Astronomy For Dummies. Maran Stephen P.

of the Great Square of Pegasus, a large asterism, are marked by three stars of the Pegasus constellation and a fourth from Andromeda. Figure 1-1 shows the Big Dipper in the night sky. (In the United Kingdom, some people call the Big Dipper the Plough.)

      Photo © Jerry Lodriguss

       FIGURE 1-1: The Big Dipper, found in Ursa Major, is an asterism.

      In addition to the roughly 30,000 professional astronomers worldwide, several hundred thousand amateur astronomers enjoy watching the skies. Amateur astronomers usually know the constellations and use them as guideposts when exploring the sky by eye, with binoculars, and with telescopes. Many amateurs also make useful scientific contributions. They monitor the changing brightness of variable stars; discover asteroids, comets, and exploding stars; and crisscross Earth to catch the shadows cast as asteroids pass in front of bright stars (thereby helping astronomers map the asteroids’ shapes). They even join in professional research efforts with their home computers and smartphones through Citizen Science projects, which I describe in Chapter 2 and elsewhere throughout the book.

      In the rest of Part 1, I provide you with information on how to observe the skies effectively and enjoyably.

      What You See: The Language of Light

      Light brings us information about the planets, moons, and comets in our solar system; the stars, star clusters, and nebulae in our galaxy; and the objects beyond.

      In ancient times, folks didn’t think about the physics and chemistry of the stars; they absorbed and passed down folk tales and myths: the Great Bear, the Demon star, the Man in the Moon, the dragon eating the Sun during a solar eclipse, and more. The tales varied from culture to culture. But many people did discover the patterns of the stars. In Polynesia, skilled navigators rowed across hundreds of miles of open ocean with no landmarks in view and no compass. They sailed by the stars, the Sun, and their knowledge of prevailing winds and currents.

      Gazing at the light from a star, the ancients noted its brightness, position in the sky, and color. This information helps people distinguish one sky object from another, and the ancients (and now people today) got to know them like old friends. Some basics of recognizing and describing what you see in the sky are

      ❯❯ Distinguishing stars from planets

      ❯❯ Identifying constellations, individual stars, and other sky objects by name

      ❯❯ Observing brightness (given as magnitudes)

      ❯❯ Understanding the concept of a light-year

      ❯❯ Charting sky position (measured in special units called RA and Dec)

They wondered as they wandered: Understanding planets versus stars

      The term planet comes from the ancient Greek word planetes, meaning “wanderer.” The Greeks (and other ancient people) noticed that five spots of light moved across the pattern of stars in the sky. Some moved steadily ahead; others occasionally looped back on their own paths. Nobody knew why. And these spots of light didn’t twinkle like the stars did; no one understood that difference, either. Every culture had a name for those five spots of light – what we now call planets. Their English names are Mercury, Venus, Mars, Jupiter, and Saturn. These celestial bodies aren’t wandering through the stars; they orbit around the Sun, our solar system’s central star.

      Today astronomers know that planets can be smaller or bigger than Earth, but they all are much smaller than the Sun. The planets in our solar system are so close to Earth that they have perceptible disks – at least, when viewed through a telescope – so we can see their shapes and sizes. The stars are so far away from Earth that even if you view them through a powerful telescope, they show up only as points of light. (For more about the planets in the solar system, flip to Part 2. I cover the planets of stars beyond the Sun in Part 4.)

If you see a Great Bear, start worrying: Naming stars and constellations

      I used to tell planetarium audiences who craned their necks to look at stars projected above them, “If you can’t see a Great Bear up there, don’t worry. Maybe those who do see a Great Bear should worry.”

Ancient astronomers divided the sky into imaginary figures, such as Ursa Major (Latin for “Great Bear”); Cygnus, the Swan; Andromeda, the Chained Lady; and Perseus, the Hero. The ancients identified each figure with a pattern of stars. The truth is, to most people, Andromeda doesn’t look much like a chained lady at all – or anything else, for that matter (see Figure 1-2).

      © John Wiley & Sons, Inc.

       FIGURE 1-2: Andromeda is also known as the Chained Lady.

      Today astronomers have divided the sky into 88 constellations, which contain all the stars you can see. The International Astronomical Union, which governs the science, set boundaries for the constellations so astronomers can agree on which star is in which constellation. Previously, sky maps drawn by different astronomers often disagreed. Now when you read that the Tarantula Nebula is in Dorado (see Chapter 12), you know that, to see this nebula, you must seek it in the Southern Hemisphere constellation Dorado, the Goldfish.

      The largest constellation is Hydra, the Water Snake. The smallest is Crux, the Cross, which most people call the Southern Cross. You can see a Northern Cross, too, but you can’t find it in a list of constellations; it’s an asterism within Cygnus, the Swan. Although astronomers generally agree on the names of the constellations, they don’t have a consensus on what each name means. For example, some astronomers call Dorado the Swordfish, but I’d like to skewer that name. One constellation, Serpens, the Serpent, is broken into two sections that aren’t connected. The two sections, located on either side of Ophiuchus, the Serpent Bearer, are Serpens Caput (the Serpent’s Head) and Serpens Cauda (the Serpent’s Tail).

      The individual stars in a constellation often have no relation to each other except for their proximity in the sky as visible from Earth. In space, the stars that make up a constellation may be completely unrelated to one another, with some located relatively near Earth and others located at much greater distances in space. But they make a simple pattern for observers on Earth to enjoy.

      As a rule, the brighter stars in a constellation were assigned a Greek letter, either by the ancient Greeks or by astronomers of later civilizations. In each constellation, the brightest star was labeled alpha, the first letter of the Greek alphabet. The next brightest star was beta, the second Greek letter, and so on down to omega, the final letter of the 24-character Greek alphabet. (The astronomers used only lowercase Greek letters, so you see them written as α, β, … ω.)

So Sirius, the brightest star in the night sky – in Canis Major, the Great Dog – is called Alpha Canis Majoris. (Astronomers add a suffix here or there to put star names in the Latin genitive case; scientists have always liked Latin.) Table 1-1 shows a list of the Greek alphabet, in order, with the names of the letters and their corresponding symbols.

TABLE 1-1 The Greek Alphabet

When you look at a star atlas, you discover that the individual stars in a constellation aren’t marked α Canis Majoris, β Canis Majoris, and so on. Usually, the creator of the atlas marks the area of the whole constellation as Canis Major and labels the individual stars α, β, and so on. When you read about a star in a list of objects to observe, say, in an astronomy magazine (see