The Shield of Sobieski

By Tom Burns - Stargazing

If you ever wonder why astronerds like me moan so constantly about the glow of city lights, consider the case of Scutum, the Shield.

The constellation lies high in the southwest in our late summer sky below the constellation Aquila, the Eagle, but you’ll have a devil of a time finding it even from the relatively rural skies of Perkins Observatory. The generalized glow of Columbus and Delaware streetlights and otherwise useless decorative lighting has seen to that.

The ancients didn’t even bother to honor its faint stars with a constellation name, which is an odd oversight. A milky cloud of stars, now called the Scutum Star Cloud (SSC for short), dominates that section of sky.

The SSC is a somewhat isolated region of the more general stream of light called the Milky Way. It looks like a puff of silver dust when viewed from dark, rural skies. The Milky Way is beautiful as a whole, but the SSC is its most splendiferous part. Edward Emerson Barnard called the region the “Gem of the Milky Way.”

Over the top of the silvery puff are placed a few stars, rendered exceedingly faint and therefore, I suppose, inconsequential by the glow of unresolved stars behind them.

As a result, the task of naming the region was reserved for Jan Hevelius late in the 17th century.

Hevelius was an old-fashioned stargazer, even by the standards of the 1600s. Other astronomers used telescopes to determine the precise locations of the stars. Hevelius used a simple quadrant to plot stellar positions.

He was also something of an artist. Over the years, he painstakingly etched star maps on copper plates. As was the custom of the time, he overlaid the constellations with elaborate portraits of their associated mythological figures.

Hevelius was renowned for his excellent eye. He continued to observe the stars well into his 70s — an age when other astronomers yielded reluctantly to their diminished vision. Even then, he could ferret out the faintest stars with his unaided eye alone. He thus rendered constellations out of the faint stars groupings that no one before had bothered to name. The region of the Scutum Star Cloud was ripe for the picking.

The area is consequential for another reason about which Hevelius was probably unaware. In the midst of the Scutum Star Cloud is the Wild Duck Cluster, one of the great telescopic showpieces of the nighttime sky. In binoculars, look for a bright, fuzzy dot near the center of the Scutum Star Cloud.

Gottfried Kirch discovered it in 1681. In 1764, Charles Messier forever cemented its fame when he included it as the 11th entry in his catalog of diffuse, comet-like objects. More than 250 years later, amateur telescopists still refer to it as M11.

In 1844, retired British admiral and stargazer extraordinaire William Smyth wrote that M11 “somewhat resembles a flight of wild ducks” in V-formation. To this day, amateur astronomers often refer to M11 as the Wild Duck Cluster even though their larger telescopes have erased the resemblance to ducks in flight.

“Open clusters,” as such star accumulations are called, are born from enormous clouds of glowing hydrogen gas called gaseous nebulae. In such clouds, stars are still condensing out of the primordial gas in many Milky Way locales as you read these lines. A similar cloud gave birth to our own sun and some of the bright, nearby stars that can be seen with your unaided eye.

In fact, we can see those exceedingly thin clouds of hydrogen precisely because they have young stars inside of them. The stars ignite the gas around them, much in the same way that a spark of electricity ignites the gas in a fluorescent light bulb, except that the fluorescent “light bulbs” are often hundreds of trillions of miles wide.

Often, every cloud contains enough hydrogen to give birth to thousands of stars. Since the nebulae are relatively small compared to the Milky Way, the stars that are born there are relatively close together to start with. Stars are born in litters, like puppies, and early in their lives, they huddle together close to their cosmic mother.

When a nebula gives birth to many stars, it uses up much of its available hydrogen gas. What remains of the gas is often blown outward into space by the powerful explosions of the young stars.

As a result, you’d expect to find some parts of the Milky Way that are dense congregations of relatively young stars without any surrounding hydrogen gas.

And that’s exactly what you’ll find when you look down and to the left from the top star in Scutum. Binoculars will show M11 as a grainy patch of light. In a small telescope, the area expands into a gorgeous, tightly packed grouping of stars.

The Wild Duck is one of the most massive open clusters known. Its total mass is equivalent to 11,000 or so stars like our below-average sun. It contains over 1,000 stars all at a distance of perhaps 6,000 light-years. (One light-year is equivalent to about six trillion miles.)

The cluster is also one of the most voluminous astronomers have observed. M11 is a mind-boggling 190 light-years wide, about the size of the gigantic gaseous nebula that gave birth to the cluster.

Given its relatively young age at only 300 million years, you are seeing young stars, newly liberated from their nebulous star mom.

Most of the stars of the rest of the Milky Way are much more distant from each other than the stars found in clusters. As cluster stars grow older, they drift apart and find their own places in our galaxy, leaving us with the Milky Way galaxy, a sight of unparalleled splendor.

Despite the beauty of the Scutum Star Cloud and its extraordinary open cluster, because of the relative faintness of its naked-eye stars, the area of the sky remained unnamed until Hevelius came on to the scene.

When his detailed and exquisitely drawn atlas of the stars was finally published in 1690, three years after his death, both the astronomical and political worlds were greeted with something of a surprise, as we shall see.

Hevelius was born in the Polish city of Gdansk and loved his native land fervently. For 200 years, the nation had elected its king, and in 1674, it chose wisely. John Sobieski served his nation well in times of trouble. As a field commander of the Polish armies, he had won great victories against the invading Cossacks and the Ottoman Turks.

In 1683, he faced his greatest challenge. The Turks swept toward Vienna, the grandest city of the Holy Roman Empire, with a massive army. Sobieski rushed to its defense with a pitiful army of 25,000. At the nearby town of Kahlenberg, he won a victory so complete that it changed the course of European history.

Afterward, a cathedral service of thanksgiving was held. In honor of the man who had saved Vienna and the western world from invasion, the officiating priest read this Biblical passage: “There was a man sent from God, whose name was John.” To his grateful nation, Sobieski seemed a truly heavenly deliverer.

Hevelius was among the most grateful. He honored King John among the heavens with the constellation Scutum Sobiescianum, the Shield of Sobieski. At its center were two faint stars, which Hevelius imagined to be the cross that decorated the great king’s coat of arms and thus his shield.

With the death of Sobieski in 1696, the tide of history turned against Poland. It has suffered many defeats over the intervening years and spent centuries under the dominion of foreign powers as one nation after another swept across its soil.

Now, at last, its people live in freedom from the tyranny of other nations. May the starry shield of its great king protect it and keep it safe as long as Scutum shines like a cloud of brilliant diamond dust against the darkness of the night.

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.