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Stars and Galaxies
Chapter 25-1
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Objectives:
Describe constellations.
Discuss circumpolar constellations.
Discuss apparent and absolute magnitude.
Constellations
• Ancient Greeks and
Romans observed patterns
of stars in the sky.
• They imagined that the
constellations represented
mythological creatures.
• In reality the stars in a
constellation may not be
near each other.
Circumpolar Constellations
• As the Earth moves,
some constellations,
such as Ursa Minor
and Ursa Major, circle
around Polaris.
• They appear to move
because the Earth is in
motion. They are
visible all year.
Apparent and Absolute
Magnitude
• The absolute magnitude of a star is the
amount of light it gives off.
• The apparent magnitude is the amount of
light that is received on earth.
• A star that is dim can appear bright if it is
close to Earth.
Brightness of Stars
The brightness of a star depends on
• Size
• Surface temperature
• Distance from the Earth (absolute and
apparent magnitude)
Measuring Star Distance
• One method of measuring the
distance to stars is called
parallax.
• Parallax refers to the apparent
change in the position of a star
in the sky.
• Measurement is based on the
triangle formed by the Sun,
Earth and star. The greater the
parallax angle, the closer the
star is to Earth.
• This method cannot be used for
a star more than 100 light-years
away.
Light Years
• A light-year is the distance
that light travels in one
year. Light travels at
300,000 km/s or 9.5
trillion km in one year.
• The nearest star to Earth,
other than the sun, is
Proxima Centuri, 4.3 light
years away.
Properties of Stars
• The color of a star
indicates its temperature.
• Astronomers study the
composition of stars by
observing their spectra as
it is split by a prism.
• The dark lines in a spectra
tell the scientist which
elements the star contains.
Chapter 25-2
• Objectives:
• Describe the layers of the Sun.
• Discuss the features of the Sun.
The Sun
• The sun is 150 million kilometers from the
Earth.
• The sun is 4.6 billion years old.
• The volume of the is 1 million times greater
than Earth.
• The sun’s density is only ¼ of the Earth
because the sun is made only of gases.
Layers of the Sun
• Corona (hottest part of
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outer layers – 1,700,000
degrees C)
Chromosphere(atmosphere 27,800 ºC)
Photosphere (surface of
the sun – 6000 ºC) at the
top of the convective zone
The radiative zone is
between the core and the
convective zone
Core- interior of sun15,000,000ºC
Sunspots
• Sunspots – cooler than the
rest of the sun’s surface
they appear as dark spots.
Their movement shows
that the sun rotates but
that the sun rotates more
quickly at the equator than
the poles. This tells us it
does not rotate as a solid
object.
Prominences
• Prominences – loops of
gas originating in the
chromosphere
Solar Flare
• Solar flares – bright
bursts of light on the
sun’s surface
CME
• CME –Coronal mass ejections are a
continuous stream of high-energy particles
released into space in all directions from
the sun. They can interfere with radio
signals and telephone communications.
Chapter 25-3
• Objectives:
• Determine how stars are classified.
• Compare the Sun to other types of stars on
the H-R diagram
• Describe how stars evolve.
Size of Stars
• Neutron star- A dying high mass star that is16 km in
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diameter.
White dwarf star- This is a dying low to medium mass star
which is earth sized, but can be as small as Asia.
Medium sized star- This low to medium mass star at birth
is 1/10 to 10 times the size of the Sun.
Giant star-This high mass star at birth is 10-100 times our
Sun.
Super giant star- This very high mass star at birth is1001000 times the size of our Sun.
Composition of Stars
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Hydrogen and helium (96-99%)
Oxygen
Neon
Carbon
Nitrogen
Calcium
Very high mass stars can fuse iron.
Surface Temperature of Stars
• The surface temperature of a star can be
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determined by the color.
Blue-white (hottest) – 25,000º C
White - 10,000º C
Yellow (Sun) - 6,000º C
Red-orange- 5,000º C
Red (coolest) - 3,000º C
The Hertzsprung-Russell
• As the absolute
magnitude increases,
the temperature also
increases.
Why do stars shine?
• Within the core of a star, gravitational
forces cause nuclear fusion.
• Four hydrogen atoms are fused to produce
one helium atom. The remaining matter is
given off in the form of heat and light
energy.
The Evolution of Stars
• New stars are born from the gases in a nebula.
• When hydrogen in the cloud reaches a certain
temperature (15,000,000 degrees C), nuclear
fusion begins. A protostar, or new star, is formed.
• The main factor that shapes the evolution of a star
is the mass it began with.
• A more massive stars have a shorter life.
Evolution of a Star
Medium-Sized Stars
• For the first few billion years, a star shines
as nuclear fusion occurs in the core.
• When most of the hydrogen is gone, the
helium core shrinks and heats up again.
• As the outer shell expands, it cools and its
color reddens and become a red-giant.
• When all of the helium atoms are fused into
carbon, the star begins to die.
White Dwarfs
• When a red-giant begins to die, gravity
causes the star’s matter to collapse inward
and become a white dwarf which is very
dense.
• The amount of time it takes the white dwarf
to die depends on the mass of the star when
it first formed. The smaller the starting
mass, the longer a star will live.
Massive Stars
• The evolution of high and very high mass stars
are the same as low and medium mass stars until
they become red giants. Low and medium mass
stars will become white dwarfs. Higher mass
stars will begin to produce oxygen, nitrogen and
iron.
• The iron atoms will not fuse and begin to absorb
energy. The star will explode in a supernova
which can light the sky for weeks.
Evolution
• Low to Medium Mass -nebula- main
sequence- red giant- white dwarf- brown
dwarf
• High Mass -nebula- main sequence- red
giant- supernova-neutron star
• Very High Mass- nebula- main sequencered giant- supernova- black hole and new
nebula
Supernova
• During a supernova explosion, the heat
reaches such high temperatures that iron
atoms can fuse to form new elements.
• These elements explode into space and form
a new nebula.
• Chinese astronomers recorded a supernova
in 1054 that lit the night sky for 23 days and
could be seen for 600 nights. (Crab Nebula)
High Mass and Very High Mass
Stars
• After the supernova explosion, a high mass
star will become a neutron star.
• After a supernova explosion, in a very high
mass star the core that remains will be so
massive, that without the energy created by
nuclear fusion to support it, the core is
swallowed by its own gravity.
• The gravity of the core is so strong that light
cannot escape – a black hole.
Detection of Black Holes
• Most black holes have companion stars.
• When the gases from the companion hole
are pulled into the hole and heated, they
give off a burst of X-rays.
Chapter 25-4
• Objectives:
• Describe the Sun’s position in the Milky
Way Galaxy
• Explain that the same natural laws apply to
our solar system also apply to other
galaxies.
Nebulae
• A nebula is a massive cloud of dust and gas.
• Nebulae are the birthplace of new stars.
• Stars are held together by gravity in
galaxies. There are three main types of
galaxies, spiral, elliptical and irregular.
Spiral Galaxies
• Galaxies contain
various star groups.
• Most galaxies are
spiral galaxies. Spiral
galaxies are shaped
like pinwheels.
Elliptical Galaxies
• Galaxies that vary in
shape from nearly
spherical to flat disks
are called elliptical
galaxies. They
contain very little dust
and gas. They are
usually older.
Irregular Galaxies
• Irregular galaxies have
no definite shape.
The Milky Way Galaxy
• The Milky Way Galaxy is a
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spiral galaxy.
Most of the older stars in the
Milky Way are found near the
nucleus of the galaxy.
The Milky Way is estimated to
be 100,000 light-years in
diameter and about 15,000
light-years thick.
The Sun is located in one of the
pinwheel arms.
The stars rotate counter
clockwise around the center.
This takes 200 million years.
Local Group Galaxy Cluster
• Astronomers have concluded that the
Universe is expanding since all the galaxies,
with the exception of other galaxies in the
Local Group Galaxy Cluster, show a red
shift. Within the Local Group, some show a
red shift and others a blue shift, since some
are moving toward the Milky Way and
others away.
The Big-Bang Theory
• Astronomers believe that the expanding universe
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is the result of an enormous explosion known as
the big bang.
The explosion occurred 15-20 billion years ago.
As the matter moved away from the explosion,
gravity caused clusters to form.
These clusters became the galaxies of the
universe.
Support comes from Red Shift and background
radiation.
Spectroscope
• Astronomers use telescopes and spectroscopes to
study the stars. A spectroscope can break up the
light from a distant star into colors.
• After the light is focused into a lens, a prism
separates light into its different colors, called a
spectrum.
• By using a spectroscope, scientists can determine
if an object is moving toward the Earth or away,
how fast the star is rotating and what elements
make up the star.
The Red Shift
• When a star is approaching the Earth, the light
waves will be compressed.
• The wavelengths are shorter and are characteristic
of blue and violet light.
• If the star is moving away from Earth, the waves
will be expanded. Longer wavelengths are
characteristic of red light.
• The spectrum of a star moving away from Earth is
shifted toward the red end. (the Red Shift)
Doppler Effect
• The Red Shift Theory
is based on the doppler
effect in which waves
light and sound,
appear to compress as
they approach the
viewer.
• They appear to
lengthen as they move
away from the viewer.
The Steady State Theory
• This theory states that the universe is and
has always been the same.
The Oscillation Theory
• In this universe, the gravitational attraction
between the galaxies will cause the
movement away from each other to slow
down. Then gravity will begin to pull the
galaxies back toward the center of the
universe until another big bang occurs. This
will continue to happen over and over.