Dying beautifully in a crowd

Contributed by
Jan 15, 2009
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The Hubble Space Telescope folks just released a spectacular and surprising picture:

Hubble picture of the planetary nebula NGC 2818. Click to way embiggen.
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

First off, whoa. It's gorgeous. What you're looking at is a planetary nebula (it doesn't have much to do with planets; these objects were named thus because they looked like planets through small telescopes). It was a star something like the Sun that reached the end of its life and blew off a strong wind of gas. Eventually, as more material left the star, deeper layers of the star got exposed. Eventually, the core was all that was left: a hot, small, dense object called a white dwarf. It flooded the nebula with UV light, ionizing the gas and lighting it up. The complex interaction of the gas and the radiation produced the shape and the different colors.

But NGC 2818, as it's called, is an oddball. When I first saw this picture I didn't even think it was a planetary nebula, I thought it was a much larger gas cloud that forms stars. The shape is not much like other planetaries! Usually they are round, or hourglass shaped. This one is squashed and weird. The colors are pretty much normal: the outer parts are loaded with nitrogen and are reddish, while the inner region is hotter, less dense (because late in the game the wind from the star got hotter and less dense), and glows blue due to oxygen. The fingers or towers pointing toward the center are due to the light and wind slamming into denser blobs of material. They're a bit like sandbars that form in a current.

In fact, NGC 2818 does appear to be a bit different. I think the star that formed it (which should be right smack in the middle, but I don't see much there; it might be hidden by one of the fingers) was more massive than the Sun. The wind speed is higher, indicative of a more massive star. The nebula itself is much larger than other planetaries; they are usually a light year or so across, and this one is well over three. The amount of different elements in the cloud also seem to say that this was a bigger and hotter star than usual, too.

What makes this guy most unusual, though, is that it appears to be inside an open cluster, a loose aggregation of stars about 10,000 light years from Earth. Most planetaries are loners, since they come from old, dying stars. The Sun has a ten billion year lifespan, enough time that were it born in a cluster it would have long ago drifted away. But NGC 2818 is located in a cluster, and the safe assumption is that this was where it was born. That means it must be from a relatively young star (or else it would have left the cluster)... and massive stars age faster and die younger than low mass stars.

So I think the star that formed this beautiful and intricate web of gas was a big one, maybe even close to the limit between where stars die this way, and explode as spectacular supernovae. I had never heard of this particular object before, and I'm glad astronomers got this image so that it can be studied more carefully. You can learn a lot looking at things that are up against the edge of two different behaviors, and investigating stars like this give us a lot of insight into what happens at that limit between going gentle into that good night, and raging against the the dying of the light.