Open Star Cluster M46 and Planetary Nebula NGC 2438. M46 is a rich open star cluster in the constellation Puppis. The stars in this cluster probably number more than 500. It is about 5400 light years from us. The tiny planetary nebula NGC 2438 is just left of center in this image and looks like a small doughnut. It's blue central star (16 th magnitude) is one of the hottest stars known with a temperature of about 75,000 K (not seen in this image).