On August 21, a solar eclipse will occur over much of the United States.  Although it won’t be a “total” eclipse in the Shenandoah Valley, more than 80% of the Sun’s face will be blocked. That will dim the daylight very noticeably. Let’s plan to do some observing if the weather is clear!

This month let’s focus on two early summer constellations: Corona Borealis and Hercules.  You can find them in the southeastern sky soon after dark this month. Start from the end of the handle of the Big Dipper and swing your gaze east (that is, to the left) a distance about equal to the length of the Dipper’s handle.  The map below should help you find them.  Remember that the Big Dipper is part of the constellation Ursa Major.[singlepic id="1551" w ="500" float="center"]

Locator Map for the Crown and the Laborer

Corona Borealis is Latin for “Northern Crown,” and it is one of the 48 constellations listed by the 2nd-Century Greek-Alexandrian astronomer Claudius Ptolemy. There are several stars that mark the crown, but none is especially bright.  The brightest of them is Alphecca; its name is derived from the Arabic word for “the bright one.”

Notice that the standard constellation names are Latin, but many of the names of individual stars are derived from Arabic. To make a long story short, for several hundred years after the Fall of Rome, it was scholars in the Islamic world (especially in the “House of Wisdom” in what is now Baghdad) who preserved and extended mathematical and scientific knowledge.  Arabic astronomers named many previously unnamed stars; those Arabic names survive on today’s star maps.

Alphecca is actually a double, or binary, star. The primary (Alphecca A, the brighter member of the pair) has about 70 times as much  radiant power (emitted light energy per second) as the  Sun. The secondary star (Alphecca B) is 100 times fainter than the primary - a yellow dwarf that puts out somewhat less light than the Sun. The two stars orbit one another every 17 days; they are closer to one another than Mercury is to the Sun.  Alphecca is an eclipsing binary: each star passes in front of the other as viewed from Earth. The Alphecca system is about 75 light-years from Earth.

The engraving below is from a 19th-Century set of constellation cards called “Urania’s Mirror.” It shows the mythological images and the naked-eye stars of these two constellations. Some constellations have bright stars in patterns that fit their names very well (for example, Orion in the winter sky). For other constellations, it takes a lot of imagination to recognize the figures.[singlepic id="1547" w ="500" float="center"]

Corona Borealis and Hercules

The constellation Hercules is named for the mythological son of the Roman god Jupiter and Alcheme, a mortal woman. Hercules was a mercenary swordsman who was driven mad by the goddess Hera. In his deranged state, he killed his wife and children.  After recovering his sanity and in deep sorrow, he consulted Pythia, the Oracle at Delphi, who sent him to serve King Eurystheus.  Hercules performed twelve difficult and dangerous labors assigned by the king -- including cleaning the Augean stables, which housed 1,000 cattle and had not been cleaned in 30 years. (I grew up on a farm, but my father never gave me a task like that!)  According to one tradition, after he completed the twelve labors, Hercules joined Jason and the Argonauts in their quest for the Golden Fleece.

Hercules is a sprawling constellation with 18 or more naked-eye stars, but none of them is as bright as Alphecca.  The most recognizable shape within the constellation is "the Keystone,” formed by four stars to the northeast of Corona Borealis. The map below shows the Keystone and also marks several star clusters and nebulae.

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Detail of Hercules Map

Let’s zoom in on two objects in Hercules.  The first is a star cluster known as Messier 13 or simply M13, after the 18th-century French comet hunter Charles Messier. It was number 13 on Messier's list of about a hundred “non-comets”: objects that should be ignored by comet hunters because they could be mistaken as the first appearance of an approaching comet.

M13 is also known as the “Beehive.” The telescopic photo below shows the the dense central region of M13.  Notice the mix of yellow and blue stars. On an especially dark night, this central part of the cluster is just barely visible to the naked eye. The entire Beehive cluster contains several hundred thousand stars and is nearly as big on the sky as a full moon. It is about 15,000 light years away. Astronomers estimate the cluster’s age at about 12 billion years, more than twice the age of the Earth.

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The Dense Core of the Beehive Cluster

The other object we’ll look at is much fainter; the size of the marker on our map of Hercules is misleading in that respect. This is the planetary nebula Abell 39, or A39 for short.  The image below was obtained with a large telescope, and includes many external galaxies, some of which can be seen shining right through the nebula. The name “planetary nebula” is a misnomer, which was applied in the 19th Century: viewed through a small telescope, some of these objects look like planets.

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The "Planetary Nebula" Abell 39

Abell 39 is about 5 light-years across and about 7,000 light years away.  Although it looks ring-shaped on the sky, studies indicate that it consists a thin spherical shell of gas surrounding the central star.  It looks brighter at the edge because we looking through more glowing gas near the edge than in the center. If we could magically do quick long-distance space travel, we would find that Abell 39 looks ring-shaped from any vantage point!  Based on what we observe, the central star ejected the shell about 20,000 years ago.  (Since Abell 39 is 7,000 light years away, the light we’re receiving now has been traveling for 7,000 years,so the ejection actually occurred about 27,000 years ago.)

The mass of the shell is about 60% of the mass of our Sun, and about the same amount of mass remains in the central star. Based on our current understanding of stellar evolution, the star probably formed more than 5 billion years ago -- before our Sun and planets formed.  Because the original mass of the central star in Abell 39 was somewhat greater than the Sun’s mass, it shone more brightly than the Sun does, and it exhausted its nuclear fuel more quickly.  The Sun is predicted to become a red giant and produce a planetary nebula about 5 or 6 billion years from now. In terms of its life cycle, the Sun is middle-aged.

That’s all for now. Next month we’ll take a look at two summer constellations of the zodiac: Scorpius and Sagittarius.

--Bill Ingham