Download Chapter 2: The Sky

Chapter 2: The Sky
The daily motion of the sky is
caused by the rotation of our
planet.
Stars
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Constellations- groups of stars
Stars we know in the Western culture originated in
Mesopotamia over 5000 years ago.
Other constellations were added by the Babylonian,
Egyptian, & Greek astronomers
48 constellations are still in use of all the ancient one
named
Stars
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Each culture grouped and named constellations differently.
According to ancient astronomers, a constellation was a loose
grouping of stars.
– Many fainter stars were not included
– Regions of the southern sky not visible to the astronomers in
the northern latitudes were not identified with constellations
– If any constellation boundaries existed, they were only
approximate.
Alpheratz was part of Pegasus or part of
Andromeda
What’s in a
name:
Arabic for “horse’s
shoulder”
Claim to
fame:
Brightest star in Andromeda. At
one tome it was part of
Pegasus.
Type of star:
White Main Sequence Star
How far
away:
72 light years away
How bright:
About as bright as the Sun
Where to
view:
In the constellation
Andromeda, the princess
When to
view:
August through December
Stars
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1928, the International Astronomical Union
established 88 official constellations with clearly defined
boundaries.
As a result, a constellation now represents an area of the
sky and any star within that region belongs to only one
constellation.
Asterisms
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A named group of stars not identified
as a constellation
The Big Dipper, which is part of Ursa
Major (the Great Bear)
The Great Square, which is three stars
from Pegasus and one from
Andromeda
Constellations & Asterisms
Most are made of stars that are not physically
associated with one another
 Some may be many time farther away than
others and moving through space in different
directions
 Only commonality among stars is they lie in
approximately the same direction from Earth
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The Names of the Stars
Names come from Greek (mythology) names
translated into Latin, which is the language of
science
 But most names come from ancient Arabic
 Sirius (the Scorched One), Capella (the
Little She Goat), Aldebaran (the Follower
of the Pleiades)
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Naming the Stars
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Johann Bayer, a lawyer by trade, published an atlas of the
sky called Uranometria, in which he assigned Greek letter to
the brighter stars of each constellation in approximate order of
brightness
Brightest star is alpha, next brightest is beta
Greek letter followed by the possessive form of the constellation
name
This method only approximately identifies a stars brightness
Greek Alphabet
a
b
g
d
e
z
h
Q
Alpha
Beta
Gamma
Delta
Epsilon
Zeta
Eta
Theta
i
k
l
m
n
x
o
p
Lota
Kappa
Lambda
Mu
Nu
Xi
Omicron
Pi
r
s
t
u
f
c
Y
W
Rho
Sigma
Tau
Upsilon
Phi
Chi
Psi
Omega
Brightness of Stars
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Brightness is measure by using a magnitude
scale
First appeared in writing 140 AD by Ptolemy,
but was probably used by Hipparchus (160127 BC)
Stars are divided into six classes: brightest
stars are first-class stars, fainter stars are
second-class and so on until the sixth class of
stars is reached
Brightness of Stars
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As telescopes were developed, fainter stars could be seen so
the magnitude scale was extended to include fainter stars.
The Hubble Space Telescope is currently (copyright of book
2005) the most sensitive astronomical telescope and can detect
stars fainter that +28 magnitude.
The brightest stars are brighter than first class stars so the scale
has been extended to include these stars also.
Example: Vega (alpha Lyrae) is almost zero magnitude at 0.04,
Sirius (the brightest star in the sky) has a magnitude of -1.42,
the sun has a magnitude of -26.5 and the moon -12.5
Apparent Visual Magnitude
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The numbers associated with the star brightness is apparent
visual magnitude (mv)
Describes how stars look to human eyes
Apparent visual magnitude ignores the effect of distance
and tells us only how bright the star looks as seen from
Earth
Magnitude & Intensity
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Magnitude depends on the physiology
of the eye and psychology of
perception.
Intensity is more accurate and precise
Intensity is a measurement of the light
energy from a star that hits one
square meter in one second
Magnitude & Intensity
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The human eye senses brightness by
comparing the ratios of the objects
intensities.
Constant intensity ratio= 2.5
See table 2-1, page 14
IA/IB=(2.512) (mB-mA)