
Figure 10 Changing the blending mode of all layers to Lighten.