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Chapter 11 The Lives of Stars

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Chapter 11 The Lives of Stars – PowerPoint PPT presentation

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Title: Chapter 11 The Lives of Stars


1
Chapter 11 The Lives of Stars
2
What do you think?
  • Where do stars come from?
  • Do stars with greater or lesser mass last longer?

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5
Stars form out of enormous volumes of dust and gas
  • Interstellar medium
  • H2 (mostly), CO, H2O, NH3, H2CO
  • Most is concentrated in giant molecular clouds

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8
Supernova explosions in cold, dark nebulae
trigger the birth of stars.
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10
Stars form in large groups called open clusters
or galactic clusters
11
When a protostar ceases to accumulate mass, it,
becomes a pre-main-sequence star.Its life
path is forever determined by its initial mass
12
H II regions harbor young star clusters
13
An OB association is where O and B class stars
are producing ionizing radiation which makes an
HII nebula glow.
14
Star formation and glowing HII regions in the
Great Orion Nebula
15
Plotting all the stars from a cluster on an H-R
diagram reveals its age
16
Plotting all the stars from a cluster on an H-R
diagram reveals its age
17
Stars spend most of their life cycle on the main
sequence
  • Main sequence stars are in hydrostatic
    equilibrium
  • outward thermal pressure is exactly balanced by
    the inward force of gravity
  • Main sequence stars are those stars fusing
    hydrogen into helium in their cores
  • Zero-age main sequence (ZAMS) is the location
    where a pre-main-sequence star fusing hydrogen in
    its core first becomes a stable object

18
The more massive a star, the faster it goes
through its main sequence phase
19
When core hydrogen fusion ceases, a main-sequence
star becomes a giant
  • When hydrogen in the core is no longer fusing
    into helium, the star can no longer support its
    weight
  • The enormous weight from the outer layers
    compresses hydrogen in the layers just outside
    the core enough to initiate shell hydrogen
    fusion.
  • This extra internal heat causes the outer layers
    to expand into a giant star.

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21
Helium fusion begins at the center of a giant
  • While the exterior layers expand, the helium core
    continues to contract and eventually becomes hot
    enough (100 million kelvins) for helium to begin
    to fuse into carbon and oxygen
  • core helium fusion
  • 3 He ? C energy and C He ? O energy
  • occurs rapidly - called the Helium Flash

22
Some Laws of Physics are important here
  • Pauli exclusion principal
  • two identical particles cannot exist in the same
    place at the same time
  • this effect in stars is called electron
    degeneracy pressure and is not dependent on
    temperature
  • the star is supported by the fact that the
    electrons cannot get any closer together

23
As stars evolve, they move on the H-R diagram -
their exact track depends on their initial mass
24
Globular clusters are bound groups of hundreds of
thousands of old stars at the edge of the galaxy
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A composite HR Diagram showing various star
clusters
27
Variable Stars
  • Change brightness because their diameter is
    fluctuating
  • (big/bright to small/dim and back again)
  • RR Lyrae variables (periods less than 24 hours)
  • Cepheid variables (periods between 1 100 days)
  • Mira variables (periods greater than 100 days)

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Cepheids enable astronomers to estimate vast
distances
  • This period-luminosity relationship is important
    because if an astronomer can find a Cepheid and
    measure its period, she can determine its
    luminosity and absolute magnitude.
  • Comparing the absolute and apparent magnitudes
    allows for the distance to be calculated.

31
What did you think?
  • Where do stars come from?
  • Stars form from gas and dust inside giant
    molecular clouds
  • Do stars with greater or lesser mass last longer?
  • Lower-mass stars last longer because the lower
    gravitational force inside them causes fusion to
    take place at slower rates compared to the fusion
    inside higher-mass stars.

32
Self-Check
  • 1 Describe the physical properties and visual
    appearances of objects associated with
    pre-main-sequence stellar evolution.
  • 2 Identify the defining characteristic of
    main-sequence stars and compare the relative
    lifetimes on the main sequence for stars of
    different mass.
  • 3 List the names of nuclear fusion reactions and
    indicate the classes of stars in which each
    reaction is thought to be active.
  • 4 Identify the physical property normally
    thought to control the life cycles of stars and
    planets.
  • 5 Explain how observations of open and globular
    star clusters contribute to the testing and
    extension of current theoretical models for
    stellar evolution.
  • 6 Identify the stages of stellar evolution in
    which mass loss is significant.
  • 8 Compare and contrast RR Lyrae and Cepheid
    variable stars in terms of period, population
    membership, luminosity, and evolutionary status.
  • 9 Describe how the identification of Cepheid
    variables can be used to determine the distance
    to a star cluster.
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