Celestial Portraits

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Well, now that you're standing there with your arm outstretched and your head full of angles, what can you do with this wealth of information?

We now have some rough tools for measuring separations and sizes in the sky, but we still need a way to anchor our altazimuth measurements, which, remember, are relative to where we happen to be standing on Earth. We need the celestial equivalent of landmarks.

Fortunately for us, our ancestors had vivid imaginations.

Human brains are natural pattern makers. We have all seen elephants and lions masquerading as clouds in the sky. Present the mind with the spectacle of 3,000 randomly placed points of light against a sable sky, and, before you know it, it will start "seeing" some pretty incredible pictures. The constellations—arbitrary formations of stars that are perceived as figures or designs—are such pictures, many of them inspired by mythological heroes, whose images (in the western world) the Greeks created by connecting the dots.

By the second century c.e., Ptolemy (whom we'll meet in Chapter 3) listed 48 constellations in his Almagest, a compendium of astronomical knowledge. Centuries later, during the late Renaissance, more constellations were added, and a total of 88 are recognized today. We cannot say that the constellations were really discovered, because they do not exist except in the minds of those who see them. Grouping stars into constellations is an arbitrary act of the imagination and to present-day astronomers are a convenience. In much the same way that states are divided into counties, the night sky is divided into constellations. The stars thus grouped have no physical relationship to one another and, in fact, are many, many trillions of miles apart. Nor do they necessarily lie in the same plane with respect to the earth; some are much farther from us than others. But, remember, we simply imagine that they are embedded in the celestial sphere as a convenience.

If the constellations are outmoded figments of the imagination, why bother with them?

The answer is that they are convenient (not to mention poetic) celestial landmarks. We all use landmarks to navigate on land. "Take a right at the

Astronomer's Notebook

Of the 88 constellations, 28 are in the northern sky and 48 are in the southern sky. The remaining dozen lie along the ecliptic—a circle that describes the path that the sun takes in the course of a year against the background stars. This apparent motion is actually due to the earth moving around the sun. (We'll revisit the term ecliptic in Chapter 11). These 12 constellations are the zodiac, familiar to many as the basis of the pseudoscience (a body of lore masquerading as fact verified by observation) of astrology. All but the southernmost 18 of the 88 constellations are at least sometimes visible from part of the United States.

gas station," you might tell a friend. What's so special about that particular gas station? Nothing—until you invest it with significance as a landmark. Nor was there anything special about a group of physically unrelated stars—until they were invested with significance. Now these constellations can help us find our way in the sky and, unless you are using a telescope equipped with an equatorial mount, are more useful than either the celestial or altazimuth coordinate system.

Astro Byte

In an age when so many objects, of necessity, are referred to by rather cold catalog names (NGC 4258, W49A, K3-50A, to name a few), it is pleasing that we can still refer to some objects by their brightness within a given constellation. Cygnus X-1 is a famous x-ray source and black-hole candidate in the constellation of Cygnus, the swan.

Astro Byte

In an age when so many objects, of necessity, are referred to by rather cold catalog names (NGC 4258, W49A, K3-50A, to name a few), it is pleasing that we can still refer to some objects by their brightness within a given constellation. Cygnus X-1 is a famous x-ray source and black-hole candidate in the constellation of Cygnus, the swan.

Star Words

An asterism is an arbitrary grouping of stars within or associated with a constellation, which are perceived to have a recognizable shape (such as a Teapot or Orion's Belt) and, therefore, readily serve as celestial landmarks.

Star Words

An asterism is an arbitrary grouping of stars within or associated with a constellation, which are perceived to have a recognizable shape (such as a Teapot or Orion's Belt) and, therefore, readily serve as celestial landmarks.

The Dippers First

Almost everybody knows the Big Dipper and maybe the Little Dipper, too. Actually, neither Dipper is a constellation, but are subsets of other constellations, Ursa Major and Ursa Minor, the big and little bears (official constellation names are in Latin). Such generally recognizable subgroups within constellations are called asterisms. The Big Dipper is not only bright, but it is easy to find in the northern sky in all seasons except fall, when it is low on the horizon. It might interest you to know that you'll find the Big Dipper between 11 and 14 hours R.A. and +50 to +60 degrees dec. Using your hand to estimate the Big Dipper's angular size, you'll see that it's about 25 degrees across the bowl to the end of the handle. But the really important thing is that its seven bright stars form a pattern that really does look like a dipper.

And that's what's so handy about asterisms. They are simpler, brighter, and more immediately recognizable than the larger, more complex constellations of which they are a subset. They will help you to find the constellations with which they are associated and generally help to orient you in the sky.

Seafarers and other wanderers have long used the Big Dipper as a navigational aid. If you trace an imaginary line between the two stars that mark the outer edge of the Big Dipper's bowl and extend that line beyond the top of the bowl, it points to Polaris, the North Star, about 25 degrees away. Polaris is very nearly at the north celestial pole (about 1 degree off), which means that it appears to move very little during the course of the night, and travelers have always used it as a compass. During the decades before the American Civil War, many Southern slaves escaped to the free North by following the North Star. For sky explorers, the combination of the Big Dipper and Polaris provide a

On the tear-out cards at the front and back of this book, you will find four "all-sky" star charts, which show the major constellations visible in the night sky during the four seasons. See Chapter 6 for tips on the best times and places to view the night sky and steps you can take to minimize the effects of light pollution.

major landmark useful in locating other constellations. One of those constellations— actually, it's another asterism—is the Little Dipper. Dimmer and smaller than the Big Dipper, it would be harder to find, except that Polaris, which we've just located, is at the tip of its handle. Like its big brother, this asterism consists of seven stars.

The Stars of Spring

Let's look at a few of the highlights of each season's sky.

With the arc of our Galaxy (the Milky Way) low and heading toward the western horizon, the spring sky offers fewer bright stars than any other season. This isn't necessarily a bad thing, because it makes identifying the three bright ones that much easier. Some 45 degrees south of the Big Dipper's bowl is the constellation Leo. If you can't quite pick out Leo, you might find it easier to identify the asterism called the Sickle, a kind of backwards question mark that forms Leo the Lion's mane. At the base of the Sickle is the bright star Regulus.

Arcturus, another bright star of spring, may be located by extending the curve of the Big Dipper's handle 35 degrees southward.

Yellow-orange in color, Arcturus is the brightest star of the constellation Bootes, the Charioteer.

Now extend the Big Dipper handle's curve beyond Arcturus, and you will find Spica (in Virgo), the third bright star of spring. In vivid contrast to the warm hue of Arcturus, Spica is electric blue. We'll find out in Chapter 17, "Of Giants and Dwarfs: Stepping Out into the Stars," that the color of a star actually tells us about its surface temperature. It can be quite a thrill looking at different stars in the sky and be able to "take their temperatures" simply from their colors.

Astronomer's Notebook

On the tear-out cards at the front and back of this book, you will find four "all-sky" star charts, which show the major constellations visible in the night sky during the four seasons. See Chapter 6 for tips on the best times and places to view the night sky and steps you can take to minimize the effects of light pollution.

"Arc to Arcturus" is a handy mnemonic often taught to astronomy students to help them easily locate the star. Following the arc of the Big Dipper's handle leads to this bright star.

Astro Byte

"Arc to Arcturus" is a handy mnemonic often taught to astronomy students to help them easily locate the star. Following the arc of the Big Dipper's handle leads to this bright star.

Summer Nights

Summer offers four bright stars. Three of them form a distinct right triangle called the summer triangle. Vega is at the triangle's biggest angle. Vega is also the brightest star in the constellation Lyra, the Lyre (or harp). South and east of Vega is the second brightest star in the triangle, Altair, which is in the constellation Aquila, the Eagle. Deneb is the third star of the summer triangle, and is in Cygnus, the Swan. Deneb is also part of the prominent asterism, the Northern Cross, and is the brightest star in that group.

Take a good long look at Deneb. Bright as it is—fourth brightest in the summer sky— it is one of the most distant stars visible to the naked eye, fifty times more distant than Vega and several hundred times farther than Alpha Centauri, our closest stellar companion.

If you are sufficiently far from sources of atmospheric and light pollution, and the night is clear and dry, you may notice that the Northern Cross lies within a kind of hazy band stretching across the sky. This band is the Milky Way, our own Galaxy, which we will explore in Chapter 21, "The Milky Way: Much More Than a Candy Bar," and whose haze is the light of some 100 billion or so stars.

Two other major summer constellations should not be missed. Sagittarius, the Archer, and Scorpius, the Scorpion, are found low in the southern sky about 30 degrees below the celestial equator. You can locate Scorpius by finding the fourth bright star of the summer sky, Antares, unmistakable for the red hue that gives it its name, which means "rival of Mars." If Scorpius is not below your horizon (and therefore out of sight), you should recognize its fishhook-shaped scorpion's tail.

One hour R.A. (15 degrees) east of Scorpius is Sagittarius. You may better recognize it by two asterisms within it: the Teapot, which looks as if it pours out on the tail of nearby Scorpius, and the Milk Dipper, called this because its dipper shape seems to dip into the milkiest part (thickest star cloud) of the Milky Way. As we will see, there is a reason that the Milky Way is thicker here, bulging slightly. Sagittarius is the direction of the center of our own Galaxy.

Fall Constellations

In the fall, the constellation Pegasus, winged horse of Greek mythology, is easy to locate. If you find it hard to imagine connecting the stars to trace out the horse, look for the highly recognizable asterism associated with Pegasus called the Great Square. At southern latitudes, by about 10 p.m. in early October, it should be directly above you. The four stars marking out its four corners aren't terribly bright, but the other stars in that area of the sky are fairly dim, so the figure should stand out clearly. The eastern side of Great Square also coincides with the 0 marking from which the hours of right ascension start, increasing to the east.

Some 20 degrees west and 5 degrees south of Markab, the star that marks the Great Square's southwest corner, is Enif, the brightest star in Pegasus. Its name means "the horse's mouth," and between Markab and Enif is the horse's neck. Look to the Great Square's northeast corner for the star Alpheratz, which is not part of Pegasus, but part of Andromeda, the Maiden in Chains.

If you trace a line from Alpheratz through Markab, continuing about 40 degrees southwest of Markab, you'll find the zodiacal constellation Capricornus, Capricorn, or the Sea Goat. Capricorn is distinguished by its brightest star, the brilliant Deneb Algiedi.

Return to the Great Square. About 20 degrees east of it, you'll find another zodiacal constellation, Aries, the Ram. This grouping is easy to identify, since it is marked by two fairly bright stars a mere 5 degrees apart.

Last to rise in the sky of fall is Perseus, slayer of snake-haired Medusa and other monsters of Greek mythology. About 45 degrees up in the northeast, it lies across the Milky Way and is marked by its brightest star, Mirfak.

Winter Skies

Winter nights, with the bright arc of the Milky Way overhead, offer more bright stars than are visible at any other season: Sirius, Capella, Rigel, Procyon, Aldebaran, Betelgeuse, Pollux, and Castor. Brightest and most readily recognizable of the winter constellations is Orion, the Hunter, which spans the celestial equator and sports the heavens' second most familiar asterism (after the Big Dipper): Orion's Belt, three closely spaced bright stars in a line 3 degrees long. The star Rigel, brightest in the Orion constellation, marks the hunter's foot, 10 degrees below and to the west of Orion's Belt. About the same distance and direction above the Belt is Betelgeuse, a reddish star, whose name is Arabic for "armpit of the giant." And that is precisely what Betelgeuse marks: Orion's armpit. If you look at the winter star chart on the tear-out card, you'll also see Bellatrix, which marks the shoulder of Orion's arm holding his shield, which is an arc of closely spaced, albeit dim stars. Suspended from Orion's Belt is a short sword, the middle "star" of which is actually a region where stars are being born. (We will discuss the Orion nebula and other regions like it in Chapter 12, "Solar System Family Snapshot.")

Saiph is Orion's eastern leg. About 15 degrees to the southeast of this star is Sirius, called the Dog Star, because it is in the constellation Canis Major, the Great Dog. Sirius is the brightest star in the heavens.

To the northeast of Orion you will readily see a pair of bright stars close together. These are Castor and Pollux, the Twins, which represent the two heads of the constellation Gemini. Moving in an arc to the northwest of Castor and Pollux, you should see another bright star, this one with a distinctly yellow-gold color. It is called Capella, which means "little she-goat," and the ancients thought the star was the color of a goat's eye. Capella is in the constellation Auriga, the Goatherd.

Astro Byte

There is an abundance of stars that still have their Arabic names, a testament to the many contributions of Arab astronomers: Aldebaran, Mizar, Alcor, and so on.

Astro Byte

There is an abundance of stars that still have their Arabic names, a testament to the many contributions of Arab astronomers: Aldebaran, Mizar, Alcor, and so on.

Return to Orion. Just to the northwest of his shield, you will find Taurus, the bull, which is marked by Aldebaran, a bright orange star that forms the constellation's bull's eye. Early sky watchers imagined Taurus eternally charging the shield of Orion, who stood eternally poised to strike the animal with his upraised club.

It is admittedly difficult to imagine the bull in Taurus, though you may at least be able to discern a V-shaped asterism called the Hyades, which is the bull's mouth. To the northwest of this feature are the Pleiades, or Seven Sisters, a strikingly beautiful cluster of seven stars that are part of an open cluster.

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