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Chapter Twenty-Seven: Stars
 27.1 The Sun
 27.2 Stars
 27.3 The Life Cycle of Stars
NGC 3532 in the constellation Carina
ESO/G.Beccari
27.2 How are stars classified?
 Astronomers classify
stars according to:
27.2 How are stars classified?
 Astronomers classify
stars according to:
 size/mass,
27.2 How are stars classified?
 Astronomers classify
stars according to:
 size/mass,
 temperature,
27.2 How are stars classified?
 Astronomers classify
stars according to:
 size/mass,
 temperature,
 color, and
27.2 How are stars classified?
 Astronomers classify
stars according to:




size/mass,
temperature,
color, and
brightness.
27.2 What is the size of
stars?

Stars come in a range
of sizes and masses.
27.2 What is the size of
stars?

Stars come in a range
of sizes and masses.

Our Sun is a mediumsized star.
27.2 What is the size of
stars?

Stars come in a range
of sizes and masses.

Our Sun is a mediumsized star.

The largest stars,
giant stars have a
mass of about 60
times the mass of the
Sun.
The sun is considered a __________ star.
a. Giant
b. Medium-sized
c. Dwarf
27.2 The size of stars

There are two types of
giant stars.
27.2 The size of stars

What are two types of
giant stars?

Red giants are cooler
than white stars.
27.2 The size of stars

What are two types of
giant stars?

Red giants are cooler
than white stars.

Blue giant stars are hot
and much more
massive than our sun.
Which star is cooler, a blue
giant or a red giant?
27.2 The size of stars
 Stars that are smaller
than the sun come in
two main categories,
dwarfs and neutron
stars.
27.2 The size of stars
 Stars that are smaller
than the sun come in
two main categories,
dwarfs and neutron
stars.
 Sirius, the Dog Star, is
the largest known
white dwarf.
27.2 How is temperature and color related
with stars?
 If you look closely at the stars on a clear night,
you might see a slight reddish or bluish tint to
some stars.
27.2 How is temperature and color related
with stars?
 If you look closely at the stars on a clear night,
you might see a slight reddish or bluish tint to
some stars.
 This is because their surface temperatures are
different.
27.2 Temperature and color
 The color of light is related to its
energy.
27.2 Temperature and color
 The color of light is related to its
energy.
 White light is a mixture of all colors at
equal brightness.
27.2 Brightness and luminosity
 Brightness, also
called intensity,
describes the
amount of light
energy per second
falling on a surface.
27.2 Brightness and luminosity
 For a distant source of light like a star,
the brightness decreases as the
inverse square of the distance.
What does the inverse square law
mean?
27.2 Brightness and luminosity
 Luminosity is the total
amount of light given off
by a star in all directions.
.
27.2 Brightness and luminosity
 Luminosity is the total
amount of light given off
by a star in all directions.
 Luminosity is a
fundamental property of a
star whereas brightness
depends on both
luminosity and distance.
27.2 Temperature and luminosity
 In the early 1900s, Danish astronomer
Ejnar Hertzsprung and American
astronomer Henry Russell developed
an important tool for studying stars.
27.2 Temperature and luminosity
 In the early 1900s, Danish astronomer
Ejnar Hertzsprung and American
astronomer Henry Russell developed
an important tool for studying stars.
 Their graph showed luminosity on the
y axis…
27.2 Temperature and luminosity
 In the early 1900s, Danish astronomer
Ejnar Hertzsprung and American
astronomer Henry Russell developed
an important tool for studying stars.
 Their graph showed luminosity on the
y axis…
 …and surface temperature on the
x axis
27.2 Temperature and luminosity
 H-R diagrams are useful because they help
astronomers categorize stars into groups:
?
27.2 Temperature and luminosity
 H-R diagrams are useful because they help
astronomers categorize stars into groups:
 Main sequence stars, like the Sun, are in a very
stable part of their life cycle.
27.2 Temperature and luminosity
 H-R diagrams are useful because they help
astronomers categorize stars into groups:
 Main sequence stars, like the Sun, are in a very
stable part of their life cycle.
 White dwarfs are hot and dim and cannot be seen
without a telescope.
27.2 Temperature and luminosity
 H-R diagrams are useful because they help
astronomers categorize stars into groups:
 Main sequence stars, like the Sun, are in a very
stable part of their life cycle.
 White dwarfs are hot and dim and cannot be seen
without a telescope.
 Red giants are cool and bright and some can be
seen without a telescope.
27.2 Temperature and luminosity
 H-R diagrams are useful because they help
astronomers categorize stars into groups:
 Main sequence stars, like the Sun, are in a very
stable part of their life cycle.
 White dwarfs are hot and dim and cannot be seen
without a telescope.
 Red giants are cool and bright and some can be
seen without a telescope.
Can you locate blue giants on the H-R diagram?
Chapter Twenty-Seven: Stars
 27.1 The Sun
 27.2 Stars
 27.3 The Life Cycle of Stars
27.3 The life cycle of stars
 A star, regardless of
its size, begins its
life inside a huge
cloud of gas (mostly
hydrogen) and dust
called a nebula.
27.3 The life cycle of stars
 A star, regardless of
its size, begins its
life inside a huge
cloud of gas (mostly
hydrogen) and dust
called a nebula.
 The Eagle Nebula is
the birthplace of
many stars.
27.3 The life cycle of stars
 A protostar is the
earliest stage in the life
cycle of a star.
27.3 The life cycle of stars
 A protostar is the
earliest stage in the life
cycle of a star.
 The Orion Nebula was
the birthplace of these
protostars.
27.3 The life cycle of stars
 A star is born when temperature and pressure
at its center become great enough to start
nuclear fusion.
27.3 The life cycle of stars
 A star is born when temperature and pressure
at its center become great enough to start
nuclear fusion.
 Once nuclear fusion begins, a star is in the
main sequence stage of its life cycle.
27.3 The life cycle of stars
 The time a star stays on the main
sequence depends on the star’s mass.
27.3 The life cycle of stars
 The time a star stays on the main
sequence depends on the star’s mass.
 High-mass stars burn brighter, and
hotter, using up their hydrogen faster
than low-mass stars.
27.2 The old age of Sun-like stars
 With no more energy
flowing outward, nothing
prevents gravity from
crushing the matter in the
core together.
27.2 The old age of Sun-like stars
 With no more energy
flowing outward, nothing
prevents gravity from
crushing the matter in the
core together.
 When hydrogen fusion
stops, the core glows
brightly and is called a
white dwarf.
27.2 The old age of Sun-like stars
 A planetary nebula forms
when a star blows off its
outer layers leaving its bare
core exposed as white
dwarf.
27.2 The old age of Sun-like stars
 A planetary nebula forms
when a star blows off its
outer layers leaving its bare
core exposed as white
dwarf.
 Planetary nebulae are one
of nature’s ways of
recycling the matter in old
stars and distributing new
elements.
27.3 Supernovae and synthesis of the
elements
 Scientists believe the early universe was
mostly hydrogen, helium and a trace of
lithium.
27.3 Supernovae and synthesis of the
elements
 Scientists believe the early universe was
mostly hydrogen, helium and a trace of
lithium.
 Heavier elements are created by nuclear
fusion inside the cores of stars.
27.3 Supernovae and synthesis of the
elements
 Scientists believe the early universe was
mostly hydrogen, helium and a trace of
lithium.
 Heavier elements are created by nuclear
fusion inside the cores of stars.
 Nuclear fusion reactions are exothermic,
releasing energy only up to iron.
27.3 Supernovae and synthesis of the
elements
 Scientists believe the early universe was
mostly hydrogen, helium and a trace of
lithium.
 Heavier elements are created by nuclear
fusion inside the cores of stars.
 Nuclear fusion reactions are exothermic,
releasing energy only up to iron.
 When the core of the star contains mostly iron,
nuclear fusion stops.
27.3 Supernovae and synthesis of the
elements
 If a star’s iron core reaches 1.4 times the
mass of the Sun, gravity becomes strong
enough to combine electrons and protons into
neutrons.
27.3 Supernovae and synthesis of the
elements
 If a star’s iron core reaches 1.4 times the
mass of the Sun, gravity becomes strong
enough to combine electrons and protons into
neutrons.
 During this brief period, heavier elements such
as gold and uranium are created, as atomic
nuclei are smashed together.
27.3 Supernovae and synthesis of the
elements
 If a star’s iron core reaches 1.4 times the
mass of the Sun, gravity becomes strong
enough to combine electrons and protons into
neutrons.
 During this brief period, heavier elements such
as gold and uranium are created, as atomic
nuclei are smashed together.
 The core of the star collapses and the result is
a spectacular explosion called a supernova.
27.3 Supernovae and synthesis of the
elements
 The Crab nebula is
the remains of a
supernova.
27.3 Supernovae and synthesis of the
elements
 The Crab nebula is
the remains of a
supernova.
 Chinese
astronomers
recorded it’s
demise 1054 AD.
27.3 Examining light from stars
 Spectroscopy is a tool of astronomy in which
the light produced by a star or other object
(called its spectrum) is analyzed.
27.3 Analyzing light from stars
 A spectrometer splits light into a spectrum of
colors and displays lines of different colors
along a scale.
27.3 Analyzing light from stars
 A spectrometer splits light into a spectrum of
colors and displays lines of different colors
along a scale.
 Each element has its own unique pattern of
spectral lines.
27.2 Analyzing light from stars
 In 1861, Sir William Huggins used
spectroscopy to determine that the
Sun and the stars are made mostly of
hydrogen.
27.2 Analyzing light from stars
 A few years later, Sir Joseph Norman
Lockyer observed a line at the exact
wavelength of 587.6 nm.
 He concluded that this must be an
undiscovered element and named it helium,
after the Greek name for the Sun, Helios.
27.2 Analyzing light from stars
 A few years later, Sir Joseph Norman
Lockyer observed a line at the exact
wavelength of 587.6 nm.
27.2 Analyzing light from stars
 A few years later, Sir Joseph Norman
Lockyer observed a line at the exact
wavelength of 587.6 nm.
 He concluded that this must be an
undiscovered element and named it helium,
after the Greek name for the Sun, Helios.