CHAPTER 2 | THE SKY 13
in the sky or to its brightness. A more useful way to identify stars
is to assign letters to the bright stars in a constellation in approxi-
mate order of brightness. Astronomers use the Greek alphabet
for this purpose. Th us, the brightest star in a constellation is usu-
ally designated alpha, the second brightest beta, and so on. Often
the name of the Greek letter is spelled out, as in “alpha,” but
sometimes the actual Greek letter is used, especially in charts.
You will fi nd the Greek alphabet in Appendix A. For many con-
stellations, the letters follow the order of brightness, but some
constellations, by tradition, mistake, or the personal preferences
of early chart makers, are exceptions (■ Figure 2-4).
To identify a star by its Greek-letter designation, you would
give the Greek letter followed by the possessive (genitive) form of
the constellation name; for example, the brightest star in the
constellation Canis Major is alpha Canis Majoris, which can also
be written Canis Majoris. Th is both identifi es the star and the
constellation and gives a clue to the relative brightness of the star.
Compare this with the ancient name for this star, Sirius, which
tells you nothing about location or brightness.In addition to the 88 offi -
cial constellations, the sky
contains a number of less for-
mally defi ned groupings called
asterisms. Th e Big Dipper,
for example, is a well-known
asterism that is part of the
constellation Ursa Major (the
Great Bear). Another asterism
is the Great Square of Pegasus
(Figure 2-2b), which includes
three stars from Pegasus plus
Alpheratz from Andromeda.
Th e star charts at the end of
this book will introduce you
to the brighter constellations
and asterisms.
Although constellations
and asterisms are groups of
stars that appear close together
in the sky, it is important to
remember that most are made
up of stars that are not physi-
cally associated with one
another. Some stars may be many times farther away than others
and moving through space in diff erent directions. Th e only thing
they have in common is that they happen to lie in approximately
the same direction from Earth (■ Figure 2-3).
The Names of the Stars
In addition to naming groups of stars, ancient astronomers gave
individual names to the brightest individual stars. Modern
astronomers still use many of those ancient names. Although the
constellation names came from Greek translated into Latin—the
language of science until the 19th century—most star names
come from ancient Arabic, though much altered by the passing
centuries. Th e name of Betelgeuse, the bright orange star in
Orion, for example, comes from the Arabic yad al jawza, mean-
ing “shoulder of Jawza [Orion].” Names such as Sirius (the
Scorched One) and Aldebaran (the Follower of the Pleiades) are
beautiful additions to the mythology of the sky.
Naming individual stars is not very helpful because you can
see thousands of them. How many names could you remember?
Also, a simple name gives you no clues to the location of the star
Great square
of PegasusPegasusAndromedaabAlpheratz■ Figure 2-2
(a) In antiquity, constellation boundaries were poorly
defi ned, as shown on this map by the curving dotted lines
that separate Pegasus from Andromeda. (From Duncan
Bradford, Wonders of the Heavens, Boston: John B. Russell,
1837.) (b) Modern constellation boundaries are precisely
defi ned by international agreement.