Planetary Nebulas in Star Clusters

In January 2009, scientists announced that they had discovered a new planetary nebula using the Hubble Space Telescope. The new NGC 2818 (pending a more imaginative name) showed a colourful mix of red, green and blue gas clouds forming in the Pyxis (Compass) constellation, thousands of light-years away from the Earth. NGC 2818 was especially exciting because it was found in an open star cluster – an unusual place for such nebulae.


A planetary nebula is a large cloud of gas which has been thrown out into space by a dying star. Despite the name, they actually have nothing in common with planets; the term comes from astronomers using early telescopes, who noticed that through their primitive devices the nebulae seemed to resemble the gas giants of our solar system, like Jupiter and Saturn. Most planetary nebulae are very short-lived, with life-spans of less than a million years; however, many of them have been located, especially through the assistance of the Hubble telescope. Our own sun, after it passes through its red giant phase in several billion years, will probably eject enough gas to form a planetary nebula.

Planetary nebulae form some of the most strikingly beautiful images we draw from outer space. Famous nebulae, usually those imaged by Hubble, include the Horsehead Nebula and the Eagle Nebula.


What made NGC 2818 unusual among planetary nebulae, however, was its location: inside an open star cluster. Open star clusters are large groups of stars (often as many as a thousand of them), which formed from the same molecular cloud at roughly the same time, and are loosely held together in a group by their overlapping gravitational forces.

NGC 2818, however, doesn’t fit what scientists expect to find in such star clusters. According to the current theories of open star clusters, the first generation of hot blue stars explode into supernovas within the first few tens of millions of years of the life of the cluster. This blows away most of the free-floating gas in the cluster, so that new stars are unable to form – hence, the reason why stars in a cluster are usually of the same age. The survivors of this first generation are the longer-lived yellow stars, but their gravitational field is weak enough that the open cluster usually flies apart – if not of its own accord, then when it passes by some other large gravity source, like a molecular cloud or another star cluster, and is disrupted by an altered gravity field.

A planetary nebula like NGC 2818, however, could only have formed when a yellow star died. Larger, hotter blue stars explode as supernova: They do not form planetary nebulas. This means that NGC 2818’s open star cluster is uncharacteristically old, probably several billion years old. Scientists did not expect an open star cluster to survive for that long.