Transcript Week 7 Notes

Stars &
Constellations
Week 7
What is a star?
• a ball of gas held together by its own gravity
• gravitational force continually tries to collapse
the star by pulling gas molecules toward its
center
• Gravity is counteracted by the pressure of the
hot gas inside the center of the star
• Gravity pulls inward, and the pressure pushes
outward
• The two forces balance each other and the star
maintains a spherical shape
The Sun
• The Sun is our own special star
• Fueled by nuclear fusion reactions
• an active star that displays
•
•
•
•
sunspots
solar flares
erupting prominences
coronal mass ejections
– These phenomena impact our near-Earth
space environment
– determine our "space weather."
Magnitude
• Invented by the ancient Greeks around
150 B.C.
• 2 kinds of magnitude
– Apparent - how bright stars look to us in the
sky from here on Earth
– Absolute – how bright a star would be if all the
stars were equal distance away from the
Earth
Classification
• Stars are classified by temperature and color
• O stars are the hottest
– Extremely bright, very rare
• M stars are the coolest
– Very common, fairly dim
• Color of star is connected to temperature
– Hot stars are blue
– Cool stars are red
– In between stars are white, yellow, or orange
• Sun has a medium temperature
– Yellow star
Classification
Star Type
Color
Average Luminosity
(Sun = 1)
Example
O
Blue
1,400,000
10 Lacertra
B
Blue
20,000
Rigel Spica
A
Blue
80
Sirius
F
Blue -white
6
Canopus
G
White - yellow
1.2
Sun
K
Orange - red
0.4
Aldebaran
M
Red
0.04
Betelgeuse
Stargazing
• How we interpret our observations depends on our point
of view
• Media gives us many points of view
• We have a better understanding of the natural world
• We don’t rely on legends to explain the movements of
the sun, moon, and stars.
• Stars hold the mystery of blazing nuclear furnaces
scattered in the vastness of space.
Historical Observations
• Early Greeks gave names to star-shaped figures from
their religious and cultural stories.
• These stories are called myths.
• For centuries man has wondered whether the
numberless specks in the sky could affect their lives.
• People who studied the stars were called astrologers
• Astrology became a religious practice.
– Became part of the the Babylonian religion
• The zodiac was a sacred pathway for the sun and the planets
• Stars also were signposts for sailors and travelers.
Historical Observations
•
•
•
•
Ancient people told time by the sky
Moon measured the month
Stars marked the year and season
Observers noticed that the stars set 4 minutes earlyier
every night.
• As people studied the sky, they observed at least four
different motions:
– Most stars rise in the east and set in the west;
– Both the sun and moon rise in the east and set in the west;
– The sun appears over the horizon farther north in summer and
farther south in winter;
– The moon does not always rise at the same time.
Constant Motion
• The Earth turns on its axis at 1,600 kilometers per hour
• The Earth moves around the Sun at 107,200 kilometers per hour
• Our solar system moves around the Milky Way at 69,200 kilometers
per hour.
• The Milky Way Galaxy is moving toward the star Vega at 20
kilometers per SECOND
• Our solar system is located in the Milky Way Galaxy
• we cannot look at the Milky Way from the outside to observe its size
and shape
• Outer space is so large that a measurement was invented to
express the large distances.
• A light year is the distance that light travels in one year.
– One light year is almost 6 trillion (6,000,000,000,000) miles
Constant Motion
• Instruments measure the speed of light as 186,000 miles
per second
• The nearest star, besides the Sun, is Proxima Centauri
– It is 4.3 light years away
Star Charts
• Constellations are star groups that seem to travel
together in space.
• Star charts are used to help follow the stars
• Each season has
its own pattern of
stars
Northern Circumpolar
Constellations
Winter Sky
• February is the best month to view stars
• Orion is the best known constellation
– 3 stars make up Orion’s belt
– Betelgeuse marks his right shoulder
• Betelgeuse has a diameter of 400 million miles
– Rigel is Orion’s left knee
• To the southeast is Sirius, the dog star
– It is the nose of Canis Major
• To the northwest is Taurus the Bull
– The horns contain Aldebran
– Pleaides, the seven sisters, are located in the shoulder of the bull
Northern Hemisphere Winter Sky
Spring Sky
• Best known constellation – Ursa Major, the big bear
– 2 end stars point to Polaris, the North Star
• North Star
– Not one of the brightest stars
– Can be used to find direction
– Is the end star in the handle of the Little Dipper
• Southeast of the arc in handle of Big Dipper leads to Arcturus.
– Arcturus is in Boötes
• East horizon is Libra
• Most prominent constellation is Leo
– Looks like a question mark
– Star Regulus is the period
Northern Hemisphere Spring Sky
Summer Sky
• The summer triangle
• Lyra, the harp – contains Vega
• East of Lyra is Cynus, the swan – The northern cross
– Brightest star is Deneb in the tail of the swan
• Below Cynus to the southeast is Aquila, the eagle
– Brightest star is Altair
• Altair, Vega, Deneb make up the summer triangle
• West of Lyra is Hercules
• Hercules’ foot is on the head of Draco, the dragon
• South of Lyra is Sagittarius, the archer
• On southwestern horizon is Scorpius, the scorpion
Northern Hemisphere Summer Sky
Autumn Sky
• Autumn sky is least spectacular of the four seasons
– Summer triangle moved off to the west
– Orion is just rising on the eastern horizon
Autumn Sky
The Perennial Sky
• Polar stars are found around Polaris
• These stars never set in the northern hemisphere
• They are called circumpolar stars
– Include constellations:
– Little Dipper, Big Dipper, Draco, Cassiopeia,
Cepheus, Camelopardalis
What is a constellation?
• a group of stars that seem to form a picture in
the night sky.
• The Greeks recognized and named forty-eight
constellations. Many of these constellations
were also recognized by the Arabs, Egyptians,
and the Babylonian.
• In 1928, the International Astronomy Union fixed
the boundaries of the eighty-eight constellations
in the sky.
• The names of the constellations are given in
Latin, because Latin was once the language of
learning.
What is a constellation?
• For convenience, the ancients named the constellations
after a certain figure that it seemed to form in the
sky. They refer to mythological or legendary men,
women, and animals.
• For thousands of years, people have used their
knowledge of the constellations to guide them from place
to place.
• By knowing the positions of the constellations, it is
possible to locate stars, planets, comets, and meteors.
• The constellations appear to move westward as the
Earth rotates around the axis. For this reason, certain
constellations can be seen only during one of the
seasons of the year.
Andromeda
• Andromeda's mother, Queen Cassiopeia, bragged that she
was prettier than the sea nymphs. The nymphs complained
to Poseidon, who in turn sent a monster to destroy her land.
The queen and her husband, King Cepheus, were told to
sacrifice their daughter to save the country.
• Andromeda was chained to
a cliff for the monster, called
Cetus. Just as the monster
was ready to bite down on
the maiden, Perseus
rescued her. Perseus and
Andromeda were put in the
sky along with Cepheus,
Cassiopeia and Cetus.
Aquarius
• In Greek myth, Aquarius was Ganymede, the young boy
kidnapped by Zeus. Zeus sent his eagle, Aquila, to
snatch Ganymede out of the fields where the boy was
watching over his sheep.
Ganymede would become
the cupbearer for the
Olympian gods.
Aries
• Aries represents the ram of the Golden Fleece. Zeus, a
god, once married a mortal woman named
Nephele. They had two children together. Their names
were Phrixus and Helle. Once as a gift, Zeus gave
Nephele a winged-ram of Golden Fleece. Soon Zeus
married another mortal woman, her name was Ino. She
hated Nephele’s children and wanted to kill them.
Aries
• To keep her children safe, Nephele sent them away on
the back of the winged-ram. The ram flew east. One
day as the ram and the children were gliding through the
air Helle fell off. She fell right between Europe and Asia
, but Phrixus made it safely to Colchis . There, Phrixus
sacrificed the ram and gave its Golden Fleece to King
Aeetes. Later, Zeus decided to put the figure of the ram
in the sky as a constellation, so he would never forget
Nephele, Helle, or Phrixus
Boötes
• Boötes
• generally considered to be a
Herdsman, as he eternally
shepherds the stars around the
North Pole.
• first recorded appearance
being in Homer's Odyssey
• squeezed between Canes
Venatici and Hercules, with
Virgo to the south
• The northern border touches
both Ursa Major and Draco
Boötes
• The association of the
Dipper with the Greater
Bear leads Boötes to be
identified with the story
of Zeus' seduction of
the maiden Callisto,
who was turned into a
bear as result of the
jealousy of Zeus'
spouse, the goddess
Hera. Callisto was the
daughter of King
Lycaon of Arcadia. She
bore the boy Arcas as
the result of her liaison
with Zeus.
Cancer
• According to an ancient
Greek legend, the figure
of a gigantic crab was
placed in the nighttime
sky by the goddess Hera
to form the constellation
Cancer. Hera swore to kill
Heracles, the most
famous Greek hero. Hera
attempted to kill Heracles
in many different ways, but each time his incredible physical
strength allowed him to survive. The Romans called him
Hercules. Hera cast a spell of madness on Heracles, causing
him to commit a great crime. In order to be forgiven, he had to
perform twelve difficult tasks. One of these tasks was
destroying the terrible nine-headed water-serpent, Hydra.
Cancer
During the battle between Heracles and Hydra, the goddess
Hera sent a giant crab to aid the serpent. But Heracles, being
so strong, killed the crab by smashing its shell with his foot. As
a reward for its service, Hera placed the crab's image in the
night sky.
Canis Major
• A magical dog which was destined never to surrender a
chase. It was first bestowed on Europa by Zeus, who
passed it to her son Minos, and from him to Procris and
Cephalus. The last of these set it to hunt down the
Teumessian fox, which was destined never to be caught. To
resolve the contrary fates of the two animals,
• Zeus placed them amongst the
stars as the constellations Canis
Major and Minor to play out the
chase unresolved for eternity.
Canes Venatici
• It represents the two dogs
Asterion and Chara, both
held on a leash by Boötes
as they apparently chase
the Great Bear around
the North Pole.
• With one exception, the
constellation's stars are
quite faint, fourth- and
fifth-magnitude stars
Capricornus
• This constellation depicts the result of the sudden
appearance of the earthborn giant Typhoeus. Bacchus
was feasting on the banks of the Nile at the time, and
jumped into the river. The part of him that was below
water was transformed into a fish, while his upper body
became that of a goat. From this point of view, he saw
that Typhoeus was
attempting to tear Jupiter
into pieces; he blew a
shrill note on his pipes,
and Typhoeus fled.
Jupiter then placed the
new shape of Bacchus in
the heavens out of
thanks for the rescue.
Cassiopeia
• A Queen of Ethiopia, mother of the
lovely Andromeda. When she
boasted that her daughter was more
beautiful than the Nereids, Poseidon
sent a sea monster to devour the girl.
She was rescued by Perseus, who
slew the beast. As a memorial the
whole family were placed amongst
the stars, but Cassiopea because of
her pride, was set to hang eternally
upside down on her throne.
Cepheus
• A King of Aethiopia and father of
the lovely Andromeda. He was
forced to sacrifice his daughter to
a sea monster because the boasts
of his wife Cassiopeia offended
the gods. But the hero Perseus
slew the beast and rescued her.
As a memorial the whole family Cepheus, Cassiopeia, Andromeda
and Perseus - were placed
amongst the stars.
Coma Berenices
• Coma Berenices is sandwiched between the Hunting Dogs,
Canes Venatici to the north, Virgo to the south, Leo on the
west border and Boötes on the east border.
• When the King went to war,
Berenice promised her hair to
Aphrodite, the Greek Goddess
of Love, if her husband
returned safely and victorious.
After the king’s return Berenice
kept her promise though the
loss was hard. Aphrodite
herself brought the hair to the
stars.
Cygnus
• When Zeus wanted to seduce
the goddess Nemesis he
transformed himself into a
swan, and bade Aphrodite in
the guise of an eagle pursue
him into her lap. As a memorial
of this successful ruse he
placed an eagle and swan in
the sky as the constellations
Cygnus and Aquila.
Draco
• circumpolar constellation
• visible all night from northern
latitudes
• winds around the little dipper
• At one time Draco was quite a bit
larger when the ancient
Mesopotamians gave the dragon
large wings which wound around
Ursa Major.
• The Greek philosopher Thales
lopped off the wings in the sixth
century BC.
Draco
• Draco the dragon is famous
throughout mythology. One of the
more popular stories involves
Heracles and the twelve labors.
Gaia gave Hera a golden apple
tree when she married Zeus. Hera
put the tree in the garden to be
guarded by the Hesperides and a
dragon called Ladon.
• Heracles asks Atlas to gather the
apples while he and Athena held
up the sky. Atlas, thinking he could
trick Heracles into holding the sky
forever, gladly accepted the
mission. When he returned with the
apples, Heracles slipped the sky
back on Atlas' shoulders.
Gemini
• The constellation represents the
twins Castor and Pollux. The
Twins are really only halfbrothers. They share the same
mother Leda, but have different
fathers. Castor's father, and
Leda’s husband, was Tyndareus,
the king of Sparta. The father of
Pollux was none other than
Zeus. Zeus visited Leda on her
wedding night disguised as a
swan. Castor was mortal and
Pollux was not. When Castor
was killed in battle, Pollux asked
if he could share his immortality
with his brother. His wish was
granted and they were placed in
the sky together.
Hercules
• Hercules is the figure of the most famous Greek hero,
Heracles. Heracles was a muscular man endowed with
extraordinary courage and physical strength. The ancient
Romans called him Hercules. Heracles was the son of
• the sky god Zeus and the queen
of Tiryns, Alcmene. When Hera
discovered Zeus' infidelity, she
decided to kill the child born from
the Zeus' relationship with
Alcmene. That child was
Heracles. Hera placed two
serpents in Heracles' cradle, but
the child strangled the snakes
with his bare hands, clearly
Hercules
• revealing his divine nature. This and many other heroics
made Heracles famous throughout the ancient world.
Hera was so annoyed at Heracles' growing fame that
she cast a spell of madness over him. Out of control,
Heracles killed his own wife and children.
• His remorse was so profound that
when he returned to his senses
he could find no peace of mind.
He visited the oracle of Delphi to
see how he could demonstrate
his remorse.
• The oracle advised him to obey
the orders of Eurystheus, the king
of Tiryns and Mycenae.
Eurystheus ordered Heracles to
Hercules
• accomplish twelve difficult tasks (the so-called Twelve Labors
of Heracles). Heracles completed the twelve labors and is
celebrated to this day for his great courage and strength.
Hydra
• Hydra lived in a swamp in a land called Lerna. The
second labor of Heracles was to kill the monster. It was
no easy task because the Hydra had 9 heads, one of
which was immortal. During the battle each time
Heracles cut off a head two more heads sprouted in its
place. Heracles was losing the battle until he took a torch
and burned each neck as he cut off the head. That
prevented any new heads from sprouting. Finally the
Hydra only had its single immortal head remaining.
Heracles cut it off but because that head could not be
killed he got rid of it by burying it under a massive rock.
And that was the end of the Hydra.
Lepus
• Lepus the hare has a number of
origins. According to one story, Orion
the famous hunter (and the
constellation right above Lepus) loved
to hunt hares, and so Lepus was
placed in the sky for Orion's benefit. In
another story, Lepus represents the
hare so often associated with the
moon. While we tend to see a man in
the moon, many other cultures have
seen a hare, and have many stories
to tell about it. The Arabs believed
that the four brightest stars in Lepus
represented four camels drinking from
the river Eridanus, another nearby
constellation. The early Egyptians
believed Lepus to be the boat of
Osiris.
Lepus
• Perseus had to fetch the head of the Gorgon Medusa whose head
could turn anyone who looked at it into stone. With the help of
Hermes, Perseus succeeded. On his voyage back to Seriphos, he met
the beautiful Andromeda in Ethiopia and married her. When his
triumph became known, he was invited to the city of Larissa to
participate in funeral games in honor of the king.
Leo
• The first on the list of Hercules' labors was the task of
killing the Nemean Lion. A giant beast that roamed the
hills of the Peloponnesian villages causing havoc.
Hercules' arrows bounced harmlessly off of the lion's
body, his sword bent in two and his wooden club
smashed to pieces. Hercules had to wrestle the beast
finally choking it to death.
•Hercules then wrapped the
pelt of the lion around his
body to protect himself from
his second labor, killing the
poisonous sea serpent
Hydra. The lion found its
way to the heavens to
commemorate the great
battle with Hercules.
Libra
• Libra is a zodiacal constellation. It
represents the balance or scales,
and is one of the oldest
constellations.
• Although now associated with
Virgo, a goddess of justice who
had scales as the emblem of her
office, it was once associated with
the fall equinox. On that day, the
days and nights are of equal
length (i.e. the moon and the sun
are in balance).
• Libra is represented in the
heavens next to the hand of Virgo.
Orion
• In Greek mythology, Orion
was a famed Boeotian
hunter. In one account he
was the son of Poseidon.
Orion was blinded by
Oenopion, whose daughter
he had raped, but his vision
was restored by the rising
sun. Stories concerning
Orion's death vary. The most
common relates that as he
attempted to assault Artemis,
or one of her nymphs, he was
stung by a scorpion sent by
Apollo or by Artemis herself.
Both the scorpion and Orion
later became constellations.
Pegasus
• In Greek mythology, when Perseus slew the Medusa drops
of the monster's blood fell into the sea. Poseidon, god of the
sea, commanded that the mixture of blood and sea-foam
give rise to Pegasus, the Winged Horse. Now this wonderful
horse was coveted by many a man but only one, the hero
• Bellerophon, was able to tame
him. Bellerophon knew that it
would be difficult to capture
Pegasus so he asked Athena,
the goddess of wisdom, for
help. Athena listened to
Bellerophon and gave him a
golden bridle whose magic
could tame the Winged Horse.
Bellerophon knew Pegasus'
Pegasus
• favorite watering hole and he waited there in hiding. When
Pegasus came to get a drink Bellerophon held out the golden
bridle and Pegasus willingly came to his new master. At first
Bellerophon found it difficult to control the flying horse but in a
short time the two soared as if they were made for each other.
Pegasus
• During his travels Bellerophon came to the country of Lycia and found the
land in complete disarray. A monster had been terrorizing the countryside
and the king of Lycia asked Bellerophon for help. The monster was the
Chimaera, part goat, part lion, and part dragon. No one had been able to kill
it and few even had the courage to try. Bellerophon accepted the king's
request. He flew on Pegasus to the Chimaera's lair and after a terrific battle
slew the monster and saved the kingdom. Pegasus and Bellerophon were to
have many grand adventures together but they were not destined to remain
companions forever. After a while Bellerophon became very prideful and
thought that he deserved a place with the gods. When he flew Pegasus
towards heaven Zeus became furious with his arrogance. Zeus sent a gadfly
to bite Pegasus. He bucked so strongly that Bellerophon fell off and
plummeted to earth. Bellerophon survived the fall and wandered about the
countryside lonely and lame until the end of his days. Pegasus continued
flying up to heaven and worked with Zeus by carrying his thunderbolts.
Perseus
• Perseus was an ancient Greek hero.
His mother was Danae, the daughter
of Acrisius, the king of Argos. When a
prophecy revealed to Acrisius that his
grandson would kill him, Acrisius
imprisoned his daughter Danae to
keep her chaste. Zeus, however,
fooled Acrisius' precautions by
entering the prison disguised as a
shower of gold. When Acrisius
discovered that Danae had given birth
to Perseus, he had the mother and the
son thrown into the sea in a chest of
wood. Luckily they reached the island
of Seriphos where the king Polydectes
offered them hospitality and protection.
Perseus was secretly raised on the
island and became a courageous
young man. He was sent to complete a
dangerous mission.
Perseus
•During the games, Perseus
threw a discus and accidentally
hit his grandfather Acrisius,
who was watching the games,
unaware of his grandson's
presence. Acrisius died fulfilling
the prophecy.
Pisces
•
One day Typhoeus, an earthborn
giant, appeared and scared all the
gods, forcing them to take different
forms to flee. Jupiter transformed
into a ram, Mercury into an ibis,
Diana into a cat, and Bacchus into
a goat and they all fled. Cupid
and his mother Venus, who were
bathing in the banks of the
Euphrates River at the time,
transformed themselves into two
fish. These two fish were later
known as Pisces the fish. Later,
to immortalize this event Minerva
placed the figures of these two
fish in the sky as a constellation.
Sagittarius
• The zodiacal constellation Sagittarius represents the centaur Chiron.
Most of the centaurs were regarded in myth as bestial--they were,
after all, half horse. However, the ancient Greeks had a great deal of
respect for the horse, and so were reluctant to make the centaurs
entirely bad. In fact, Chiron was renowned for his gentleness. He
was an excellent archer, musician, and physician, and tutored the
likes of Achilles, Jason, and Hercules.
•
Chiron, however, was accidentally shot
and wounded by Hercules. The arrow,
which had been dipped in the poison of
the Lernaean Hydra, inflicted great
suffering on Chiron--so great, in fact,
that even the talented physician could
not cure himself. In agony, but as an
immortal unable to find release in death,
Chiron instead offered himself as a
substitute for Prometheus.
Sagittarius
• The gods had punished Prometheus for giving fire to man by chaining
him to a rock. Each day an eagle would devour his liver, and each night it
would grow back. Jupiter, however, had at the request of Hercules
agreed to release Prometheus if a suitable substitute could be found.
Chiron gave up his immortality and went to Tartarus in place of
Prometheus; in recognition of his goodness, Jupiter placed him in the
stars
Scorpius
•
•
To the ancient Greeks, the constellation Scorpius
was the image of a scorpion. The constellation
was related to the death of the giant hunter Orion
by an enormous scorpion. There are several
different stories about Orion's death. According to
one variant, Orion once met the goddess of the
chase, Artemis while he was sailing to the island
of Crete. Artemis loved hunting as well as Orion.
They began hunting together and enjoying each
other's company. Orion became so infatuated
with Artemis that he tried to rape her. The
goddess avenged herself and set a scorpion on
him. No matter how hard he tried, Orion could not
defeat the scorpion.
When Orion tried to escape, the scorpion stung
him to death with its poisonous tail. As a reward
for its services, Artemis placed the image of the
scorpion among the stars. To this day, the
constellation Scorpius appears in the sky to be
ever pursuing the constellation Orion.
Taurus
• Zeus, a god, fell madly in love
with a mortal woman named
Europa. He wanted to meet
her, but he didn’t want her to
know his true identity, just
yet. He went to her in the form
of a snowy white bull. At first
she was scared, but when she
saw the bull meant no harm
she made a wreath of flowers
and put it around his
neck. Then they started to
play with each other. Soon
Europa became very
comfortable with the bull. She
climbed onto the bull’s back
and he swam out to sea with
her.
Taurus
•
He took her to Crete and revealed his true identity to
her. They had three children together. Eventually Europa
died. Zeus never wanted to forget her so he put the figure
of a bull in the sky as a constellation to always remember
he was in the form of a bull when he first met her.
Ursa Major
•
•
was a river goddess.
–Callisto
.
Callisto was the favorite
companion of the moon goddess
Diana. She used to accompany
Diana on the hunt. One day the
god Jupiter saw the beautiful
Callisto and fell in love with her.
They had a boy named Arcas.
Jupiter's wife Juno was mad and
changed Callisto into a bear.
One day, Arcas shot an arrow at
Callisto because he thought she
was a bear. At the last moment,
just before the arrow struck,
Jupiter tossed Callisto and her son
Arcas into the heavens as the star
formations Ursa Major and
Boötes. Ursa Major is known as
the Great Bear, with Boötes, the
Bear Warden. Arcas is always
standing next to her.
•contains the Big Dipper. The handle of
the Dipper is the Great Bear's tail and
the Dipper's cup is the Bear's flank
Ursa Minor
• In Greek myth, Zeus was
having an affair with the
lovely Callisto. When his
wife, Hera, found out she
changed Callisto into a
bear. Zeus put the bear in
the sky along with the
Little Bear, which is
Callisto's son, Arcas.
• contains the Little Dipper.
The handle of the Dipper is
the Little Bear's tail and the
Dipper's cup is the Bear's
flank.
Virgo
• Named for the Greek goddess Demeter, the
Earth-goddess, and is associated with the
arrival of spring and bringer of the growing
season. Hades, the God of the
Underground, fell in love with Demeter's
daughter, Persephone. He promised
himself that he would marry Persephone,
making her his queen. Demeter would not
stand for this. One day Hades, in a black
chariot drawn by four great black horses in
golden harness and reins, rode up to
Persephone and carried the girl off with him
back to the Underworld. Demeter became
worried when her daughter did not return
home. She sent out to search for her
daughter. Demeter searched with out food
or sleep. When Zeus pleaded with her to
return to Olympus and accept Persephone's
marriage to Hades, she refused and
continued her search.
Virgo
•
People everywhere were starving and
could not understand how their Earthgoddess could be so cruel. Zeus sent
Hermes, Messenger of the gods, to visit
Hades and tell him that Persephone must
return with him. Hades, knowing that the
gods of heaven were stronger than he,
agreed to let Persephone go. There was a
happy reunion, but Persephone told her
mother that she loved her husband
Hades. Zeus solved the problem by saying
that Persephone would spend half of her
time in the Underworld with Hades and the
other half on Olympus with her mother. In
this way, winter comes when Persephone
goes down to the Underworld to be with
Hades. When Persephone returns to
Olympus, the winter cloak of death melts
and there is a rebirth of life over the land
and the crops begin to grow.
Geocentric Theory
• The idea that the Earth was the center of the universe
Aristotle’s view
Ptolemy’s view
Aristotle
• Greek philosopher Aristotle
supreme authority in scientific
matters for almost 2000 years
after his death
• Geocentric theory – earth is the
center of the universe
– The planets are fastened to a
series of crystalline spheres
concentric with the earth.
– Contradicted his observations
Ptolemy
• Modified Aristotle’s model
• Also thought that the Earth was
fixed at the center of the heavens
• Sun, moon and planets revolved
around Earth
• Explained that each planet moves
in a small circle
• The center of this small circle is
carried on the circumference of a
larger circle
• The center of the larger circle is
Earth
Meteors
• A bright tail or streak of light that appears
in the night sky when a meteoroid enters
the Earth’s atmosphere. The friction with
the air causes the rock to glow with heat.
•A piece that manages to get through
the atmosphere and hits the ground is
known as a meteorite
•Avg speed of a meteoroid is 85,000
mph
•2 tons of meteoric dust falls on the
Earth every 24 hours
Meteors
• Most dust is made of iron & nickel
• Meteor showers occur when a large
number of meteoroids appear together
and seem to come from the same area in
the sky.
• They are named according to the
constellations from which they appear to
fall.
Red Shift
• a certain kind of change observed in the
spectrum of most stars and galaxies
• The wavelengths of light in the spectra of many
types of nearby stars are well known.
• When spectra are observed of the same types of
stars lying at a great distance, however, the
wavelengths usually appear longer.
• That is, they are shifted toward the red (longerwavelength) end of the spectrum
Hubble Constant
• Developed by Edwin
Hubble in 1920s
– Discovered that galaxies
were heading AWAY from
the Earth in all directions
– Showed that more distant
galaxies were moving away
from Earth faster than closer
galaxies
– He found that the red shift
was greater for distant
galaxies
Hubble Constant
• Hubble Constant is the relationship
between distance and velocity.
– Expressed in kilometers per second per
megaparsec
These six galaxy clusters are a subset
of the 38 that scientists observed with
Chandra, with distances ranging from
1.4 to 9.3 billion light years from Earth,
to help determine the Hubble constant.
Cepheid Variables
• Cepheid variables can help us to determine distances to celestial
objects.
• Cepheids are referred to as "cosmic mileposts" or "standard
candles" of the universe because they help us measure interstellar
distances.
• Cepheids get bright rapidly...then dim gradually over periods of time
ranging from a few days to a few months.
• The period of time a Cepheid takes to complete one pulsation is
directly related to the star's intrinsic brightness.
• Its intrinsic brightness helps us find the distance to the star. If we
know how bright the star really is and we can see how bright the star
appears to be, then we can measure its distance.
– If a Cepheid appears brighter than it should, it's closer to us.
– If it appears dimmer than it should, it's farther away.