Encountering the ephemeral unicorn


To the ancients, the constellations were collections of bright stars associated with figures out of their mythologies. They cared little for the patches of faint stars that filled in the blank spaces between their villains and heroes.

Even when the stars became useful for navigation, only a few bright stars were necessary for that purpose.

A startling advance in technology made the whole sky interesting. In the early part of the 17th century, along came the telescope. All of a sudden, the almost empty patches were packed with stars and other objects of scientific interest.

Slowly, the constellations evolved from stick figures to patches of sky, and those formerly “empty” patches got names so that the entire sky was covered.

Such was the case with Monoceros, the Unicorn. It didn’t get its name until Dutch celestial cartographer Petrus Plancius came along in 1613, just a few years after Galileo published his first telescopic observations in 1610. A revolution was happening, and the Unicorn was one of its first results.

Like the constellation’s mythological namesake, the Unicorn’s stars are just as dim and ephemeral as they ever were. With the growth of light pollution around cities, the poor unicorn is effectively invisible to urban and suburban stargazers. Even diehard astronerds rarely look in its direction.

However, once every year, it behooves even the most casual of stargazers to seek out a dark rural sky and lie out in the cold and the dark to stare for an hour or two in the Unicorn’s direction.

Late on Nov. 21, Earth will pass through the debris trail of an unnamed and unknown comet. As Earth plows at seven miles per second through the cloud of cosmic detritus, its tiny particles will leave streaks of light called meteors. Each meteor lasts for only the blink of an eye, and many of them will be too faint to be seen under the glow of city lights.

The Alpha Monocerotids meteor shower, as it is called, makes up for the faintness of its meteors by their sheer quantity. The resulting meteor shower will probably result in 100 or more meteors during its peak hour. Careful observers have reported as many as 400 during years when the Earth passes through the thickest part of the debris trail.

Another strange quality of the Monocerotids is the small diameter of the debris path and the resulting brevity of its peak period. Some meteor showers last for days or even weeks. The peak of the Monocerotids invariably lasts less than an hour.

As a result, if the sky is clear, you should be out and looking toward the eastern sky by 11 p.m. EST or so. Meteor mavens predict the peak to happen at around 11:50 p.m.

Monoceros is nestled between the constellations Canis Major and Orion. Therefore, you should look in that general direction, i.e., low in the southeast and to the left of Orion. However, try to use as much of your peripheral vision as possible. Meteors might appear all over the sky.

The shower gets its name because Earth is zipping along in its orbit in the general direction of the Unicorn’s brightest star, which astronomers have named “Alpha.” Thus, its meteoric streaks will seem to radiate from a spot near that star.

Since meteor-shower meteors tend to appear all over the sky, you might have trouble distinguishing them from meteors that result from stray hunks of cosmic debris, called sporadic meteors, that happen to get caught by Earth’s motion around the sun.

Draw an imaginary line in the opposite direction of the meteor’s motion. All the Monocerotids’ lines will seem to intersect at the same point to the left of Orion.

Despite the dimness of the Unicorn’s stars, good reasons exist to give Monoceros a little attention during the winter months if you happen to own a telescope.

First, there’s the Rosette nebula, just below the nose of the unicorn and marked as “2237” on the accompanying star map.

It is one of the largest of the so-called “emission nebulae,” with a quantity of hydrogen gas and dust more than 11,000 times greater than our sun’s.

At a distance of about 5,200 light-years from Earth, it is about 130 light-years in diameter (a light-year is about six trillion miles). A beam of light would take 130 years just to cross it.

The Rosette consists of a cluster of stars surrounded by a faint glow in the shape of a Christmas wreath.

The star cluster is easily visible in binoculars as two parallel rows of three stars. A small telescope will reveal a dozen stars or so.

The glowing gas is more difficult to see. I saw it faintly in binoculars when I was observing from the dark skies of Arizona.

Around cities like Columbus, you’ll need at least a medium-sized telescope equipped with a “nebular filter,” which blocks out some of the “skyglow” caused by outside lighting and lets through the light from the nebula.

The Rosette is a stellar nursery, a place where stars are being born. Some of its gases have condensed into stars, which accounts for the cluster of young stars at its center.

Scattered through the Rosette are small areas of dark gas and dust, visible only on long-exposure photographs. They are called Bok globules after the American astronomer Bart Bok. He first recognized them as “protostars,” stars in the earliest stage of their formation.

The gases in the Rosette Nebula started out as very cold and very spread out. The denser portions of gas began to collapse into even denser regions and started to heat up.

When those “globules” are dense enough to become opaque and block the light from the glowing gases behind them, they receive the official title of “protostars.”

Globules have a temperature of about 440 degrees below zero, which doesn’t sound like much, but they are still warmer by a few degrees than the gases in the nebula that surrounds them.

Over a period of millions of years, they slowly condense until they reach a temperature and density sufficient for a thermonuclear reaction to occur. Hydrogen will begin to form to helium. Energy will be released in massive quantities. Boom, stars!

The area of the star cluster at the center of the Rosette is mysteriously devoid of the glowing gas that makes up the rest of the nebula, giving the Rosette its distinctive doughnut shape.

This central cavity is a remarkable 70 light years in diameter, which leaves a ring of gases only 30 light years thick.

What became of all the gas in the center?

When the stars in the cluster formed, they used up a lot of the gases in that area. The star cluster is composed of hot, new stars, which, like most youngsters, are extremely energetic. The temperature of their outflowing gasses reaches a temperature 10 times greater than the surfaces of the stars. The result is an intensely hot and powerful “stellar wind” of almost 22,000 miles per hour, which blew the remaining gases into the outer ring of the nebula.

If that sounds intense, you ain’t heard nothing yet.

Just up and to the left from the Rosette is Plaskett’s Star. It’s just under naked-eye visibility, so it takes binoculars to see it as a pale blue point of light.

Plaskett’s Star was formed out of some of the same material that makes up the Rosette. It is a “binary star,” two stars orbiting each other around the same center of gravity.

Those stars are huge. Each one has fifty times the “starstuff” of our sun and shines with 3,600 times the brightness. At a surface temperature of 50,000 degrees Fahrenheit, they are at least five times hotter than the sun.

With a separation of only 50 million miles, they are only half the distance from each other than our sun is from Earth.

Combining great mass and size with proximity produces weird effects. Like Siamese twins, their substance is literally flowing between them as giant hot halos around the stars and gigantic streamers of hot gas that shoot from one star to the other.

Taken together, they may be the biggest, meanest, nastiest stars of the 300 billion stars in our Milky Way galaxy.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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