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Astronomy
8th grade Science
Earth in Space
Ancient astronomers and Egyptians were among
the first people to study the stars and movement
of the sun.
It seemed to them as though Earth were
standing still and the sun and moon were
moving.
Actually, the Earth’s rotation on its axis makes it
seem as if the Sun and moon move across the
sky each day.
Earth in Space
Rotation - spin
Earth
moves
through
space in
two major
ways:
rotation
and
revolution.
Revolution - orbit
Earth in Space
 Earth spinning on its
axis is called rotation.
 Earth’s ROTATION
causes day and night.
 It is day on the side of
Earth facing the sun.
 Sunlight can’t reach
the side of Earth facing
away from the sun, so
it is night there.
 How long does it take
Earth to rotate on its
axis once?
Earth in Space
 The movement of the Earth around the sun is called
revolution.
 One complete revolution around the sun is called a year
and takes 365 ¼ days. (Every 4 years, leap year has an
extra day that makes up for the ¼ day.)
 Earth’s path as it revolves around the sun is called its
orbit.
Earth in Space
 Near the equator, the
sunlight strikes Earth’s
surface more directly
than near the poles.
 That is why areas near
the equator have a
warmer climate and
areas near the poles
have a colder climate.
Earth in Space
 The imaginary line that passes through the
North and South poles is called Earth’s
axis.
 The north end of the axis currently points
toward a point in space near the North Star.
 Most places outside the tropics have four
distinct seasons: winter, spring, summer,
and fall.
 Earth has seasons because it is tilted on its
axis as it revolves around the sun.
 If Earth’s axis were straight up and down,
temperatures would remain fairly constant
year round - there would be no seasons.
Earth in Space
When the north end of Earth’s axis is pointed toward the sun,
the Northern Hemisphere gets more direct sunlight, so it is
summer. June 21 is the summer solstice (longest day of the
year).
Earth in Space
A few months later, the north end of Earth’s axis points neither
toward nor away from the sun. This is autumn in the Northern
Hemisphere. September 22 is the fall equinox, and day and
night are both 12 hours long.
Earth in Space
Three months later, the north end of Earth’s axis is pointed
away from the sun, and the Northern Hemisphere gets less
direct sunlight, so it is winter. December 21 is the winter
solstice (shortest day of the year).
Earth in Space
Three months later, the north end of Earth’s axis is pointed
neither toward nor away from the sun. This is spring in the
Northern Hemisphere. March 21 is the spring equinox, and day
and night are each 12 hours long.
Earth in Space
http://macmillanmh.com/science/2008/s
tudent/na/scienceinmotion/Common/SIM.
html?Module=../Grade4/Chapter8EarthRevolvesAroundTheSun/
Telescopes
A scientist who studies the moon, stars, and other objects in
space is called an astronomer.
Most of the objects in space that astronomers study are too
far away to visit. So in order to decide what a distant star
or galaxy is made of, how hot it is, and how far away it is,
astronomers study the light (electromagnetic radiation) that
the object gives off.
Image taken by
Hubble Space
telescope.
Telescopes
Telescopes are instruments that gather and focus
electromagnetic radiation. They make distant objects
appear larger and brighter.
A telescope that uses lenses or mirrors to collect and
focus visible light is called an optical telescope.
Because the Sun is so bright during the day, optical
telescopes can only be used at night.
Telescopes
Devices that detect radio waves given off by objects in space
are called radio telescopes. Radio telescopes can be used
during the day or the night because visible light from the sun
does not interfere with radio waves.
Other telescopes are designed to detect microwave radiation,
ultraviolet radiation, infrared radiation, x-rays, or gamma rays.
(The hottest objects in space give off x-rays).
Radio telescope
Telescopes
An observatory is a building that
contains one or more telescopes.
Many large observatories are
located on mountaintops or even
in space because Earth’s
atmosphere makes objects in
space look blurry.
Telescopes
Most ultraviolet radiation, all x-rays, and all gamma rays
are blocked by Earth’s atmosphere. To detect these
wavelengths, astronomers have placed telescopes in
space.
Hubble Space
Telescope
Compton
Gamma Ray
Observatory
Chandra X-ray
Observatory
Spitzer
Space
Telescope
Characteristics of Stars-Color &
Temperature
• Astronomers classify stars by their physical characteristics:
color, temperature, size, composition, and brightness.
• The color of a star reveals its surface temperature:
• The coolest stars appear reddish in the sky.
• Medium temperature stars appear yellow.
• The hottest stars appear blue.
Omega Centauri cluster in
the Milky Way (image
taken by Hubble Space
telescope)
Characteristics of Stars Size
Stars vary greatly in size. Giant stars are typically 10 to
100 times larger than the sun and more than 1,000 times
the size of a white dwarf.
Characteristics of Stars Composition
When a star’s light is passed through a spectroscope (prism),
thin black lines appear in the spectrum.
Characteristics of Stars Composition
Particular elements in the star create particular patterns of
black lines. Once scientists knew the element responsible for
each pattern, they knew which elements were present in the
star. Most stars are made of about 73% hydrogen, 25 % helium,
and 2% other elements.
Characteristics of Stars - Composition
Copy the table on a sheet of notebook paper. Follow the teacher’s
instructions to complete the analysis.
Star
Chemical Composition
A
B
C
D
Write and answer the following questions.
1.
Why do astronomers need to study the light emitted by
stars?
2. Explain how astronomers can determine the composition
of a star by studying the light it emits.
3. Did any of the stars in your analysis have the same
chemical composition?
Characteristics of Stars
- Brightness
• The brightness of a star depends on both its size and
temperature. Larger, hotter stars shine more brightly.
• Because distances in space are so large, astronomers use a
unit called the light-year. A light year is the distance that
light travels in one year, or about 6 million million miles.
• Light travels at 186,000 miles per second.
Characteristics of
Stars
• When viewed from
Earth, stars may
appear close together
in space, but they
generally are not.
•
To measure the
distance to nearby
stars, astronomers
often use parallax.
• Parallax is the
apparent change in
position of an object
when you look at it
from different places.
OA
Get out your green textbook and turn to
page 718.
Look at the spectra for stars A, B, and
C.
Compare each star’s spectrum to the
spectra of the known elements above.
Write the chemical composition of
stars A, B, and C.
Star Systems and
Galaxies
Stars do not occur in space at completely arbitrary
places. Some stars, such as our Sun, are single stars.
However, most stars are members of small groups of
two or more stars called star systems.
Double star system - two stars
with planets orbiting both
(image taken by Kepler space
observatory)
Star Systems and
Galaxies
Other stars belong to a larger grouping of stars that all
formed at about the same time and are about the same
distance from Earth. This is called a cluster.
There can be from tens to millions of stars in a cluster.
Star Systems and
Galaxies
A galaxy is a huge group of single stars, star systems, star
clusters, dust, and gas all bound together by gravity.
Astronomers classify most galaxies into the following types:
Spiral galaxy: appears to have a bulge in the middle and
arms that spiral outward like a pinwheel. Our solar
system is located in a spiral galaxy call the Milky Way.
The galaxy’s spiral structure is visible only from above or
below.
Elliptical galaxy: looks like round or flattened balls.
Most elliptical galaxies contain only old stars.
Irregular galaxy: galaxy without a regular shape; these
are typically smaller than other galaxies and have many
bright, young stars.
Star Systems and
Galaxies
Artist’s drawing of the Milky Way.
Look up at night and you can see
the Milky Way galaxy.
Star Systems and
Galaxies
This spiral galaxy,
M101,is 170,000
light years across,
about twice the
size of our Milky
Way galaxy. It is
estimated to
contain at least
one trillion stars.
(Image taken by
Hubble space
telescope)
Star Systems and
Galaxies
Another image of M101
combines data from four of
NASA’s space telescopes:
•Infrared (Spitzer Space
Telescope)
•Visible (Hubble)
•Ultraviolet (GALEX)
•X-ray (Chandra)
Star Systems and
Galaxies
This elliptical galaxy,
Centaurus A, is
60,000 light years
across. It can be
seen without
binoculars from
earth. (Image taken
by the Canada France
Hawaii Telescope on
the top of Mauna Kea
volcano in Hawaii.)
Star Systems and
Galaxies
An irregular galaxy.
Star Systems and
Galaxies
Galactic clusters are compact clusters of galaxies
bound together by gravity.
Each bright object is
a galaxy.
Star Systems and
Galaxies
Astronomers define the universe as all of space and everything in it.
Hierarchical structure of the universe:
Stars  star systems  galaxies  galactic clusters  universe
The Expanding
Universe
• Astronomers theorize that the universe formed in an
instant in an enormous explosion. This theory is called
the Big Bang Theory. Before the explosion, astronomers
believe that the universe we see now was no larger than
a dot. Since the big bang, the size of the universe has
been increasing rapidly.
• There are two key pieces of evidence that support the
Big Bang Theory.
The Expanding
Universe
• In the 1920’s, American astronomer Edwin Hubble discovered
that the light given off by a star or galaxy gets “stretched” if it
is moving away from us. This causes the light being given off to
have a longer wavelength and the object to appear redder than
it really is. This is called the red shift. Using Hubble’s idea,
astronomers found that all distant galaxies are moving rapidly
away from our galaxy and each other.
Hubble’s Law states that the
farther away a galaxy is, the
faster it is moving away from
us.
The Expanding
Universe
• In 1965, two American
physicists accidentally
detected radio waves being
given off from all directions in
space. Scientists later
concluded that this glow, called
cosmic background radiation, is
the leftover energy from the
big bang.
The Expanding
Universe
• Since astronomers can measure approximately how fast the
universe is expanding now, they can infer how long it has been
expanding. Astronomers estimate that the universe is about
13.7 billion years old.
• About 5 billion years ago, gravity caused a giant cloud of dust
and gas to collapse and form our solar system (the sun and
planets).