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Earth Science Notes
Stars and Galaxies
What is a Star?
A star is a massive, luminous ball of
plasma that is held together by gravity.
 Stars shines due to nuclear fusion in its
core releasing energy that radiates into
space.
 When stars form they are composed of
about 70% hydrogen and 28% helium by
mass
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There is a small fraction of heavier elements.
Classifying Stars

Stars are classified by their luminosity
(brightness) and the temperature.
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Hetzsprung and Russell developed a graph
that shows the relationship between
luminosity and temperature.
Three main classification
Main sequence
 Dwarfs
 Giants

Classifying Stars
The Hertzsprung-Russell Diagram
Classifying Stars
The
HertzsprungRussell
Diagram
Classifying Stars
Main Sequence Stars
 Run diagonally across H-R diagram
 Brightness increases as temperature
increases
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The sun is near the middle of the sequence
Around 90% of all stars are main
sequence
Classifying Stars
Dwarf Stars
 White dwarfs – hot but small (bottom-left
of H-R diagram)
 Red Dwarfs – cool but small (bottom-right
of H-R diagram)
Classifying Stars
Giant Stars
 Star with substantially
larger radius and
luminosity than a main
sequence star of the same
surface temperature
 Some Giants are large but
not hot
 Super-Giants are even
larger
Evolution of Stars
All stars begin as begin as nebulas
 Nebulas – large clouds of gas
and dust
 As particles in the nebula
contract they increase in
temperature until the reach 10
million K. This is when fusion
begins.
 Energy given off from the fusion
process powers the star
Evolution of Stars
Two series for stars
 Nebula  high mass star  main
sequence  red supergiant  supernova
 black hole or neutron star
 Nebula  low mass star  main
sequence  red giant  white dwarf
Evolution of Stars
Evolution of Stars
Main Sequence to Giant Stars
 When hydrogen in a star is depleted, the
fusion process begins to stop.
 The core collapses and heats up.
 The outer layers of the star begin to cool
and expand. This stage of the life cycle is
called a giant.
Evolution of Stars
White Dwarfs
 When a stars core uses up its fuel it
contracts even more and its outer layers
escape to space.
This leave behind a hot, dense core
 At this stage the stars becomes a white
dwarf
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White dwarfs are about the size of the
Earth. They are dim and hot stars.
Evolution of Stars
Supergiants
 In massive stars, the core heats up to
extremely high temperatures and heavier
elements form via fusion, the star expands
into a supergiant.
Evolution of Stars
Supernovas
Once an iron core forms the star collapses and a
shockwave travels through the star. The outer
portion of the star explodes and creates a
supernova.
Evolution of Stars
Neutron Stars
 Sometimes when the core of
a supernova collapses it will
shrink from 3 times the size of
the sun to about 20 km. Only
neutrons can exist at this
density.
 One teaspoon of a neutron
star is about 600 million
metric tons
Evolution of Stars
Black Holes
 Sometimes a supernova’s core can
collapse to a point. The gravity at this
point is so great that is pulls everything in,
even light.
Evolution of Stars
Black Holes
Evolution of Stars
Constellations
Constellations are patterns made of stars
in the sky
 Stars appear close together in the sky,
however, they are actually light years from
each other
Constellations

Some constellations are visible only in the
northern or the southern hemisphere
Circumpolar constellations appear to rotate
around Polaris (north star)
Some constellations are only visible during
certain times of the year
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Circumpolar Constellations
Constellations
Some stars in constellations appear brighter
than others.
This can be due to two things:
 The stars actual luminosity (brightness)
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Actual magnitude – how much light is given off by
star
The stars distance
Apparent magnitude – how much light is received on
Earth
 Stars far away
appear dimmer

Measurements in Space

Parallax – the apparent change in position
of object when viewed from different
perspectives
Measurements in Space

Light year – measurement of distance

The distance light travels in 1 year
 9.5
trillion km / year
Astronomical Unit – distance from Earth
to Sun = 1 AU
 Solar Mass – the mass
of our sun = 1 solar mass

Measurements in Space

Wavelengths of light can be measured to
determine the composition of stars

Spectra of elements are like fingerprints, they
will tell you what elements are present
Galaxies and The Universe
Galaxy – large group of stars, gas, and dust
held together by gravity
 Galaxies are separated
by great distances
millions of light
years
Galaxies and The Universe
Our galaxy is the Milky Way
 We are located on one of the spiral arms
 There is a black hole located at the center
The Milky Way
Galaxies and The Universe
Three types of Galaxies
 Spiral – have spiral arms that swing out
from the center (Like Milky Way)
Galaxies and The Universe

Elliptical – football shaped
Galaxies and The Universe

Irregular – have many different shapes
Origin of the Universe
Three Models the Universes Origin
 The Steady-State Theory
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The Oscillating Model
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Suggest universe is the same as it has always been
Suggest that the universe expands and contracts over
time
The Big Bang Theory

Suggest universe began as an explosion and has
expanded ever since
Origin of the Universe
The Big Bang Theory
 Evidence suggest that the
universe is expanding.
Scientist have theorized
that it has expanded from
one point
 According to this theory the
universe began with an
enormous explosion
Expansion of the Universe

As the universe expands, some stars are moving
toward us and some move away. This causes
an apparent change in the light we see
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Red shift – light moving toward Earth appears more
red (toward the red end of the spectrum)
Blue shift - light moving away from Earth appears
more blue (toward the blue end of the spectrum)
Expansion of the Universe
Redshift and
Blueshift
Summary
Stars are large bodies of plasma held
together by gravity
 The Hurtzsprung-Russel relates
brightness to temperature of stars
 There are three main classifications of
stars
 There are three models for the origin of
the universe, the leading model is the big
bang theory
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