Helps to create start trails from series of photos. It uses ImageMagick.

compose_star_trails.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'tmpdir'

@compose_method = 'LightenIntensity'
@processes_count = 4
@intermediate_image_format = "tga"

class Numeric
  def duration
    secs  = self.to_int
    mins  = secs / 60
    hours = mins / 60
    days  = hours / 24

    if days > 0
      "#{days} days and #{hours % 24} hours"
    elsif hours > 0
      "#{hours} hours and #{mins % 60} minutes"
    elsif mins > 0
      "#{mins} minutes and #{secs % 60} seconds"
    elsif secs >= 0
      "#{secs} seconds"
    end
  end
end

@start_time = Time.now

input_files=Dir.glob('*.jpg').sort.uniq
input_files_size = input_files.size


Dir.mktmpdir('startrails-tmp') do |tmp_dir| 


@processes_count.times do |n|
  `convert #{input_files[0]} -fill black -draw 'rectangle 0,0 8000,8000' #{tmp_dir}/out#{n}.#{@intermediate_image_format}`
end

puts

remain_str = ''

prev_string_size = 0

s=0
threads=[]
input_files.each_slice(@processes_count) do |files| 
  files.each_with_index do |file, n|
    threads[n] = Thread.new{`convert #{tmp_dir}/out#{n}.#{@intermediate_image_format} #{file} -compose #{@compose_method} -composite #{tmp_dir}/out#{n}.#{@intermediate_image_format}`}
  end
  files.each_with_index do |file, n|
    threads[n].join
  end
  s+=files.size
  if s > 0
    elapsed_time = (Time.now - @start_time).to_f
    time_per_one_file = elapsed_time / s.to_f
    remain_files = input_files.size - s 
    remain_time = remain_files*time_per_one_file
  end
  remain_str = "| #{remain_time.round.duration} remains" if remain_time > 0 && s > @processes_count
  print "\r#{' '*prev_string_size}" # clear prev status string in terminal
  out_str = "\r#{(100.0*s.to_f/input_files_size).round(2)}% (#{s}/#{input_files_size}) #{remain_str}" # status string
  print out_str
  prev_string_size = out_str.length
end
puts

puts "Writing final output file..."

`convert #{input_files[0]} -fill black -draw 'rectangle 0,0 8000,8000' #{tmp_dir}/out.#{@intermediate_image_format}`
@processes_count.times do |n|
  `convert #{tmp_dir}/out.#{@intermediate_image_format} #{tmp_dir}/out#{n}.#{@intermediate_image_format} -compose #{@compose_method} -composite #{tmp_dir}/out.#{@intermediate_image_format}`
end

`convert #{tmp_dir}/out.#{@intermediate_image_format} startrails_#{Time.now.strftime("%Y-%m-%d_%H-%M")}.png`  


end
puts "Done"
duration = Time.now - @start_time
puts "Duration: #{duration.duration} (#{duration.to_f.round(2)}s)"