For this week's APOD, I chose the picture called Planetary Nebula NGC 2438, uploaded on April 7, 2011. The image was taken from the Teide Observatory in Canary Islands, in Spain. In the photo, there is a halo-like ring of flowing gas surrounding the nebula, that spans about 4.5 light years long. Other various photos have shown a halo around a planetary nebula quite like this one as well, and they are made during the early active phases of aging central stars. In other words, planetary nebulae are a short but glorious final stage in stellar evolution, in which the gaseous shrouds are ionized by a hot central source while the core shrinks as the star runs out of energy for nuclear fusion.
This particular planetary nebula lies in the constellation Puppis, or the Stern, which is about 3,000 light years away and in the giant constellation of Argo. NGC 2438 has a visual brightness of 10.8 magnitude, and was discovered by William Herschel in 1786. It also appears to lie on the edge of a luminous, young, open, star cluster: M46. NGC 2438 is not much older than M46, but it does move at a different pace than the cluster which makes them differ. From estimations, NGC 2438 appears closer than M46, so the planetary nebula is the one that appears in the foreground in comparison to the young star cluster.
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