Stack layers and increment opacity to create a fading star trail effect.

Star Trails.jsx

// Create a file in the path "Adobe Photoshop 2022/Presets/Scripts/Star Trails.jsx" and paste the contents there to use
// Star Trails, author Mika Kuitunen

// DEBUGGING STUFF

function ConsoleLog() {
  // Seeing logs: macOS: nc -lvk 8000, Win: install Ncat, then ncat -lk 8000
  this.socket = new Socket();
  this.hostPort = "127.0.0.1:8000";
}

ConsoleLog.prototype.log = function(logMessage){
  if (!debug) return;
  
  if (this.socket.open(this.hostPort)){
    this.socket.write (logMessage + "\n");
    this.socket.close();
  }
};

var debug = false;
var console = new ConsoleLog();

// on localized builds we pull the $$$/Strings from a .dat file
$.localize = true;

// Put header files in a "Stack Scripts Only" folder.  The "...Only" tells
// PS not to place it in the menu.  For that reason, we do -not- localize that
// portion of the folder name.
var g_StackScriptFolderPath = app.path + "/"+ localize("$$$/ScriptingSupport/InstalledScripts=Presets/Scripts") + "/"
										+ localize("$$$/private/LoadStack/StackScriptOnly=Stack Scripts Only/");

$.evalFile(g_StackScriptFolderPath + "LatteUI.jsx");

$.evalFile(g_StackScriptFolderPath + "StackSupport.jsx");

$.evalFile(g_StackScriptFolderPath + "CreateImageStack.jsx");

/************************************************************/
// loadLayers routines

loadLayers = new ImageStackCreator(localize("$$$/AdobePlugin/Shared/LoadStack/Process/Name=Load Layers"),
										  localize('$$$/AdobePlugin/Shared/LoadStack/Auto/untitled=Untitled'));

// LoadLayers is less restrictive than MergeToHDR
loadLayers.mustBeSameSize			= false;	// Images' height & width don't need to match
loadLayers.mustBeUnmodifiedRaw		= false;	// Exposure adjustements in Camera raw are allowed
loadLayers.mustNotBe32Bit			= false;	// 32 bit images
loadLayers.createSmartObject		= false;	// If true, option to create smart object is checked.
loadLayers.mustNotBeSmartObj		= false;

loadLayers.stackLayers = function()
{
	var result, i, stackDoc = null;
	
	stackDoc = this.loadStackLayers();
	if (! stackDoc)
		return;
	
	// Nuke the "destination" layer that got created (M2HDR holdover)
	stackDoc.layers[this.pluginName].remove();
	
  var layers = stackDoc.layers;
  var max = layers.length;
  var k = 0;
  for (var j = layers.length - 1; j >= 0; j--) {
    layers[j].blendMode = BlendMode.LIGHTEN;
    layers[j].opacity = (100.0 / (k + 1)) * (k / max + 1);
    k++;
  }
}

// "Main" execution of Merge to HDR
loadLayers.doInteractiveLoad = function ()
{
	this.getFilesFromBridgeOrDialog(localize("$$$/private/LoadStack/LoadLayersexv=LoadLayers.exv"));

	if (this.stackElements)
		this.stackLayers();
	else
		return 'cancel';
}

loadLayers.intoStack = function(filelist, alignFlag)
{
	if (typeof(alignFlag) == 'boolean')
		loadLayers.useAlignment = alignFlag;
		
	if (filelist.length < 2)
	{
		alert(localize("$$$/AdobeScripts/Shared/LoadLayers/AtLeast2=At least two files must be selected to create a stack."), this.pluginName, true);
		return;
	}
	var j;
	this.stackElements = new Array();
	for (j in filelist)
	{
		var f = filelist[j];
		this.stackElements.push(new StackElement((typeof(f) == 'string') ? File(f) : f));
	}
		
	if (this.stackElements.length > 1)
		this.stackLayers();
}

if (typeof(loadLayersFromScript) == 'undefined')
	loadLayers.doInteractiveLoad();