var canvas = document.getElementById("canvas"),
	context = canvas.getContext("2d"),
	w = canvas.width = window.innerWidth,
	h = canvas.height = window.innerHeight,
	
	hue = 217,
	stars = [],
	count = 0,
	maxStars = 1600;//星星数量
    
	var canvas2 = document.createElement("canvas"),
	context2 = canvas2.getContext("2d");
	canvas2.width = 100;
	canvas2.height = 100;
	var half = canvas2.width / 2,
	gradient2 = context2.createRadialGradient(half,half,0,half,half,half);
	gradient2.addColorStop(0.025,"#CCC");
	gradient2.addColorStop(0.1,"hsl("+hue+",61%,33%)");
	gradient2.addColorStop(0.25,"hsl("+hue+",64%,6%)");
	gradient2.addColorStop(1,"transparent");
	
	context2.fillStyle = gradient2;
	context2.beginPath();
	context2.arc(half,half,half,0,Math.PI*2);
	context2.fill();
	
	function random(min,max){
		if(arguments.length < 2) {
			max = min;
			min = 0;
		}
		if (min > max) {
			var hold = max;
			max = min;
			min = hold;
		}
		return Math.floor(Math.random()*(max-min+1))+min;
	}
	
	function maxOrbit(x, y){
		var max = Math.max(x,y),diameter = Math.round(Math.sqrt(max*max+max*max));
		return diameter/2;//星星移动范围，值越大范围越小，
	}
	
	var Star = function(){
		this.orbitRadius = random(maxOrbit(w,h));
		this.radius = random(60,this.orbitRadius)/8;//星星大小
		this.orbitX = w/2;
		this.orbitY = h/2;
		this.timePassed = random(0,maxStars);
		this.speed = random(this.orbitRadius)/50000;//星星移动速度
		this.alpha = random(2, 10)/10;
		count++;
		stars[count] = this;
	}
	
	Star.prototype.draw = function(){
		var x = Math.sin(this.timePassed)*this.orbitRadius+this.orbitX,
		y = Math.cos(this.timePassed)*this.orbitRadius+this.orbitY,
		twinkle = random(10);
		if(twinkle === 1 && this.alpha > 0){
			this.alpha -= 0.05;
		}else if(twinkle === 2 && this.alpha < 1){
			this.alpha += 0.05;
		}
	
		context.globalAlpha = this.alpha;
		context.drawImage(canvas2,x-this.radius/2,y-this.radius/2,this.radius,this.radius);
		this.timePassed += this.speed;
	}
	
	for (var i = 0; i < maxStars; i++) {
		new Star();
	}
	
	function animation() {
		context.globalCompositeOperation = "source-over";
		context.globalAlpha = 0.5; //尾巴
		context.fillStyle = "hsla("+hue+",64%,6%,2)";
		context.fillRect(0,0,w,h)
	
		context.globalCompositeOperation = "lighter";
		for(var i = 1, l = stars.length;i < l;i++) {
			stars[i].draw();
		};
		window.requestAnimationFrame(animation);
	}
	window.setTimeout(animation,400);