THE SKY
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Since ancient times,
people have named
groups of the stars for
heroes, gods and
mythical beasts.
These groups of stars
are called constellations.
Different cultures
named groupings of
stars different names
but every star is a
member of one and only
one constellation.
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While there are 88 official
constellations, the sky also
contains asterisms, which
are star groupings that are
more loosely defined.
The Big Dipper is an
asterism that is part of the
constellation Ursa Major
(The Great Bear).
Constellations and
asterisms aren’t
necessarily closely
associated with one
another, they just lie in
approximately the same
direction from Earth.
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Most constellation names
come from Greek
translated into Latin from
the fall of Rome to the 19th
century. Most are named
for mythological creatures
and heroes.
However, most star names
come from ancient Arabic.
Betelgeuse, the bright red
star in Orion comes from
the Arabic yad al jawza,
which means the armpit
of Jawza (Orion).
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Variable stars
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RR Lyrae
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Cepheid variables
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Red giants
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Brown dwarfs
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White dwarfs
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Supergiants
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Neutron stars
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Variable stars are stars that
vary in brightness over
time. These changes
usually occur very slowly,
over a period of months or
years, but sometimes it can
take just hours.
Some types of variable stars
are red giants, eclipsing
binaries, RR Lyrae and
cepheid variables.
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A binary star is a
double star system in
which two stars orbit
one another around a
central point of gravity.
An eclipsing binary
occurs when the plane
of a binary’s orbit is
nearly edgewise to our
line of sight.
Sirius α and β are binary
stars.
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RR Lyrae variables are
periodic variable
stars, commonly
found in globular
clusters, and often
used as standard
candles to measure
galactic distances.
Because of their age,
they are relatively
dim.
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Cepheid variables as seen
from Hubble telescope
Cepheid stars oscillate
between two states: In one
of the states, the star is
compact where large
temperatures and pressure
gradients build up in the
star. When the star is in its
expanded state, the pressure
weakens and the star
contracts back to a
compressed state.
Cepheid variable stars have
masses 5-20 times the mass
of our star. The more
massive stars are luminous
with extended envelopes
that are outer layers of gas.
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The most common of the variable stars, red
giants are stars of average size, similar to our
sun, in the final stages of life.
In the last several million years of its life, the star
will become alternately brighter and dimmer,
spending about a year in each phase until it
eventually runs out of fuel.
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Brown dwarfs are objects which
have a size between that of a giant
planet like Jupiter and that of a
small star.
It is thought they are made of gas
and dust but lack the size it takes
to produce enough pressure to
begin nuclear fusion. In theory,
an object with less than 8% of the
mass of the sun cannot become a
star.
So why would we care about
brown dwarfs? It is possible that a
great deal of the mass in the
universe is in the form of brown
dwarfs, and since they do not give
off much light, they could
constitute part of the "missing
mass" problem faced by
cosmology.
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About 1,600 light-years away,
two dense white dwarfs in the
J0806 binary star system orbit
each other once every 321 seconds.
When they reach the end of their
long evolutions, smaller stars
typically become white dwarfs.
White dwarfs are formed
from stars about the size
of our sun when their
hydrogen is used up.
In the absence of fusion,
gravity takes over and
causes the star to collapse
upon itself.
The bigger the original
star, the smaller the white
dwarf it becomes. Due to
the strong gravitational
field, the larger stars
collapse more completely
than smaller stars.
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Supergiants represent
the final stages of life
of only the most
massive stars and
hence are quite rare.
It’s believed that at
the end of a
supergiant’s life, there
is a supernova which
then creates a neutron
star or a black hole.
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Blue supergiants are rare but burn bright
and hot at surface temperatures of 20,00050,000o C
Blue supergiants represent a slower burning
phase in the death of a star.
Rigel, the brightest star in the Orion
constellation, is a blue supergiant
You can catch Orion in the east before dawn
in late summer, but on January evenings
Orion is riding at its highest in the midevening sky. Look for Orion high in the
south on these Northern Hemisphere winter
evenings. By early March, Orion – with
blue-white Rigel in its midst – is high in the
south as soon as the sun sets. By early May,
it is setting before the sky has a chance to
get really dark.
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The red supergiant is a star
that is bigger and more
massive than the red giant.
Red supergiants usually have
a diameter which would be
several hundred times that of
the Sun and they would
weigh 10 times more than the
Sun.
The red supergiant phase is
very short lived, they only
last for a hundred to a
million years before
exploding into a supernova.
Although red supergiants are
very rare, clusters of them do
exist in space.
More than two dozen stars, appearing yellow near the
center of this Spitzer Space Telescope image of the
cluster RSGC2 are red supergiants. Warm dust in the
region glows red. The blue oval with a pink outline at
top left may be the result of an ancient supernova and
the larger blue patch below center is a region of current
star formation.
Image credit: B. Davies/RIT/NASA
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A neutron star is a type of
stellar remnant that can
result from the gravitational
collapse of a massive star
during a Type II, Type Ib or
Type Ic supernova event.
In Chandra's image (right),
the colors of red, green, and
blue are mapped to low,
medium, and high-energy
X-rays. At the center, the
bright blue dot is likely the
neutron star that
astronomers believe formed
when the star exploded.
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A nova occurs when one
member of a binary star system
temporarily becomes brighter.
Most often the brighter star is a
shrunken white dwarf and its
partner is a large star, such as a
red giant.
From time to time (50 years or
so), matter is transferred from
the larger star to its smaller
partner, initiating a nuclear chain
reaction on the smaller star’s
surface.
When the reaction ceases, the
material blows off the star,
causing it to glow brightly.
Days or weeks later the star
fades and the process begins
again.
This is the aftermath of supernova 1987A – a shock wave of material unleashed in the
blast slammed into a ring of debris likely shed by the star 20,000 years previously.
Image: NASA, ESA, P. Challis and R. Kirshner (Harvard-Smithsonian Center for
Astrophysics)
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In 1603, Bavarian lawyer
Johann Bayer published an
atlas of the sky called
Uranometria. He assigned
lower-case Greek letters to the
brighter stars of each
constellation.
The brightest star is usually
designated α (alpha), the
second brightest β (beta), γ
(gamma) and so on.
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Sirius, or Isis, is an alpha star
and the brightest we can see.
Sirius α is about 8.5 ly away and
heading toward Earth at many
1000’s of miles per second. Its
annual heliacal rising exactly
matches Earth’s solar year at
365.25 days.
Ancient Egyptians knew this
and marked the rising of Isis
with the start of their calendar
year.
New Year’s marks the return of
Sirius to the midheaven position
at midnight.
Sirius B is a white dwarf star.
Sirius β spins on its axis 23 times
per minute, creating an immense
magnetic field.
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Betelgeuse
Polaris
Sirius
Rigel
Aldebaran
Spica
Vega
Alpha Centuri
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Translation of name
Age of star
Type of star
Size of star relative to our sun
Magnitude of star
Constellation
Distance from Earth
When you can best see it from
Grand Junction.
Star speed of rotation/Time it
takes to rotate
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“armpit of Orion”
7x106 years old
Red super giant
Size: 700-900 x the size of
the sun
Magnitude: 0.42
Constellation: Orion
Distance: 642 ly
Best seen: Jan-mid March
Speed of rotation: 5 km/s
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“The Bear’s Tail”
Age: 7 x10 7 years
Super Giant
Size: 46 x the size of Earth
Magnitude: 1.985
Constellation: Ursa minor
Distance from Earth: 434
light years
Best seen: year round
Speed of rotation: 17
km/s
Rotates in 19 hours
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“Glowing or Scorcher”
“Dog Star”
Age: 4-6 million years
Type: White dwarf, blue
in color
Size: 2 x larger than our
sun
Magnitude: -1.46
Constellation: Canis
major
Distance: 860 light years
Best seen: winter and
spring
Speed of rotation: 16
km/s
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“Central one” or “Right
foot”
Age: < 3.4 x 109 years
Type: Blue super giant
Size: 60 x larger
Magnitude: .12
Constellation: Orion
Distance from Earth:
77.28 light years
Best seen: Jan-mid March
Speed of rotation: 40
km/s
6th Brightest star—40,000 x
brighter than our sun
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“The follower”
Age: 10 million
Type of star: Orange giant
Size: 44 x bigger than our
sun
Magnitude: 0.87
Constellation: Taurus
Distance from Earth: 65 light
years
Best seen: summer
Rotation: 5.2 km/s
Rotates in 643 days
The fiery eye of the bull, it
burns 153 x brighter than our
sun
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“Ear of corn”
Age: 275 million years old
Type of star: blue dwarf
Size: 7-10 x larger than our
sun
Magnitude: 0.98
Constellation: Virgo
Distance from Earth: 275 ly
or 80 parsecs
Best seen: spring to late
summer
Speed of rotation: 199 km/s
It is a whirling binary star.
15th brightest star from Earth
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Name: Swooping
eagle/falling
Age of star: 3.4 x 108 years
Type of star: Variable
white dwarf
Size: 2x the size of our
sun
Magnitude: 0.03
Constellation: Lyra
Distance from Earth: 25 ly
Best seen: summer
Speed of rotation: 236
km/s
Time of rotation: 12.5 hrs
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Name: Brightest star in
Centaurus
Age: 5-6 x 109 years
Type of star: Yellow star
Size: 110% the size of our
sun
Magnitude: -0.27
Constellation: Centaur
Distance: 4.367 ly
Best seen: only in southern
regions of U.S.
Speed of rotation: 2.7 km/s
4th brightest star
Closest star to Earth
Embryonic Stars Emerge from Interstellar "Eggs“
“Evaporating Gaseous Globules”
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Ancient astronomers divided
the stars into six magnitudes
with the brightest called firstmagnitude stars and the least
visible sixth-magnitude stars.
Modern astronomers have a
magnitude scale that starts with
the brightest stars with negative
numbers. This is because
Hubble can detect stars as faint
as 30th magnitude so the scale
was readjusted.
Sirius, the brightest star in the
sky, has a magnitude of -1.44,
the sun is -26.5 and the moon is
-12.5.
These numbers are apparent
visual magnitudes that describe
how the stars look to human
eyes observing from Earth.
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Because light brightness
is subject to the
physiology of the eye
and psychology of
perception, an accurate
reference to starlight is
flux. It is the measure
of the light from a star
that hits 1 square meter
in 1 minute. This
measurement is directly
related to the intensity
of the light.
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Star brightness is
determined by
comparing the
intensities of two stars,
Ia and Ib by calculating
their ratios:
Ia ÷ Ib
For further details on
how to calculate
modern magnitudes,
please refer to page 17
of the textbook.
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The celestial sphere
seems to rotate the stars,
moon and sun in
westward around Earth.
You can only see the area
above the horizon from
any place on Earth.
The celestial equator lies
between the north and
south celestial poles with
the east-west points in
between.
The zenith marks the top
of the sky above your
head and the nadir marks
the bottom of the sky
underneath your feet.
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Astronomers measure distance
across the sky as angular
distances in degrees, minutes
and seconds of arc.
A minute of arc is 1.60th of a
degree and a second of arc is
1/60th of a minute of arc.
Angular diameter is the angular
distance from one edge to
another.
Circumpolar constellations
never rise or set. These include:
Ursa Major, the Big Bear
(includes the Big Dipper)
Ursa Minor, the Little Bear
Cassiopeia, the Queen of
Ethiopia
Cepheus, the King of Ethiopia
Draco the Dragon
Secrets of the Sun (53:06)
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The seasons are not caused by any variation in the
distance from Earth to the sun. What are they
caused by?
They are caused by the changes in the solar energy
that Earth’s northern and southern hemispheres
receive at different times of the year.
The two equinoxes mark the beginning of what
seasons?
Vernal equinox is the beginning of Spring and the
autumnal equinox is the beginning of Fall.
What two seasons are marked by the solstices?
Winter and Summer
Earth’s lighting and the
winter solstice
Earth’s lighting and the
summer solstice
Passive solar design
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The ecliptic is the
apparent path of the
sun in its yearly motion
around the sky.
It is a projection of
Earth’s orbit on the sky.
You can also define it as
extending the plane of
Earth’s orbit out to
touch the celestial
sphere.
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The Zodiac is a band 18” wide
centered on the ecliptic; a
“highway” that the planets follow.
Divided into 12 segments, they are
named for the constellations and
represent the signs of the zodiac.
Horoscopes show the location of the
sun, moon and planets among the
zodiacal signs with respect to the
horizon at the moment of a person’s
birth as seen from that longitude
and latitude.
Horoscopes are specific to an
individual unlike the ones you may
read in the newspapers.
Astrology is a pseudoscience that
depends on belief rather than
evidence as true science does.
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In 1920, Yugoslavian meteorologist Milutin
Milankovitch proposed that changes in the
shape of Earth’s orbit, precession and
inclination affect Earth’s climate and
trigger ice ages.
The elliptical shape of Earth’s orbit varies
every 100,000 years. Currently, we are 1.7
% closer than average to the sun during
northern hemisphere winters and 1.7 %
farther away in northern hemisphere
summers, making the northern climate
slightly less extreme.
Second factor is precession which causes
Earth’s axis to sweep around a cone with a
period of about 26,000 years which changes
the location of the seasons around Earth’s
orbit. In 13,000 years, northern summers
will occur on the side of Earth’s orbit
where it is slightly closer to the sun and
summers will be warmer.
Third factor is the inclination of Earth’s
equator that varies from 22o to 24o with a
period of about 41,000 years. When the
inclination is greater, seasons are more
severe.