Download The Milky Way

Guidepost
The previous chapter took you on a cosmic zoom
through space and time. That quick preview only sets
the stage for the drama to come. Now it is time to return
to Earth and look closely at the sky and answer four
essential questions:
• How do astronomers refer to stars and compare their
brightness?
• How does the sky appear to move as Earth rotates?
• What causes the seasons?
• How can astronomical cycles affect Earth’s climate?
As you study the sky and its motions, you will be
learning to think of Earth as a planet rotating on its axis.
The next chapter will introduce you to some of the most
dramatic cycles in the sky.
Constellations
In ancient times, constellations only referred to the
brightest stars that appeared to form groups. We
continue to use many of the same groupings today
Constellations
The stars in constellations are not
physically close to each other in space.
They were believed to represent great heroes
and mythological figures. Their position in the
sky seemed to tell stories that were handed
down from generation to generation over
thousands of years.
Different cultures grouped stars differently.
Example: The Pawnee Indians knew the
constellation Scorpius as 2 groupings
-The long tail was the snake
-The two bright stars at the scorpion’s
tail were the swimming ducks
Scorpius Constellation
Constellations
Projection
Effect:
The stars of a
constellation
may be located
at very different
distances from
us.
Example:
Cassiopeia
Northern Hemisphere
Constellations
Typically named after animals and mythological beings
Example-Greeks, Northern Asians, and Native
Americans
Constellations (5)
Stars are named by a Greek letter (a, b, g) according to
their relative brightness within a given constellation +
the possessive form of the name of the constellation:
Orion
Betelgeuse
Rigel
Betelgeuse = a Orionis
Rigel = b Orionis
Constellations (6)
Some examples of easily recognizable
constellations and their brightest stars
Greek Letter Alphabet
The Magnitude Scale
First introduced by Hipparchus (160 - 127 B.C.):
• Brightest stars: ~1st magnitude
• Faintest stars (unaided eye): 6th magnitude
More quantitative:
• 1st mag. stars appear 100 times brighter than 6th mag.
stars
• 1 mag. difference gives a factor of 2.512 in apparent
brightness (larger magnitude => fainter object!)
The Magnitude Scale (Example)
Betelgeuse
Magnitude = 0.41 mag
For a magnitude difference
of 0.41 – 0.14 = 0.27, we
find an intensity ratio of
(2.512)0.27 = 1.28.
In other words, Rigel is 1.28
times brighter than
Betelgeuse.
Rigel
Magnitude = 0.14 mag
The Magnitude Scale (2)
The magnitude scale system can be extended
towards negative numbers (very bright) and
numbers greater than 6 (faint objects):
Sirius (brightest star in the night sky): mv = -1.42
Full moon: mv = -12.5
Sun: mv = -26.5