Scutum the Shield

Scutum the Shield
Vienna, September 1683. For two months the city had been besieged by an army of the Ottoman Empire, and couldn't hold out much longer. But what, you may wonder, does this have to do with astronomy? Answer: It's the small constellation Scutum which represents the shield of the Polish king John III Sobieski. The king was the chief commander of the army that arrived in the nick of time to save Vienna from the invaders. The same king had also helped the astronomer Johannes Hevelius rebuild his observatory after a terrible fire, and Hevelius created the constellation in his honor.

The constellation's original name was Scutum Sobiescianum (Sobieski's Shield) which you can see on a page from Hevelius's star atlas. The name, along with most other two-word names, was later shortened. There have been other tributes to historical figures created among the stars, but Scutum is the only one to survive.

It's something of a surprise that Scutum is a recognized constellation. John Flamsteed (1646-1719) ignored it in his Atlas Coelestis, though he included some of its brighter stars as part of Aquila (the Eagle). Over a period of a few centuries celestial cartographers revived Scutum and removed Scutum, but somehow it survived to be on the of the 88 official constellations adopted by the International Astronomical Union (IAU) in 1922.

Alpha Scuti
Scutum is small and dim. It's the fourth smallest constellation, and even its brightest star Alpha Scuti is only fourth magnitude. The higher the magnitude, the dimmer the star appears from Earth. Yet Alpha Scuti is a variable orange giant star. It seems dim to us because it's 174 light years away. In fact, it's over a hundred times more luminous than the Sun.

Delta Scuti
Delta Scuti seems to be an unimpressive fifth magnitude star. But that's just how it looks from 200 light years away. It's of interest to astronomers because it's a variable star that gives its name to an entire class of variable stars that help determine distances. Surveys have found nearly 3000 Delta Scuti variables in the the Large Magellanic Cloud, one of the satellite galaxies of our Milky Way.

It also seems that if Delta Scuti maintains its current orbit around the Milky Way, one day it will be a neighbor of the Solar System, only ten light years away. And if it maintains its current luminosity, it will overtake Sirius as the brightest star in Earth's sky. However that's not calculated to happen for over a million years, so we won't around to check the maths.

UY Scuti
If you went outside and looked into a clear dark sky, you couldn't see UY Scuti, even though it's one of the biggest stars we know. It's a red supergiant, but it's also 9500 light years away, making it too faint to see with the unaided eye. UY Scuti is about 1700 times the radius of the Sun. If it were placed where the Sun is, it would reach out to nearly the orbit of Jupiter.

Deep sky objects
M11 – open star cluster
Although Scutum is small it has some superb deep sky objects. The best known is probably M11, an open star cluster of about 3000 stars that formed about 150 million years ago. It's some twenty light years in diameter and several thousand light years away. If you were on a planet near the center of M11, the sky would be full of luminous stars. Over fifty of them would be much brighter than Sirius. Many of them would be so bright that astronomers would have to revise the magnitude scale to accommodate them.

German astronomer Gottfried Kirch discovered the nebulous object in 1681, and it went into Charles Messier's catalogue in 1764. English astronomer and naval officer William Henry Smyth gave it the nickname Wild Duck Cluster in 1844 because in the telescope eyepiece he thought the brightest stars resembled a flock of ducks in flight. But I admit I couldn't find a single photograph that reminded me of wild ducks in flight.

NGC 6712 – globular cluster
Globular clusters are large star clusters with enough mass to pull them into a somewhat spherical shape. They're also usually quite far away – NGC 6712 is 22,500 light years distant. It was originally discovered by French astronomer Le Gentil in 1749, and then 35 years later, independently, by William Herschel. Herschel described it as a “rounded nebula”, but in the 1830s his son John Herschel identified it as a globular cluster.

IC 1295 – planetary nebula
There's a beautiful green planetary nebula (IC 1295) 3000 light years away in Scutum. Planetary nebulae don't have anything to do with planets. They're created when a dying star sheds its outer layers. The star eventually collapses into a white dwarf which may be detected in the middle of the nebula. However nobody knew this in the eighteenth century and some such nebulae showed a disk in the telescopes of the day. This reminded William Herschel of the disk of a planet.

Infrared clusters
There are also a number of large star clusters that we can't see in visible light. They were discovered using infrared light, and some of them are in Scutum.

One of the Milky Way's most massive star clusters is ssc2006-3a which is nearly 19,000 light years away. Here is an image of ssc2006-3a from the Spitzer Space Telescope. The cluster contains fourteen stars so massive that their ends will come soon as they use up all their fuel and explode as supernovae.

You Should Also Read:
Johannes Hevelius
ABC of Astronomy – D Is for Double Star
Galaxy or Star Cluster

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