CHAPTER 2 | THE SKY 15
almost zero. A few are so bright the modern magnitude scale
must extend into negative numbers (■ Figure 2-6). On this scale,
our Favorite Star Sirius, the brightest star in the sky, has a mag-
nitude of −1.47. Modern astronomers have had to extend the
faint end of the magnitude scale as well. Th e faintest stars you
can see with your unaided eyes are about sixth magnitude, but if
you use a telescope, you will see stars much fainter. Astronomers
must use magnitude numbers larger than 6 to describe these faint
stars.
Th ese numbers are known as apparent visual magnitudes
(mV), and they describe how the stars look to human eyes observ-
ing from Earth. Although some stars emit large amounts of
infrared or ultraviolet light, human eyes can’t see those types of
radiation, and they are not included in the apparent visual mag-
nitude. Th e subscript “V” stands for “visual” and reminds you
that only visible light is included. Apparent visual magnitude
also does not take into account the distance to the stars. Very
distant stars look fainter, and nearby stars look brighter. Apparent
visual magnitude ignores the eff ect of distance and tells you only
how bright the star looks as seen from Earth.Magnitude and Intensity
Your interpretation of brightness is quite subjective, depending
on both the physiology of human eyes and the psychology of
perception. To be accurate you should refer to fl ux—a measure
of the light energy from a star that hits one square meter in one
second. Such measurements precisely defi ne the intensity of star-
light, and a simple relationship connects apparent visual magni-
tudes and intensity.
Astronomers use a simple formula to convert between mag-
nitudes and intensities. If two stars have intensities IA and IB,
then the ratio of their intensities is IA/IB. To make today’s mea-
surements agree with ancient catalogs, astronomers have defi ned
the modern magnitude scale so that two stars that diff er by fi ve
magnitudes have an intensity ratio of exactly 100. Th en two stars
that diff er by one magnitude must have an intensity ratio
that equals the fi fth root of 100, 5 √____
100 , which equals about
2.512—that is, the light of one star must be 2.512 times more
intense.were fainter, second-magnitude. Th e scale continued downward
to sixth-magnitude stars, the faintest visible to the human eye.
Th us, the larger the magnitude number, the fainter the star. Th is
makes sense if you think of the bright stars as fi rst-class stars and
the faintest visible stars as sixth-class stars.
Ancient astronomers could only estimate magnitudes, but
modern astronomers can measure the brightness of stars to high
precision, so they have made adjustments to the scale of
magnitudes. Instead of saying that the star known by
the charming name Chort (Th eta Leonis) is third mag-
nitude, they can say its magnitude is 3.34. Accurate
measurements show that some stars are actually brighter
than magnitude 1.0. For example, Favorite Star Vega
(alpha Lyrae) is so bright that its magnitude, 0.04, is
■ Figure 2-5
Favorite Stars: Locate these bright stars in the sky and learn why they are
interesting.
Betelgeuse AldebaranSirius RigelTaurusOrionCanisMajorPolaris
DipperBigDipperLittleCygnusVega
LyraSpicaVirgoCentaurusCentauriAlpha Crux
SouthernCrossSirius Brightest star in the sky Winter
Betelgeuse Bright red star in Orion Winter
Rigel Bright blue star in Orion Winter
Aldebaran Red eye of Taurus the Bull Winter
Polaris Th e North Star Year round
Vega Bright star overhead Summer
Spica Bright southern star Summer
Alpha Centauri Nearest star to the sun Spring, far south
■ Figure 2-6
The scale of apparent visual magnitudes extends into negative
numbers to represent the brightest objects and to positive num-
bers larger than 6 to represent objects fainter than the human
eye can see.
Apparent magnitude (mv)SunFull
moonVenus at
brightest
SiriusPolaris
Naked
eye limitHubble
Space
Telescope
limitBrighter Fainter–30 –25 –20 –15 –10 –5 0 5 10 15 20 25 30