Stalking the elusive unicorn

By Tom Burns - Stargazing

As I stared upward at the winter constellations recently, I thought about the marvelous device that allowed me to do so.

When it comes to experiencing our world and the vast universe that envelops it, no instrument is more important than the human eye. Out of the universe streams energy in the form of tiny particles called photons, which we refer to collectively as “light.”

Over many millions of years, the human eye developed into a complex system that collects photons through an opening called the iris, brings them to a focus with a lens called the cornea, and projects them onto a detecting screen called the retina.

The retina’s detecting cells transmit a signal to a computer called the brain, a vast set of interconnected storage cells that remember and interpret the information.

As detectors go, the human eye has its deficiencies. Its tiny opening means that it can gather only a minuscule amount of the light that streams at us from the universe.

More significantly, photons come in a wide variety of forms depending on how energetic they are. Those energies are spread out over a band called the electromagnetic spectrum. They range from low-energy radio and infrared photons to very high-energy photons like x-rays and gamma rays.

Close to the middle of the spectrum is a tiny band of energy that our eyes can detect. For most of human history, visible light is all human beings were capable of seeing. The rest of those energies — virtually all of the universe — was hidden from our sight.

Given the weaknesses of our detecting system, humans did remarkably well at figuring out the stars. One of their first accomplishments was to organize the cacophony of stars into constellations.

In fact, astronomers have been using the same constellation groupings for so long that many people have come to believe that their names were lowered down from heaven on a string or invented by Og and Grog, the first stargazers.

The 88 constellations we use these days are simply subdivisions of the sky established by the first General Assembly of the International Astronomical Union in 1922.

During the preceding millennia, things were a bit more confused. The ancients concerned themselves with only the brighter stars in the heavens, and whole patches of sky were left unnamed.

The first truly revolutionary improvement over the flawed human eye was the invention of the telescope. By increasing the size of the opening through which photons could stream, astronomers could now collect far more of them. They soon developed bigger and bigger buckets that could gather light in ever-increasing quantities.

After the telescope came along, astronomers began to invent constellations that covered undesignated sections of the sky. If they found something new, they wanted to say where it was. Thus, the constellations slowly transformed from the stick figures of ancient heroes to the patches of celestial real estate we see today.

The patch of faint stars to the left of much brighter Orion is a good example. In 1613, very soon after Galileo’s first use of the astronomical telescope, Dutch cartographer Petrus Plancius produced a map of the sky that filled in the undesignated space near Orion with Monoceros, the Unicorn.

Plancius was better known as a theologian than as an astronomer. He knew that the unicorn is mentioned several times in the Old Testament. He figured it was about time that somebody named a constellation after it.

Even so, it might be very old. Plancius may have borrowed the Unicorn from an ancient Persian constellation of a horse. RH Allen suggests that ancient Chinese stargazers placed an equine constellation somewhere in the area. Add a horn, and you have Monoceros.

Pity then the poor Unicorn. Imagine a pair of them, as many people have done, standing forlornly on the shore as Noah’s arc receded into the distance. Besides, the gentle creature is outshone by bright and famous constellations like Gemini to the north, Orion to the west, and Canis Major with its bright star Sirius to its south.

The whole constellation is barely visible to the naked eye, and most urbanites and even suburbanites have never seen it. Western astronomers didn’t even give that dim slice of the sky a name until Plancius came along.

Despite the dimness of the Unicorn’s stars, a good reason exists to give Monoceros a little attention during the winter months — if you happen to own a telescope, that is.

The foremost target for most telescopists is the Rosette Nebula, just below the nose of the unicorn and marked as “NGC 2237” on most star maps.

The Rosette 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 reveals 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 the light from the nebula through.

Nebular filters represent another leap in our ability to see the universe. The visible light from the Rosette is exceedingly faint. Luckily, it does not glow equally in all the colors of the visible part of the spectrum. It contains a small amount of glowing ozone, which shines fairly brightly in a tiny portion of the spectrum’s blue part. A nebular filter blocks most of the other visible light, including much of the glow from city streetlights, and lets the ozone light through.

Astronomers have developed much more powerful techniques to see objects like the Rosette Nebula. They can filter the visible light, and they have access to huge energy-gathering telescopes to do so. They have also designed telescopes that can see the universe in all the parts of the electromagnetic spectrum. The universe thus looks much different to us than it did to the ancients.

But the eye is still the ultimate detector. As complex and detailed as an internet image of the Rosette might be, no picture matches the dumbstruck glory of seeing it with your own two eyes through a $40 set of binoculars from a remote mesa in Arizona on a still, clear night in winter.

If you are one of the happy few who have such an experience, I hope you will take a moment to remember the long history that led you to that place and the marvelous devices that opened up the universe to your awestruck gaze.

By Tom Burns


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

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