Tom Burns: Here’s one for the Astronerds


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.

Slowly, the constellations evolved from stick figures to patches of sky, and the 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.

The Unicorn’s stars are just as dim as they ever were, so casual stargazers rarely look in its direction. However, if you have a set of binoculars and a dark, rural sky, there are good reasons to give Monoceros a little attention.

First, there’s the Rosette nebula, just below the nose of the unicorn and marked as “NGC 2237” on most star maps. It is one of the largest of the so-called “emission nebulae,” with a quantity of hydrogen gas and dust that is more than 11,000 times greater than our sun’s. At a distance of about 2,600 light years from Earth, it is about 93 light years in diameter (a light year is about six trillion miles). A beam of light would take 93 years just to cross it.

It 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 sky glow 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, who 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 these “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 millions of years, they will slowly condense until they reach a temperature and density sufficient for a thermonuclear reaction to occur. Hydrogen will begin to form to helium. Heat 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 leaving a ring of gases only 12 or so light years wide.

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 are intensely active. Such heat and activity created a “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 was formed out of some of the same material that makes up the Rosette. It is a “double star,” two stars orbiting each other around the same center of gravity.

These stars are huge. Each one has 50 times the “starstuff” of our sun and shine with 3,600 times the brightness. At a surface temperature of 50,000 degrees Fahrenheit, they are eight times hotter than the sun.

With a separation of only 50 million miles, they are only half the distance from each other that 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 among the biggest, meanest, nastiest stars of the 300 billion stars in our Milky Way galaxy.

Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

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