Transcript Stars: from Adolescence to Old Age

Stars:
From Adolescence
to Old Age
4 August 2005
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Mass Determines Life Stages
The mass of a star determines the stages it goes
through and how long it lasts in each stage
Massive stars evolve faster than small stars
Higher mass requires higher pressure to balance it, so
that hydrostatic equilibrium is maintained
Higher pressure in turn is produced by higher
temperature
The higher the temperature
inside a star, the faster it uses
up its hydrogen fuel
Although massive stars have
more fuel, they burn it so
quickly that their lifetimes are
much shorter than those of
low-mass stars
This also explains why the most
massive main-sequence stars are the most luminous
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Lifetime of Main-Sequence Stars
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Stellar Collapse
On the main sequence, a star’s inward gravity is
balanced by the outward pressure
The pressure is due to the nuclear fusion in the core
All the hydrogen in the core eventually gets used up
in the fusion into helium
In other words, the fusion of hydrogen inside the core
eventually stops
The core now contains only helium
The star is no longer on the main-sequence
Then gravity takes over and the (helium) core shrinks
The energy of the inward-falling material
in the core is converted to heat
The heat flows outward, raising the
temperature of the hydrogen just
outside the core
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Shell Burning
The shell layer outside the core becomes hot enough for
hydrogen fusion to start
This fusion in the layer just outside the core is called shell
burning
The helium core continues to contract, creating more heat
in the shell around it, leading to more fusion
This causes the star’s luminosity to increase beyond its
main sequence value
With all the new energy pouring outward,
the star’s size expands significantly
This causes the star’s outer layers to
cool down
Thus the star becomes a red giant
Its surface gets redder as it gets cooler
It is very luminous because of its
huge surface area and increased
luminosity
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Comparing the sizes of the Sun, the giant star
Delta BoÖtis (orange sphere), and the
supergiant Xi Cygni (red sphere)
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Comparing the Sun to a Supergiant
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End of Life on Earth …
When the Sun becomes a red giant, it will swallow
Mercury, Venus, and perhaps the Earth, too
Or conditions on the Earth’s surface will become
impossible for life to exist
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Time to Reach Giant Stage
Theoretical calculations
suggest that the time
for a main-sequence
star to reach the giant
stage is
short for a high-mass
star
as low as 10 million
(=107) years
long for a low-mass
star
up to 10 billion
(=1010) years
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Characteristics of Star Clusters
We saw that stars tend to form in clusters
The stars in a cluster have different masses but about
the same age
The different stars in a cluster provide a test for
theories of stellar evolution
Three types of clusters are
globular clusters, containing very old stars
open clusters, containing young to middle-aged stars
stellar associations, containing very young stars
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Testing Theory: Relatively Young Stars
A comparison of the prediction for the stars of a hypothetical
3-million-year-old cluster (left) with measurements of the stars in
cluster NGC 2264 (right)
The theory is roughly consistent with observation
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Testing Theory: An Older Cluster
A comparison of the prediction for a hypothetical
4.24-billion-year-old cluster (left) with measurements of stars in
the globular cluster 47 Tucanae (right)
The theory appears to be roughly consistent with observation
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Further Aging: Helium Fusion
As a star becomes a red giant, its (helium) core continues
to shrink, causing its temperature to continue increasing
When the core temperature reaches 100 million K, the
helium nuclei can fuse to form carbon nuclei through a
process called the triple-alpha process
In this reaction, three helium nuclei are fused into a single
carbon nucleus
As the triple-alpha process begins, the entire core is ignited
in a quick burst called the helium flash
After this, the star becomes stable, its surface temperature
increases, and its luminosity and size decreases
At this stage, carbon nuclei sometimes fuse with helium nuclei
to form oxygen nuclei
But this new period of stability does not last very long
As the helium is quickly used up in the fusion into carbon
and oxygen, gravity will once more take over
The situation is analogous to the end of the main sequence
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Near Death of Stars
Like the Sun
Then the star becomes a
red giant again, but only
briefly
The core now contains
only carbon and oxygen
At this stage, a star
similar in mass to the Sun
has exhausted its inner
resources and will soon
begin to die
The star’s luminosity may
pulsate for a time due to
its pressure and gravity
being out of sync
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Planetary Nebulae
When stars become giants, they begin to
shed their outer layers
exposing hot inner layers
losing a substantial fraction of their mass into
space
The shells of gas ejected by such stars are
called planetary nebulae
They looked like planets in early telescopes,
but have nothing to do with planets
The nebulae are glowing because the gas is
heated by the ultraviolet radiation of the
dying central stars
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Images of
Planetary
Nebulae
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Dying Process of Massive Stars
At the end of the helium-fusion stage, a star with
a mass greater than about 8 solar masses has not
yet exhausted its inner resources
Such a star is massive enough to cause more
contraction that can trigger other kinds of fusion
in its center
Carbon can fuse into still more oxygen as well as
neon, sodium, magnesium, and finally silicon
After each of the possible sources of nuclear fuel
is exhausted, the star contracts until it reaches a
temperature high enough to lead to the fusion of
still heavier nuclei
Depending on the star’s mass, this continues until
the star has used up all of its energy supplies
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Creation of Chemical Elements
The creation of heavier elements from lighter
ones by nuclear fusion is called nucleosynthesis
Theoretical calculations suggest that all elements
up to iron can be built up by nucleosynthesis
Stars like our Sun produce elements up to carbon
and oxygen
Very massive stars can produce elements up to iron
Elements heavier than iron are believed to be
produced in the supernova explosions of very
massive stars
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