Astronomy For Dummies. Maran Stephen P.

in the style of Alpha Canis Majoris or even α Canis Majoris. Instead, to save space, the magazine prints it as α CMa; CMa is the three-letter abbreviation for Canis Majoris (and also the abbreviation for Canis Major). I give the abbreviation for each of the constellations in Table 1-2.

TABLE 1-2 The Constellations and Their Brightest Stars

      Astronomers didn’t coin special names such as Sirius for every star in Canis Major, so they named them with Greek letters or other symbols. In fact, some constellations don’t have a single named star. (Don’t fall for those advertisements that offer to name a star for a fee. The International Astronomical Union doesn’t recognize purchased star names.) In other constellations, astronomers assigned Greek letters, but they could see more stars than the 24 Greek letters. Therefore, astronomers gave some stars Arabic numbers or letters from the Roman alphabet, or numbers in professional catalogues. So you see star names such as 61 Cygni, b Vulpeculae, HR 1516, and more. You may even run across the star names RU Lupi and YY Sex. (I’m not making this up.) But as with any other star, you can recognize them by their positions in the sky (as tabulated in star lists), their brightness, their color, or other properties, if not their names.

      When you look at the constellations today, you see many exceptions to the rule that the Greek-letter star names correspond to the respective brightness of the stars in a constellation. The exceptions exist because

      ❯❯ The letter names were based on inaccurate naked-eye observations of brightness.

      ❯❯ Over the years, star atlas authors changed constellation boundaries, moving some stars from one constellation into another that included previously named stars.

      ❯❯ Some astronomers mapped out small and Southern Hemisphere constellations long after the Greek period, and they didn’t always follow the lettering practice.

      ❯❯ The brightness of some stars has changed over the centuries since the ancient Greeks charted them.

      A good (or bad) example is the constellation Vulpecula, the Fox, in which only one of the stars (alpha) has a Greek letter.

      Because alpha isn’t always the brightest star in a constellation, astronomers needed another term to describe that exalted status, and lucida is the word (from the Latin word lucidus, meaning “bright” or “shining”). The lucida of Canis Major is Sirius, the alpha star, but the lucida of Orion, the Hunter, is Rigel, which is Beta Orionis. The lucida of Leo Minor, the Little Lion (a particularly inconspicuous constellation), is 46 Leo Minoris.

      Table 1-2 lists the 88 constellations, the brightest star in each, and the magnitude of that star. Magnitude is a measure of a star’s brightness. (I talk about magnitudes in the later section “The smaller, the brighter: Getting to the root of magnitudes.”) When the lucida of a constellation is the alpha star and has a name, I list only the name. For example, in Auriga, the Charioteer, the brightest star (Alpha Aurigae) is Capella. But when the lucida isn’t an alpha, I give its Greek letter or other designation in parentheses. For example, the lucida of Cancer, the Crab, is Al Tarf, which is Beta Cancri.

      

If you’re a long-time Astronomy For Dummies reader (possessing at least one of the three previous editions of the book as well as this edition), you may notice some changes in Tab1e 1-2. In 2016, the International Astronomical Union issued a list of official names for bright stars. Seven stars in Table 1-2 were affected, with minor changes in spelling or a whole new name. In one case, a star was named after its constellation: Alpha Pavonis, in Pavo the Peacock, was itself named Peacock.

      Identifying stars would be much easier if they had little name tags that you could see through your telescope. If you have a smartphone, you can download an app to identify the stars for you. Just download a sky map or planetarium app (such as Sky Safari, Star Walk, or Google Sky Map) and face the phone toward the sky. The app generates a map of the constellations in the general direction your phone is facing. With some apps, when you touch the image of a star, its name appears. (I describe more astronomy apps in Chapter 2; for the full scoop on stars, check out Chapter 11.)

What do I spy? Spotting the Messier Catalog and other sky objects

      Naming stars was easy enough for astronomers. But what about all those other objects in the sky – galaxies, nebulae, star clusters, and the like (which I cover in Part 3)? Charles Messier (1730–1817), a French astronomer, created a numbered list of about 100 fuzzy sky objects. His list is known as the Messier Catalog, and now when you hear the Andromeda Galaxy called by its scientific name, M31, you know that it stands for number 31 in the Catalog. Today 110 objects make up the standard Messier Catalog.

      

You can find pictures and a complete list of the Messier objects at The Messier Catalog website of Students for the Exploration and Development of Space at messier.seds.org. And you can find out how to earn a certificate for viewing Messier objects from the Astronomical League Messier Program website at www.astroleague.org/al/obsclubs/messier/mess.html.

      Experienced amateur astronomers often engage in Messier marathons, in which each person tries to observe every object in the Messier Catalog during a single long night. But in a marathon, you don’t have time to enjoy an individual nebula, star cluster, or galaxy. My advice is to take it slow and savor their individual visual delights. A wonderful book on the Messier objects, which includes hints on how to observe each object, is Stephen James O’Meara’s Deep-Sky Companions: The Messier Objects, 2nd Edition (Cambridge University Press).

      Since Messier’s time, astronomers have confirmed the existence of thousands of other deep sky objects, the term amateurs use for star clusters, nebulae, and galaxies to distinguish them from stars and planets. Because Messier didn’t list them, astronomers refer to these objects by their numbers as given in other catalogues. You can find many of these objects listed in viewing guides and sky maps by their NGC (New General Catalogue) and IC (Index Catalogue) numbers. For example, the bright double cluster in Perseus, the Hero, consists of NGC 869 and NGC 884.

The smaller, the brighter: Getting to the root of magnitudes

      A star map, constellation drawing, or list of stars always indicates each star’s magnitude. The magnitudes represent the brightness of the stars. One of the ancient Greeks, Hipparchos (also spelled Hipparchus, but he wrote it in Greek), divided all the stars he could see into six classes. He called the brightest stars magnitude 1 or 1st magnitude, the next brightest bunch the 2nd magnitude stars, and on down to the dimmest ones, which were 6th magnitude.

      Notice that, contrary to most common measurement scales and units, the brighter the star, the smaller the magnitude. The Greeks weren’t perfect, however; even Hipparchos had an Achilles’ heel: He didn’t leave room in his system for the very brightest stars, when accurately measured.

      So today we recognize a few stars with a zero magnitude or a negative magnitude. Sirius, for example, is magnitude –1.5. And the brightest planet, Venus, is sometimes magnitude –4 (the exact value differs, depending on the distance Venus is from Earth at the time and its direction with respect to the Sun).

      Another omission: Hipparchos didn’t have a magnitude class for stars that were too dim to be seen with the naked eye. This didn’t seem like an oversight at the time because nobody knew about these stars before the invention of the telescope. But today astronomers know that billions of stars exist beyond our naked-eye view. Their magnitudes are larger numbers: 7 or 8 for stars easily seen through binoculars, and 10 or 11 for stars easily seen through a good, small telescope. The magnitudes reach as high (and as dim) as 21 for the faintest stars in