Tom Burns: The Clown Face Nebula


Tom Burns - Stargazing



Yes, it’s winter and when no moon brightens the sky, the dark surrounds you like a tattered blanket. It’s easy enough to stay indoors and warm yourself by the glow of your screen, whatever screen it happens to be.

It is even more difficult to drag a telescope out into the cold and dark. That’s, of course, why we have places like Perkins Observatory. We’re glad to do the dragging and pointing for you.

I got my first real taste of serious telescoping during the dead of winter, so I know all of its pains and its glories.

The only sound is the wind whistling past your ears and the faint squeak of telescopes slowly moving. And it’s cold. The stiff breeze feels like a dull razor scraping against your skin. The metal of your telescope numbs your fingers. As you stare through the eyepiece, you can feel the frozen glass sucking the heat from your eye.

“No wonder they call it the ‘dead of night,’ ” you whisper. You shouldn’t have spoken. Your breath has left a thick coat of frost on the eyepiece lens.

“There has got to be a better hobby,” you think, “preferably one that can be done in front of a roaring fire with a glass of hot cider.”

You need to cheer up, and fast. So you take a look at an object that is guaranteed to bring a smile to your chapped lips.

It’s the Clown Face Nebula, the dimly glowing remnant of a dying star.

To find it, first go out and get a decent set of star maps and about $400 worth of telescope. Then look for Gemini, riding almost overhead in the evening sky. Look for two bright stars called Castor and Pollux, sometimes called the “heavenly twins” because they are about equal brightness.

Starting from Pollux, the left-hand star, sweep down and to the right to Delta, marked with a “d” on most star maps. It is about midway down the constellation. The Clown Face, marked as NGC 2392, is about half a binocular field down and to the right from Delta.

You are looking at a “planetary nebula,” so called because some 19th century astronomers believed they were distant planets in our own solar system. It’s easy to see why.

In a small telescope, the Clown Face looks like a tiny dish about the size of Jupiter. Because it glows a brilliant blue-green, it is visually similar to distant planets like Uranus and Neptune.

At the center of the Clown Face, like some cosmic bull’s-eye, is a very old star surrounded by a globe of gas, or nebula.

The Clown Face is much farther away than the planets. At 18,000 trillion miles away, it is 6 million times the distance of Neptune, the most distant planet.

In a small telescope, it is one of the best examples of an effect called “averted vision.” The screen on the back of your eye, your retina, is more sensitive at the edges than the center. If you stare right at the central star, the faint light of the nebulosity can’t be seen. Only the bright star remains.

If you look to the side of the object, the sensitive portion of the retina reveals the faint light of the nebulosity, and the star disappears, lost in the wash of nebulous light.

The central star of the Clown Face is an old dwarf that has reached the end of its life. It first swelled to enormous size as its supply of hydrogen fuel became depleted. It then collapsed to a few thousand miles in diameter and got hot. At 40,000 degrees at its surface, it is now six times the temperature of our sun. The high temperature is the result of smashing down a giant star to relatively tiny size.

The combination of heat and density in a star the size of the Clown Face is inherently unstable. A larger star might explode from the pressure. A smaller star like our sun would simply expire without much fuss.

This particular star could not abide having so much stuff condensed to such a small volume. So about 1,700 years ago, it sent part of its substance into space at a tremendous velocity — 250,000 miles per hour.

And it continues to expand at that rate. It is already 3.6 trillion miles in diameter.

As it continues to grow, its substance gets spread out very thinly. It is now, in fact, almost a perfect vacuum. That we can see it at all is a tribute to the dying star. It causes its shell of gas to fluoresce like a spark of electricity causes the gas in a fluorescent bulb to glow.

Eventually, the gas of the nebula will disappear, lost in the vast emptiness of space. It is as if the star recognizes its end and tries bravely to create a tribute to itself. As L.H. Aller wrote, these nebulae are “wreaths placed by Nature around dying stars.”

The Clown Face has captured the imagination of stargazers since its discovery by William Herschel in 1787. In his Celestial Handbook, Robert Burnham writes that the nebula resembles comedian W.C. Fields.

Most observers think the Clown Face looks more like its other nickname, the “Eskimo,” which makes it particularly appropriate for cold-weather observing. In a large amateur telescope, the central star glows like a nose at the center of the nebulosity. A ring of dark gas and dust about halfway out makes the face look like it is surrounded by a furry parka, the ghostly headgear of a dying star.

I hope that one day you will also be haunted by its shimmering, sapphire splendor.

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Tom Burns

Stargazing

Tom Burns is director of the Perkins Observatory in Delaware.

Tom Burns is director of the Perkins Observatory in Delaware.