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UNIT NINE: Matter and Motion
in the Universe
 Chapter 26 The Solar System
 Chapter 27 Stars
 Chapter 28 Exploring the Universe
Chapter Twenty-Seven: Stars
 27.1 The Sun
 27.2 Stars
 27.3 The Life Cycle of Stars
Chapter 27.3 Learning Goals
 Describe the events that occur at different
stages in the life cycle of stars.
 Apply knowledge of nuclear fusion to explain
how elements are created in stars.
 Discuss the significance of spectroscopy in
classifying 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.
 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.
 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.
 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.
 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
 As our sun grows old, its core begins to run out
of hydrogen fuel.
 Gravity then causes the core to contract, raising
the temperature.
 The hotter core pushes the outer layers of the
star away, then star expands into a red giant.
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.
 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.
 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.
 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.
 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.
 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.
Investigation 27B
Star Life Cycles and the H-R Diagram
 Key Question:
How can we use an H-R diagram to know where
a star is in its life cycle?
New Pathways in Science
 Katherine Johnson began her
career at the National Advisory
Committee on Aeronautics
(which later became NASA) in
1953. Johnson faced many
hurdles. At the time, women
weren’t permitted to attend
briefings or put their names on
reports.