Contest Entry: Planetary Nebulae

[This entry is from Brian Ventrudo, Thanks Brian!]

If you have a small telescope, you may have turned it toward M57, the famous Ring Nebula in the constellation Lyra. This beautiful object looks like a ghostly smoke-ring set against a dazzling backdrop of summer stars. It’s perhaps the best known example of a planetary nebula, a glowing shell of hot ionized gas blown into space by a dying star.

While M57 is impressive, many planetary nebulae appear dim and uninspiring in small scopes. But they’re worth contemplating because they give you a glimpse into our Sun’s future and our own origins in the innards of some distant and long forgotten star.

The name “planetary nebula” came from William Herschel who suggested these disk-like nebulae looked like planets, especially the blue-green disk of Uranus. Herschel should know. He discovered Uranus in 1781.

Of course, Herschel understood these objects weren’t planets at all because they didn’t move against the background stars from month to month. But he had no idea what they were. Not until the early 20th century was the mystery solved. While working out the details of how stars evolve, astronomers discovered that planetary nebulae are the final gasp of life of modest-sized stars like our Sun.

The basic idea behind planetary nebulae isn’t hard to understand. Here are the basics…

In their youth and middle age, stars generate energy from nuclear fusion, the transmutation of light elements like hydrogen into heavier elements like helium. Fusion happens mostly in the core of a star where the temperature is highest, about 15 million degrees. Fusion of hydrogen to helium occurs over tens of millions to billions of years depending on the mass of the star. The more massive the star, the faster it burns its fuel.

As the star ages, its core starts to run out of hydrogen. Fusion slows and the core contracts, driving the central temperature higher, up to 100 million degrees. At this point helium starts fusing into carbon and oxygen, releasing more energy and stopping the core’s contraction. But the core is so hot that it begins pushing out the star’s outer layers. The star balloons in size by a hundred times or more. At this point, the star has become a cool and luminous red giant, swallowing up any of its nearby planets in the process.

Now here’s where it gets interesting.

After a time, the helium runs out and the core contracts again. But in small to mid-sized stars, the core doesn’t get hot enough to burn the carbon and oxygen, so fusion in the core stops. But a thin shell of helium around the core continues to burn for a short time at an unstable rate, and the star begins to pulsate, driving its outer layers a light-year or so into interstellar space where they escape forever. We see this glowing shell of ejected gas—heated and ionized by the star’s scorching-hot core—as a planetary nebula.

Within 50,000 years or so, a short time in the life of a star, the gas ejection stops and the planetary nebula fades from view. What’s left of the star settles down into a white dwarf, a dense carbon-rich ember that glows like a lump of charcoal from its own residual heat for billions of years more.

When a star sheds mass as a planetary nebula, it ejects trace amounts of heavier elements like carbon, nitrogen, and oxygen into space. Some of these atoms may coalesce over time into dense clouds that form new stars and planets. In a way, this is how the galaxy recycles itself… a giant built-in “blue box” in interstellar space.

This means that some atoms of the lighter elements in your body may have been shed by a planetary nebula billions of years ago. Most of your heavier elements (iron, calcium, magnesium) were likely produced in a supernova explosion, not in a planetary nebula. But that’s a story for another day.

Like most stars in our galaxy, our Sun will become a planetary nebula after it gobbles up Mercury and singes Venus and the Earth, boiling away our oceans and scorching our biosphere. It’s not something to immediately worry about… the Sun’s got another 5 billion years before it runs out of hydrogen and things get truly nasty.

Most of the 1,500 known planetaries in our galaxy are dim and unimpressive in backyard telescopes. But some present a wonderful sight, and with a little practice you’ll come to enjoy seeking out their subtle and intricate detail.

One famous planetary, the Eskimo nebula (NGC 2392), looks like a face shrouded by a hooded parka.

Another looks like a cat’s eye (NGC 6543).

Still another (NGC 6826) appears to blink off and on as you alternate between direct and averted vision.

There are dozens more planetaries worth examining in an inexpensive 4″ to 6″ telescope. Of course more aperture and higher magnification almost always helps.

To get the best view, use a special “OIII” (oh-three) filter that passes the blue-green light from the ionized oxygen in the nebulae and blocks light pollution scattered in the surrounding sky. You’ll see an amazing improvement in contrast when you look at planetary nebulae with these filters. Well worth the investment of a hundred dollars or so.

But the best part of observing planetary nebulae—like most astronomical objects—lies not in what you see, but in what you imagine. When I look at a planetary, I can’t help but wonder what stories have played out in the center of the smoky green disk in my telescope’s field of view. Were there intelligent beings on planets around the central star? How did they face the end of their world? Did they have an advanced civilization and enough technical skill to escape their fate?

And when our sun swells and scorches the surface of our fair planet, how will the inhabitants of the Earth—whoever or whatever they may be—face the certain destruction of our planet?

Things to think about in the dark of night as you peer through your eyepiece. Which is why astronomy, when you use your imagination, will never, ever get dull.