The Universe on a Large Scale
Farther Away
• At a distance of 15 million light years we find other small galaxy
• The nearest galaxy to the Milky Way is 75,000 light
years away (controversial)
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•
Toward the constellation Sagittarius
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• Magellanic Clouds
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groups
At 50 million light years there is the Virgo Cluster
Many thousands of galaxies
• Our Local Group and the Virgo Cluster are part of supercluster that
Two small galaxies that appear to be clouds to the naked eye
has a diameter of 60 million light years
• Nearest large galaxy is
Andromeda Galaxy, M31
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Q
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• Farther away are quasars
A spiral galaxy similar to the
Milky Way
2 million light years away
Part of a cluster of 40 galaxies
called the Local Group
ISP 205 - Astronomy Gary D. Westfall
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X
cosmic microwave radiation
left over from the Big Bang
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ISP 205 - Astronomy Gary D. Westfall
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1 atom/cm3 in our galaxy
1 atom/m3 between galaxies
helium (by weight)
• The remaining 90 elements are relatively rare
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centers of stars
• These elements were ejected when stars used up
their hydrogen fuel and exploded
• There are 92 naturally occurring elements (atoms)
• Evidence has been found for man-made elements
up to 118
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Carbon, oxygen, calcium, etc.
• Hydrogen and helium come from the Big Bang
• The remaining elements were “cooked” in the
Most of the mass of an atom is in the atomic nucleus
The electrons around the nucleus are at a distance
10,000 times the size of the nucleus
ISP 205 - Astronomy Gary D. Westfall
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• The universe is roughly 75% hydrogen and 25%
• Air has about 1019 atoms/cm3
• Even atoms are mostly empty
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Composition of the Universe
• Most of the universe is empty
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Black holes?
• Beyond the quasars is the
The Universe of the Very Small
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Energetic centers of galaxies
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Supernova
ISP 205 - Astronomy Gary D. Westfall
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The Celestial Sphere
Celestial Poles
• When we look up it seems that the Earth is the
• The points where the axis of
center of the universe
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the Earth intersects the
celestial sphere are called
the north and south celestial
poles
• The celestial equator lies
halfway between the two
poles
• The circumpolar zone is
always above the horizon,
day and night
Geocentric
• The point above your head is called the zenith
• Where the sky-dome intersects the ground is the
horizon
• The sky seems to rotate around us
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Celestial sphere
• We know that the Earth’s rotation on its axis
every 24 hours causes this illusion but the concept
of the celestial sphere is still useful
ISP 205 - Astronomy Gary D. Westfall
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Rising and Setting of the Sun
ISP 205 - Astronomy Gary D. Westfall
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Tilt of Earth’s Axis
• The sun does more than just rise and set
• It changes position with respect to the background
stars
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Approximately 1 degree per day to the east
• The sun moves through the celestial sphere along
the ecliptic
• The ecliptic is not aligned with the equator of the
Earth because the Earth’s axis is tilted 23 degrees
with respect to the plane of its orbit around the
sun
• The Earth rotates on an axis tilted 23 degrees from
the vertical
ISP 205 - Astronomy Gary D. Westfall
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ISP 205 - Astronomy Gary D. Westfall
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Planets and Stars
Constellations
• Not only does the Sun move but the stars, the
• The stars form a fixed background
• Distinctive patterns of stars have been recognized
moon, and the planets move as well
• The moon and the planets move with respect to
the stars on the celestial sphere
• The moon and the planets move on paths close to
the ecliptic as well as the sun
• The Sun, the moon, and the planets are all found
in a 18 degree wide belt in the sky called the
zodiac
ISP 205 - Astronomy Gary D. Westfall
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throughout the ages
• These patterns were usually embellished and
given names and drawings
• These constellations are not necessarily nearby
stars but merely appear to be related
• Today the term constellation refers to 88 sectors
in the sky
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Constellations in the Sky
ISP 205 - Astronomy Gary D. Westfall
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Galactic Equator
• The galactic equator is defined by the Milky Way
galaxy
• The ecliptic and zodiac are not aligned with the
Galactic equator nor with the celestial equator
• In fact, the Galactic equator is very different from
the ecliptic and the celestial equator
ISP 205 - Astronomy Gary D. Westfall
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ISP 205 - Astronomy Gary D. Westfall
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Locating Objects on a Skymap
Locating the Celestial North Pole
• The Celestial North Pole is currently located near the star
•
Galactic Equator
•
Ecliptic
•
•
ISP 205 - Astronomy Gary D. Westfall
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ISP 205 - Astronomy Gary D. Westfall
Early Greek and Roman Cosmology
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The Earth was round
The Moon was a sphere
The Sun was farther away than the Moon
•
• The Greeks and Romans thought that the Earth
•
was the center of the universe and all heavenly
objects circled the Earth
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Geocentric
ISP 205 - Astronomy Gary D. Westfall
•
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• The Greeks measured the size of the Earth
• Eratosthenes noticed that on the first day of summer,
and origin of the cosmos
• The Greeks and Romans knew
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Measurements of the Earth
• Cosmology is the concept of the basic structure
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Polaris (aptly named!)
Locate the constellation Ursa Major,
the Big Dipper
Locate the outer two stars of the
dipper
Those 2 stars point at Polaris
The distance to Polaris is about 5
times the spacing between the two
stars
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sunlight struck the bottom of a vertical well in Syene at
noon
To Sun
At the same time in Alexandria,
at noon
June22
the Sun was not directly overhead
To zenith
but made an angle of 7 degrees
at Alexandria
To Sun
360 degrees divided by 7 degrees
and zenith
7°
at Syene
is about 50
50 times the distance between
Syene
Alexandria
7°
Syene and Alexandria would be
the circumference of the Earth
50 times 5000 stadia is 250,000
stadia
ISP 205 - Astronomy Gary D. Westfall
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Hipparchus
Ptolemy
• Roman astronomer around 140 AD
• Most important contribution was a geometric model of
• Roman astronomer, 150 BC
• Compiled detailed star catalog
• Invented the system of magnitudes to describe the
brightness of stars
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•
•
1 is the brightest, 5 is the dimmest, with each
magnitude about a factor 2 dimmer
In modern astronomy, each magnitude varies by a
factor 2.512
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• By comparing with previous observations,
describing the motions of the planets with superposition
of circles
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Precession
ISP 205 - Astronomy Gary D. Westfall
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Epicycles
Center of
eccentric
Earth
Equant point
Deferent
ISP 205 - Astronomy Gary D. Westfall
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Usually the planets move east light the moon and sun but
occasionally stop and move westward for some time
• He invented a complicated mathematical model
Hipparchus showed that the Earth’s north pole
changes
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the solar system capable of predicting the positions of
planets
This model lasted more than 1000 years
One puzzling observation that he explained was
retrograde motion of the planets
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Epicycles
ISP 205 - Astronomy Gary D. Westfall
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