CW = 420
CH = 330

myUniverse = {}
sinus = {}
cosinus = {}
littleStarsMaximumDistance = 300

function initUniverse(howManyStars, minSpeed, maxSpeed)

  for i=1,howManyStars do
    local newStar = {
                      position = { X=0, Y=0 },
                      angle = math.random(0,359),
                      speed = math.random()*(maxSpeed-minSpeed)+minSpeed,
                                                         -- BBGGRR
                      color = ((math.random() > 0.66) and 0xffff00 or
                               ((math.random() > 0.66) and 0xffa000 or 0xa0a000)),
                    }
    myUniverse[i] = newStar
  end

  -- caluculate sinus and cosinus for all angles (0-359). So we don't have slowdowns later (in animate function).
  for i=0,359 do
    sinus[i] = math.sin(i / 180 * 3.141592)
    cosinus[i] = math.cos(i / 180 * 3.141592)
  end

end

function animate(timer)
  -- clean scene
  canvas_fillRect(canvas, 0,0,CW,CH)

  -- move stars
  for i=1,#myUniverse do

    -- get star attributes
    local currentPos = myUniverse[i].position
    local angle      = myUniverse[i].angle
    local speed      = myUniverse[i].speed

    -- move star
    currentPos.X = currentPos.X + cosinus[angle] * speed
    currentPos.Y = currentPos.Y +   sinus[angle] * speed

    -- is star outside window? If yes, re-use it.
    if math.abs(currentPos.X) > CW/2 or math.abs(currentPos.Y) > CH/2 then

      currentPos.X = math.random(0,10)-5 -- move star to the center. We can move to (0,0).
      currentPos.Y = math.random(0,10)-5 -- But, I think using random is better (x=-5,5 y=-5,5)
      myUniverse[i].angle = math.random(0,359) -- change angle
    end

    -- draw your stuff
    local distance = math.sqrt(currentPos.X*currentPos.X+currentPos.Y*currentPos.Y)
    if distance > littleStarsMaximumDistance then
      pen_setColor(pen, myUniverse[i].color)
      canvas_ellipse(canvas, currentPos.X+CW/2-1, currentPos.Y+CH/2-1, -- draw bigger star
                             currentPos.X+CW/2+1, currentPos.Y+CH/2+1 )
    else
    canvas_setPixel(canvas, currentPos.X+CW/2   ,currentPos.Y+CH/2   , myUniverse[i].color) -- draw normal star
    end

    myUniverse[i].position = currentPos -- store star position
  end
end

if animateTimer~=nil then object_destroy(animateTimer); animateTimer = nil; end
animateTimer = createTimer(nil,false)
timer_setInterval(animateTimer,20)        -- 20 milliseconds
timer_onTimer(animateTimer,animate)

initUniverse(400,1,5)                     -- 400 stars, speed: minimum=1, maximum=5
timer_setEnabled(animateTimer,true)       -- start animation