Category Archives: planetary nebulae

Where Star Stuff Comes From

Bounty from a Planetary Nebula

The planetary nebula NGC 2818, courtesy Hubble Space Telescope.
The planetary nebula NGC 2818, courtesy Hubble Space Telescope.

I love me some planetary nebulae.

They provide many of the most evocative images in astronomy. This new image from the Hubble Space Telescope folks shows NGC 2818, a planetary nebula that lies about 10,000 light-years away from us and is nestled inside the open star cluster NGC 2818A.

It’s pretty rare to find such planetary nebulae inside open clusters because most clusters are pretty loosely bound and they tend to disperse (spread out over space) in the course of a few hundred million years. A star that forms a planetary is usually billions of years old, and so you wouldn’t expect to find one as part of an open cluster. Yet, here’s this cluster — which itself is about a billion years old (pretty ancient for an open cluster) — sporting a planetary nebula.  Pretty slick. And, astronomers will eventually figure out just how this is possible.

So, planetaries are made up of stars that are aging, or if you want to get poetic about it, thrashing about amid their death throes.  What’s a stellar death throe?  For stars like the Sun (which are the most likely to become planetary nebulae) death throes begin when the star begins losing more mass than usual as it gets older. That stuff blows off the star as a stellar wind, and it accumulates in a shell of gas and dust.  The star at the center continues to age by expanding and contracting and heating up the surrounding clouds of its former self. They glow in a kind of ghostly tribute to the slowly contracting white dwarf that remains behind.

Eventually that glowing cloud expands out to space and becomes part of the interstellar mix of “stuff” from other planetary nebulae and supernova explosions that will become — some millions or billions of years from now — the seeds of new stars, planets and — dare we say it? — life. Take a good look at NGC 2818 up there — some of the stuff in your body may well have come from a star that went through exactly what this one did more than 5 billion years ago.

Showing off What Hubble Does Best

HST on orbit
HST on orbit

I just finished work on a planetarium show about Hubble Space Telescope discoveries. I’ve written other shows about HST before, and this is sort of the “latest and greatest” one, and one where I really don’t know the ending. We’ve all been talking about the last HST servicing mission being cancelled, thus sentencing HST to its fate a few years earlier than everybody expected. Now it appears that Congress really does have the last say about this, and several folks have called for a re-investigation of the decision. So, the story’s not over yet. And, up there in orbit around Earth, HST continues on its merry way, sending back great images and science data (not mutually exclusive) for all of us to study and enjoy.

Well, rather than focus on the political aspects of HST’s “human side,” I spend all my time in this planetarium show talking about the great science it has done. It’s not an easy task. There’s a LOT to talk about, and a lot more to come. In fact, the most difficult thing about an HST planetarium show is choosing what NOT to show. There’s only so much time in the program, and in most planetaria, there are only so many slides one can cycle through in the course of a show. Sure we can throw in some video, for those who HAVE video projection capability, but for those who don’t, we’re kind of limited by the slides. I’ve chosen nearly 200 really great images and told a story of cosmic exploration using them as illustration. As I spend time looking at the sights that HST has seen for us, I’m impressed again with just how marvelous this machine has been. And what a wonderful time the astronomers who use it must be having when they open their data sets. Are they like kids opening presents? I like to think so. Or at least HOPE so.

The Eskimo Nebula (planetary nebula)
The Eskimo Nebula (planetary nebula)

One of the images I’ll be using in the show is a study of a planetary nebula that lies about 5,000 light-years away from Earth. It’s called “The Eskimo” Nebula because it looks like an intricate furry hood that an Eskimo might wear. The “parka” is really a disk of material surrounding a dying, Sun-like star. Inside the cloud is a ring of comet-shaped objects, with their tails streaming away from the central, dying star. The “face” consists of a bubble of material being blown into space by the central star’s intense “wind” of high-speed material. The story behind this apparition is fascinating. The star that formed this cloud began to lost much of its mass to space about 10,000 years ago. Before that time it had gone through what’s called the “red giant” phase, breathing out a ring of dense material that collected around the star. That ring is actually moving out from the star at about 115,000 kilometers per hour. Hot on its heels (so to speak) are high-velocity stellar winds, moving out from the star at 1.5 million kilometer per hour. They are shoving material above and below the star, creating elongated bubbles. Each bubble is about one light-year long and about half a light-year wide.

This is just one of a dozen or so planetary nebulae I’m presenting in my show, and while I can’t talk about them in excruciating detail, I can at least show people just what our Sun might look like in 5 or 6 billion years when it starts down the path toward planetary nebula-hood. Fun stuff!