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Stars and Galaxies
Composition of Stars
• Chemical elements have a characteristic
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spectrum in a given range of temperatures .
The colors and lines in a spectrum of a star
indicate the elements that make up the star.
Most common element of stars is hydrogen.
Helium is the second most common element
of stars.
Carbon, oxygen, and nitrogen make up
remaining mass of stars.
Spectroscope
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We can tell a lot about a star by
observing it using a spectroscope.
The spectroscope breaks the light
from a star into a spectrum.
Scientists can look at the spectra of
a star and tell its composition,
which direction it is traveling, its
age, and how fast it is rotating.
Stars that are moving away from
Earth produce a red shift. The lines
are shifted toward the red end of
the spectrum.
Temperatures of Stars
• Surface Temperature of a Star is indicated by the
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star’s color.
Blue shine with hottest temperatures > 30,000 C
Blue- White
10,000-30,000 C
White
7,500-10,000 C
Yellow-White
6,000- 7,500 C
Yellow
5,000- 6,000 C
Orange
3,500- 5,000 C
Red shine with the coolest temperatures < 3,500 C
• Blue stars are hotter
• Blue light has a shorter wavelength, this means
it has more energy
• Red stars are cooler
• Red light has a longer wavelength, this means it
has less energy
Sizes and Masses of Stars
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Stars vary in size and mass.
Some dwarf stars are the size of Earth
Sun is a medium-sized star.
Giant stars can have diameters 1,000 times
Sun’s diameter.
Constellations
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A star is a ball of mostly hydrogen and
helium gas that shines extremely
brightly. Our Sun is a star.
Most stars have partner stars. A group
of two stars are known as binaries.
Ancient Greeks and Romans observed
patterns of stars in the sky 
CONSTELLATIONS
They imagined that the constellations
represented mythological creatures.
In reality the stars in a constellation
may not be near each other.
Some famous constellations are Orion,
Big Dipper and Ursa Minor.
Circumpolar Constellations
• Circumpolar Stars are always
visible in the night sky.
• As the Earth moves, some
constellations, such as Ursa
Minor and Ursa Major, and
Cassiopeia circle around Polaris
or North Star.
• They appear to complete one
full circle in the sky in 24 hours
because the Earth rotates. They
are visible all year.
Stellar Motion
• Apparent Motion #1
• Apparent Motion #2
- Earth’s revolution around - Caused by movement of Earth
the Sun causes stars to
- Stars appear to move
appear to move
- Visible stars appear to shift
slightly to west every night
- As Earth orbits Sun,
different stars become
visible during different
seasons
counterclockwise around a
central star called Polaris or
the North Star
Circular pattern caused by
rotation of Earth on axis
Actual Stellar Motion
• Stars rotate on an axis
• Stars may revolve around another star
• Stars move away or toward our Solar System
Doppler Effect
• The Light Spectrum of a Star that is moving toward
or away from Earth appears to shift.
• The apparent shift in the wavelength of light
emitted by a light source ( the star) moving toward
or away from an observer is called the DOPPLER
EFFECT.
• The light from stars is shifted based on the star’s
movement in relationship to Earth.
Doppler Effect
• Light from stars that are moving away from Earth is shifted slightly
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toward the red end of the spectrum.
Light from stars that are moving toward the Earth is shifted slightly
toward the blue end of the spectrum.
When a source of light (like a star) is stationary, the wavelengths of
light remain the same distance apart.
When a source of light is moving toward you, the wavelengths of light
get closer together. This is called blueshift.
When a source of light is moving away from you, the wavelengths of
light get farther apart. This is called redshift.
This is how scientists find out if the universe is growing, shrinking, or
staying the same size
Doppler Effect
• The Doppler Effect causes the absorption spectrum
of a star to shift toward the red or blue.
Distances to Stars
• Distances between stars and Earth are
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measured in light-years.
A light-year is the distance that light travels
in one year.
The speed of light is 300,000 km/s
Light travels about 9.46 trillion km. per
year.
The light you see when you look at a star
left that star sometime in the past.
Parallax
• We measure how far a star is
from the Earth using a method
known as parallax.
• Parallax = the apparent shift in
a star’s position when viewed
from different locations.
• Parallax is determined by
observing the same star when
the Earth is at two different
points in its orbit around the
Sun. The star’s position
relative to more distant
background stars will appear to
change. This creates an angle
which can be measured.
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.
• Magnitude values are assigned to stars.
• Lower numbers mean brighter stars.
• While the Sun’s absolute magnitude is 4.8, its apparent brightness
is -26, because we are so close.
Brightness of Stars
The brightness of a star depends on
• Size
• Surface temperature
• Distance from the Earth (absolute and
apparent magnitude)
The Sun
• The sun is 150 million kilometers from the
Earth.
• The sun is 4.6 billion years old.
• The volume of the Sun 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.
Solar Flares, Prominences and
CME
• Solar flares – bright bursts
of light on the sun’s
surface.
• Twisted loops of gas that
originate in the
chromosphere are called
Prominences.
• Coronal mass ejections
(CME) interfere with
communication on Earth.
Classifying Stars
• Astronomers have developed theories about the
evolution of stars by studying stars in different
stages of development.
• Astronomers plot the surface temperature of stars
against their luminosity , the energy given off each
second in a graph called the H-R diagram which
describes the LIFE CYLCE of a star.
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
H-R Diagram
• Graph of Temperature vs.
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Luminosity of stars
Temperature on x-axis
Luminosity on y-axis
As the absolute magnitude
of a star increases, the
temperature of the star also
increases.
The exceptions are dying
stars like red-giants or
white dwarfs.
H-R Diagram
• H-R Diagram describes the Life Cycle of a star
• Highest temperatures are plotted on the left of x-axis
• Highest luminosities are plotted on the top of y-axis
H-R Diagram
• The temperatures and luminosities for most stars falls
within a band that runs diagonally through the middle
of the H-R diagram.
• Band extends from cool, dim, red stars at the lower
right to hot, bright, blue stars at the upper left 
known as Main Sequence.
• Stars within this band are called main sequence stars.
ie. Sun
Why do stars shine?
• In the main sequence of a stars life, 96-99% of the
star’s composition is hydrogen and helium.
• Due to the extreme mass of the Sun, 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.
Star Formation
• A star begins as a nebula, a cloud of gas and dust.
• 70% hydrogen, 28% helium, 2% heavier elements.
• Outside force ( like explosion of star ) compresses
cloud of gas, particles move closer to each other and
bulled together by gravity.
• Regions of dense matter build up
• Newton’s Law of Universal Gravitation = all
objects in universe attract each other though
gravitational force.
Protostars
• Gravity makes dense regions of matter more compact
• Protostar is central concentration of matter
• Gravitational energy converted to heat and
temperature of protostar increases.
• Continues over several million years
• Plasma forms; Plasma is a hot, ionized gas consisting
of free moving positive ions and electrons.
A Star is Born
• Protostar temperature
increases to 10,000,000 C
and nuclear fusion begins
• Nuclear Fusion occurs at extremely high temps and
pressure cause atomic nuclei to combine to form
larger nuclei and enormous amount of energy are
releases
• Hydrogen fuses into helium.
• Process can continue for billions of years.
Main Sequence Stage
• Second and longest stage in the life of a star
• During this stage, energy continues to be generated a
s hydrogen fuses to helium
• A star enters its third stage when almost all of the
hydrogen atoms in its core have fused to helium
atoms.
• Without hydrogen fuel, the core of the star contracts
under the force of its gravity.
• Outer shell of the star expands greatly.
Giant Stars
• As the star expands, the
star’s shell of gases grows
cooler
• Stars glow as large red stars
called red giants.
• As these stars become larger,
more luminous and cooler,
they move off the main
sequence.
• Giant stars are above the
main-sequence on the H-R
Final Stages of a Sunlike Star
• As star’s outer gases drift away, remaining
core heats these expanding gases.
• Planetary nebula is a cloud of gas that forms
around a sunlike star that is dying.
White Dwarf
• As planetary nebula disperses, gravity
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causes remaining matter of the star to
collapse inward.
A hot, extremely dense core of matter –
a white dwarf – is left.
White dwarfs shine for billions of years
before they cool completely.
White dwarfs are hot but dim in lower
left of H-R diagram. Final stage of life.
White dwarf that no longer gives off
light becomes a black dwarf.
Novas and Supernovas
• Nova – a star that suddenly becomes brighter; large
explosion as a white dwarf revolving around a red
giant captures gases ( binary system ); pressure
builds up and explosion occurs.
• Supernova – a white dwarf in a binary system that
has a tremendous explosion and blows itself apart.
Final Stages of Massive Stars
• Only a small percentage of white dwarfs
become supernovas.
• However, massive stars become supernovas
as part of their life cycle.
Neutron Stars and Pulsars
• A neutron star is a massive star that
has collapsed under gravity to the
point that the electrons and protons
have smashed together to form
neutrons.
• Neutron stars rotate very rapidly.
• A pulsar is a rapidly spinning
neutron star that emits pulses of
radio and optical energy.
Black Holes
• A black hole is an object so
massive and dense that even
light cannot escape its
gravity
• Black holes can be observed
by its effect on a companion
star – matter of the
companion star is pulled into
the black hole.
• Matter that is absorbed
becomes so hot that it emits
X-rays that can be detected.
Size of Stars
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Neutron star- A dying high mass
star that is16 km in 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 is100-1000 times
the size of our Sun.
Evolution of a Star
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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 Stars
• Low to Medium Mass -nebula- main sequencered giant- white dwarf- black dwarf
• High Mass -nebula- main sequence- red giantsupernova-neutron star and nebula
• Very High Mass- nebula- main sequence- red
giant- supernova- black hole and new nebula
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.
Theories of Universe Formation
• The first theory about the Universe was known as the
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Steady State Theory. This theory stated that the universe
was always the same.
Astronomers believe that the expanding universe 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.