Stars

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Stars

• The Life Cycle of Stars

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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.

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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.

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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.

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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.

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The life cycle of stars

• The time a star stays on the main sequence
  depends on the stars mass.
• High-mass stars burn brighter, and hotter, using
  up their hydrogen faster than low-mass stars.

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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.

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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.

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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
  a white dwarf.
• Planetary nebulae are one of natures ways of
  recycling the matter in old stars and
  distributing new elements.

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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.

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Supernovae and synthesis of the elements

• If a stars 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.

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Supernovae and synthesis of the elements

• The Crab nebula is the remains of a supernova.
• Chinese astronomers recorded its demise 1054 AD.

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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.

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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.

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Analyzing light from stars

• In 1861, Sir William Huggins used spectroscopy to
  determine that the Sun and the stars are made
  mostly of hydrogen.

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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.

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Investigation
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?

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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 werent permitted to
  attend briefings or put their names on reports.