August 2000

The Ghosts of Stars

One type of object that we always try to show in the Visitor Center Telescope is a planetary nebula. If you've never heard the term before, take a guess at what it might mean.

A nebula which is about to form a planet? A nebula made up of planets? I give up.

A planetary nebula is a reminder that nothing lasts forever.

They are dead stars. When a normal star comes to the end of its life, it swells up into a red giant, and the outer layers of gas are kicked off in a series of "puffs." These layers expand out, too far to be deterred by gravity, in ever-widening hollow spheres. The core of the star collapses, and becomes tiny and hot. At this stage, it is called a white dwarf, and it continues to shine for a little while (about 100,000 years), and the light it emits scatters off the expanding gas layers. When we look at the whole thing from Earth, we see a cloudy, luminous ball.

My favorite analogy for a planetary nebula is a Chinese lantern. A Chinese lantern is a small light bulb, or candle flame, surrounded by a shell of fragile, dyed paper. The light from the bulb scatters off the paper, and makes the paper glow. A planetary nebula works the same way. The collapsed core is the tiny bulb inside, and the ejected gas is the colored paper.

So where do planets come in?

Nowhere.

Here is the story: William Herschel conducted a very thorough study of these mysterious objects some time back, and he thought they looked like planets when he viewed them in his telescope. Specifically, he thought they looked like Uranus, since they usually glow blue. Bill Herschel was the guy who discovered Uranus. You can just imagine how discovering a planet could go to your head. (I bet all his friends called him "The Planet Guy.")

So Bill started calling these round blue clouds "planetary nebulae," and it stuck, thus cursing astronomy educators to explain the odd name for centuries to come.

Two notes. First, you've probably heard of supernovae, or exploding stars. Only the most massive stars in the universe do this; maybe one in a thousand. The other 999 don't blow up. They make planetary nebulae, which is more of a "puffing off" rather than "exploding."

Second, astronomers are way too fancy to use an "s" to indicate the plural. They use an "ae" for certain words; I bet you noticed. For example, one nebula, many nebulae. One supernova, many supernovae. That last syllable is a long "e," as in "supernoveeee." Really. Don't say "supernovay," or you'll sound like a goof.

What was that about planetary nebulae (eeee...) being reminders that nothing lasts forever? If you know some constellations, then you know the satisfaction and sense of peace that comes from seeing that old familiar, unchanging sky. Those stars will hold their places for your whole life, unfaltering, everconstant. You'll be able to teach those constellations to your grandchildren.

But they won't last forever. We know the constellations shift as stars orbit the galactic core, but planetary nebulae tell us that even the stars die.

And, of course, the Sun is a star. It will make a planetary nebula too. It will swell, and redden, and gently heave its outer parts into space. As those layers of thin, hot gas drift though the solar system, they will bake the planets, and possibly cause their orbits to decay. Whatever the status of humanity, our home solar system will cease to exist.

So when I admire the Ring Nebula, or the Cat's Eye Nebula, or look across the light years at any planetary nebula, I feel a twinge of mortality. Everything changes. Out with the old... and in with the new.

PPS: Adam, who took Classical Latin, informs me that the proper pronunciation of "ae" is actually like "I," or "eye." The long "a" pronounciation, ay, is "ecclesiastical." Adam says "NOWHERE is it pronounced as 'ee' as in 'bee'." Okay. But who's going to tell the astronomers that? -S

Steve White
Nightly Observing Program
Kitt Peak Visitor Center

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Telescope Operators/Guides for Nightly Program:
Adam Block (ablock@noao.edu) Lead Observer
Steve White (swhite@noao.edu)
Flynn Haase (fhaase@noao.edu)
Ethan Gargiulo (egargiulo@noao.edu)
Richard Barchfield (richardb@noao.edu)

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Updated: 08/01/2000