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Chapter 3
• Many people theorize how the universe was
formed. There has not been one explanation that
has been made into scientific law pertaining to
this topic. 3 popular theories are:
– Steady state theory: universe always existed as it is
today and it always will. It does not grow
– Oscillating model: the universe began with expansion,
over time the expansion slowed and the universe
contracted then the cycle began over again
– Big bang theory: the universe started with a big bang
and has been expanding ever since
• Scientist think that all solar systems begin as
clouds of materials that accretion slowly joins
and gravity holds them together
• Accretion is the process of building something
up gradually by the gathering together of
smaller pieces.
• Gravity, together with accretion make solar
systems, stars, planets, and holds them all
together
• The stars in constellations all belong to a
galaxy. The galaxy in which we live in is called
the milky way galaxy
• A galaxy is a large collection of stars, gas, and
dust held together by gravitation.
• The sun and our solar system are inside the
milky way galaxy. It is believed that the milky
way has one trillion stars!!
– Many other galaxies also exist in the universe
• See page 102
• GRAVITY!!!
• Newtons law of universal gravitation states that every
object in the universe attracts every other object!
• Earth orbits the sun because of the suns gravitational
pull!
• The strength of the gravitational pull depends on the
mass of each object. Gravity is also affected by
distance. (the farther away the less gravity)
• Intertia keeps everything from crashing into each other.
See page 106
• Gizmo Gravity Pitch
• A constellation is a pattern of stars that is
named as a group.
– The stars that make up constellations in reality,
have no relation to each other. (look this way
because earth is round like a convex lenses)
– They form an imaginary shape or figure in the sky.
• Modern astronomy divides the sky into 88
constellations, many of which you may know (big
dipper, little dipper, etc)
• Polaris, is also known as the North Star, it is used
to guide travelers and as a point of reference in
the sky. It is positioned directly over the north
Pole!
• As Earth rotates, Ursa major, Ursa
minor, and other constellations in
the northern sky rotate around
Polaris. Because of this, they are
called circumpolar constellations.
– The constellations appear to move
because earth is in motion. They
complete one full circle in the sky
in about 24h
• As earth orbits the sun, different
constellations come into view
while others disappear. Because
circumpolar constellations always
orbit Polaris they are visible ALL
year long while others are not.
• Some star systems have two stars or double
stars called binary stars
• Often, one is much brighter and more massive
than the other.
• Astronomers can detect this by observing the
effect of gravity one has on the other. The dim
stars gravity causes the bright star to wobble!
• When you look at constellations, you’ll notice that
some stars are brighter than others. Why do you think
this happens?
• The Absolute magnitude of a star is the measure of the
amount of light it gives off
• Apparent magnitude is a measure of the amount of
light received on Earth
• A star that is dim can appear bright if it is close to
Earth, and a start that’s bright can appear dim if its far
away.
• If two stars are the same distance away, what might
cause one of them to be brighter than the other?
• How do scientist determine the distance from Earth to
nearby stars?
• Parallax- the apparent shift in the position of an object
when viewed from two different positions.
• One eye closed thumb out demonstration
• The nearer an object is to the observer, the greater its
parallax is.
• Astronomers can measure the parallax of relatively close
stars to determine their distances from Earth. Knowing the
angle that is changing and the size of Earth’s orbit,
astronomers can calculate the distance of the star from
Earth.
• Look at the diagram
• Because space is so vast, a special unit of
measure is needed to record distances. Distance
between stars and galaxies are measure in light
years.
• A light year is the distance that light travels in one
year.
– Light travels at 300,000 km/s or about 9.5 trillion km
in one year.
– The nearest start to Earth other than the sun is
Proxima Centauri which is 4.3 light years away or
about 40 trillion km!!!
• A Star is a giant ball of gas, primarily hydrogen
and helium, which undergoes nuclear fusion.
Our sun is a star
• A planet is an object that orbits a star, is large
enough to have become rounded by its own
gravity and has cleared an area for orbit.
• Solar systems contain both planets and stars
and can also be referred to as planetary
systems.
• The color of a star indicates its temperature.
– hot stars are a blue-white color
– Cool stars are orange or red
– Stars with the same temperature as our sun have a yellow
color
• Astronomers study the elements in stars by observing
their spectra.
– A spectroscope is an instrument that separates light into
its component colors.
– A stars spectra is like its fingerprint it is unique to that star
only. It identifies the elements in the star; most stars are
composed mainly of hydrogen and helium
• Star Spectra
• Like people, stars are very
different from each other.
They vary in age, size,
temperature and
brightness.
• In the early 1900’s Ejnar
Hertzsprung and Henry
Russell observed that in
general, stars with brighter
temperatures also have
brighter absolute
magnitudes
• The following diagram is
called the HertzsprungRussell (H-R) diagram and
it shows the relationship
between temperature,
brightness and magnitude.
• Most stars on the H-R diagram fit into a diagonal band.
This band is called the main sequence. It contains hot,
blue, bright stars in the upper left and cool, red, dim
stars in the lower right.
• 90% of all stars are main sequence stars but there are a
few that don’t fall into this “line” what are they?
• White dwarfs are stars that are dying. They have lost all
of their energy and gas for different reasons and so
they are small, hot, but not bright
• In contrast, super giants, and giants are stars that
extremely large and red. They are usually called red
giants.
• H-R Diagram
• Reactions between the nuclei of atoms create the
energy and light in stars.
– Our sun has very high concentrations of hydrogen,
given its’ extremely high temperature. In 1930,
scientist theorized that hydrogen fused to make
helium because of the extreme temperature thus
releasing tremendous amounts of energy.
– Years earlier, Albert Einstein theorized that mass could
be converted into energy. He created the famous
equation E=mc2
• E= energy, M= mass, and C= speed of light. The small
amount of mass “lost” when hydrogen atoms fuse to form a
helium atom is converted to a large amount of energy.
• Like all living things, nothing lives on forever.
Since stars are powered by hydrogen,
eventually it will run out and the star will die.
– Our sun has a life span of about 10 billion years it
has only lived half of its life.
• Most of the theories suggest that the universe
is still expanding or has expanded but how do
we prove it?
– Doppler shift: similar to Doppler effect; when stars
move away from Earth the wavelengths of light
are stretched. If you analyze them thru a
spectroscope you see shifts in the dark lines.
When a star is moving towards earth the lines
shift to the blue side and when a star is moving
away from earth the lines shift to the red side.
• In 1929, Edwin Hubble questioned the
meaning of the red shift of light coming from
other stars to earth. All galaxies beyond the
local group show a red shift in their spectra. If
all galaxies outside the local group are moving
away from Earth, then the entire universe
must be expanding!!
• Balloon demo
• With the discovery of the red shift, scientist
confirmed their belief in The big bang theory.
Today, it is the leady theory about the formation
of the universe.
• Whether the universe will expand forever to stop
expanding is still unknown. If enough matter
exist, then eventually the universe will begin to
contract to a single point. However, studies of
dark energy indicate that the universe is
expanding faster !