Transcript Constellations, Star Names, and Magnitudes

Lecture 3 – The Sky
Constellations, Star Names, and Magnitudes
Announcements
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Homework Assignment 1 is due now.
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Homework 2 – Due Monday, February 5
Unit 5: Problem 2, Test Yourself 3
 Unit 6: Review Questions 1, 3
 Unit 9: Review Question 2, Test Yourself 1
 Unit 11: Review Question 2, Test Yourself 3
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The Constellations
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Origins
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Star patterns named by
ancient peoples after
gods, goddesses, animals,
monsters, and mythic
heroes.
The constellations used
by Western Culture today
originated in
Mesopotamia around
3,000 B.C.
The Constellations
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Only certain stars were part
of the pattern and belonged
to the constellation. Faint
stars were not part of any
constellations.
Frequently constellations
were named for their
brightest star.
Of the constellations defined
by the Babylonians,
Egyptians, and Greeks, 48
are still used today.
The Constellations
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The ancient civilizations
we got the 48 classical
constellations from
couldn’t see the entire
southern sky.
During the Age of Sail,
European explorers
added 40 modern
constellations in the
“uncharted” areas of the
sky.
The Constellations
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The 48 classical and the 40 modern constellations make
up the 88 official constellations used by astronomers
today.
The term constellation also now has a new meaning:
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No longer refers to the pattern of stars itself.
Now refers to a well defined region of the sky that contains
the traditional star pattern.
Everything inside that region of the sky is now part of the
constellation, like a “celestial state”.
The Constellations
What Is An Asterism?
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In addition to the 88
official constellations
there are several
unofficial but popular
star patterns.
Called asterisms.
Examples:
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The Big Dipper
The Little Dipper
The Northern Cross
The Great Square
Daily Grade 3 – Question 1
1. Which statement below most accurately
describes modern constellations?
A. They are 88 well defined regions in the sky.
B. They are 88 connect-the-dot mythological sky
figures.
C. They are 13 connect-the-dot mythological sky
figures along the ecliptic.
D. They are 13 well defined sky regions along the
ecliptic.
3D Constellations
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The stars of a
constellation only
appear to be close to
one another
Usually, this is only a
projection effect.
The stars of a
constellation may be
located at very
different distances
from us.
The Celestial Sphere
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Many ancient cultures,
including the Greeks and
Romans, assumed that the
Earth was stationary in the
center of the Universe.
Given this assumption, they
concluded that the Sun,
Moon, planets, and stars were
attached to gigantic
crystalline spheres that
surrounded the Earth.
Today we call this imaginary
globe the Celestial Sphere.
Celestial Sphere Concepts
THE CELESTIAL SPHERE REPRESENTS THE
APPARENT VIEW OF THE UNIVERSE AS SEEN
FROM THE EARTH.
 A huge globe of stars surrounding the Earth.
 An ancient concept of the Cosmos.
IT IS A USEFUL CONCEPT TODAY BECAUSE:
 it is understandable in terms of spherical geometry.
 it can be used to extend map concepts for the Earth to the sky.
 it is useful in navigation.
RELATIONSHIP TO THE VISIBLE SKY
 The visible sky is 1/2 the celestial sphere.
 Half of the celestial sphere is above the horizon, while the
other half is below the horizon.
The Celestial Sphere
Zenith = Point on the
celestial sphere directly
overhead
Nadir = Point on the
c.s. directly underneath
(not visible!)
Celestial equator =
projection of Earth’s
equator onto the c. s.
North celestial pole =
projection of Earth’s
north pole onto the c. s.
Angular Measure In The Sky
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360o = A
COMPLETE
CIRCLE
1o = 60’ (minutes)
1’ = 60” (seconds)
Latitude Affects Visible Sky
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What we see in the sky depends on our
latitude.
At the north pole, the north celestial pole is
at the zenith.
At the equator, the north celestial pole is at
the horizon.
The altitude of the north celestial pole =
latitude.
The Sky From Abilene – 32.4o N
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North Celestial Pole is 32.4o above the northern
horizon.
Celestial Equator is 57.6o above the southern
horizon.
Circumpolar stars lie within 32.4o of the North
Celestial Pole.
Rising & Setting stars are visible from 32.4o away
from the South Celestial Pole.
Stars within 32.4o of the South Celestial Pole are
never visible from Abilene.
Looking North
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The stars appear to circle around the North Celestial Pole
(near Polaris) once in 24 hours.
Stars within the angle of the observer's latitude away from
the North Celestial Pole never rise or set.
They are always above the horizon (circumpolar stars).
Circumpolar Constellations
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Never Rise Or Set
Are Always Above The Horizon
Circumpolar Region Depends On Latitude
Daily Grade 3 – Question 2
2. At what location on Earth is an observer that
has the South Celestial Pole directly overhead?
A.
B.
C.
D.
At Earth's equator (0 degrees latitude).
At Earth's North Pole (90 degrees North latitude).
At Earth's South Pole (90 degrees South latitude).
At 45 degrees North latitude.
Naming The Stars
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The brightest stars have had proper names for
thousands of years.
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Typically from Arabic
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Islamic astronomers produced many detailed star charts
during the middle ages.
Also many stars have names that are from the Greek
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Greek astronomers also produced many star charts.
Naming the Stars
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Proper names are often a literal description of the star’s
location in the constellation:
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Betelgeuse – “Shoulder of the giant”
Rigel – “Foot”
Deneb – “Tail”
Procyon – “Before the dog”
Algol – “Eye of the ghoul”
Star proper names also sometimes describe the star.
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Sirius – “Scorching”
Antares – “Rival of Mars”
Kochab – “Star”
Scam Alert!
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NO organization has been
given official legal power to
name the stars.
Astronomers consider the
names designated by the
International Astronomical
Union official, but the IAU
has no true legal authority to
name stars (or demote
planets, for that matter).
Companies that sell star
names as gifts are a SCAM!
Bayer Letter Names
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Johann Bayer
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Bavararian lawyer
Published a sky atlas:
Uranometria.
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Assigned lower case greek
letters to brighter stars in each
constellation.
Usually assigned in rough
order of brightness.
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Alpha = brightest star.
Beta = second brightest
Gamma = third brightest
And so on…
Astronomers continue to use
these “Bayer letters”
Bayer Letter Names
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A star’s Bayer Letter Name is:
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It’s greek letter first…
…followed by the possessive form of the constellation name.
Example: The star Rigel Kentarus is the brightest star in the
constellation Centarus:
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Alpha Centauri
Daily Grade 3 – Question 3
3. What is the most likely Greek letter name
of the second brightest star in the
constellation Lyra?
A.
B.
C.
D.
Alpha Lyrae.
Beta Lyrae.
Gamma Lyrae.
Delta Lyrae.
The Magnitude Scale
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Invented by the Greek
Astronomer Hipparchus in
the 2nd century B.C.
Called the brightest stars in
the night sky “stars of the
first magnitude.”
Second brightest were “stars
of the second magnitude.”
All the way down to “stars of
the sixth magnitude.”
The Magnitude Scale
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Very useful system, but not perfect
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Technically, it refers to Apparent Visual Magnitude
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Brightness in visible wavelengths as seen from Earth
Does NOT tell how bright the star actually is
(absolute magnitude) because it does not take into
account how far away the star is from us.
 Still useful because it is related to the brightness that
you “see” (even through a telescope).
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The Magnitude Scale
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Used for centuries, and then refined by modern
astronomers.
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1st magnitude stars are 100 times brighter than 6th magnitude
stars.
So 5 “steps” in the magnitude scale corresponds to a 100×
difference in brightness.
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What brightness difference corresponds to 1 step in magnitude?
Need to know what number you have to multiply by itself five times
to get 100.
That number is 2.5118864315095…. (usually rounded to 2.512 when
used in calculations).
So 1 step in magnitude corresponds to a 2.512× difference in
brightness.
The Magnitude Scale
If Star A is has a magnitude
this much less than Star B…
1
…then Star A is this many
times brighter than Star B.
2.512×
2
6.310×
3
15.85×
4
39.82×
5
100×
The Magnitude Scale
The magnitude scale system can be extended towards negative
numbers (very bright) and numbers > 6 (faint objects):
Sirius (brightest star in the sky): mv = -1.42
Full moon: mv = -12.5
Sun: mv = -26.5
The Magnitude Scale
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Here’s the actual equation:
IA/IB =
(m
-m
)
(2.512) B A
This thing in
parentheses here is
an exponent. So
say mB – mA is a
number we’ll call X.
You would read this
equation as “2.512
raised to the X
power.”
This number right here is “how bright star A is
compared to star B”
If it is >1, then star A is brighter than star B.
If it is <1, then star A is fainter than star B.
Daily Grade 3 – Question 4
4. The apparent visual magnitude of star A is 2
and the apparent visual magnitude of star B is
1. Based on this information which statement
below must be true?
A.
B.
C.
D.
Star A emits more light than star B.
Star B emits more light than star A.
Star A is closer than star B.
Light output and distance cannot be determined
from a star's apparent visual magnitude alone.
Announcements
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Read Units 9 and 11
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Test 1 is already approaching
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Wednesday, February 7