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Observing Stellar Evolution
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How can we see stellar evolution in action?
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Stellar Clusters, a group of coeval stars, I.e. all
born at the same time, but with different masses
(hence different life time)
How can one estimate the age of a stellar
cluster?
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By looking at the HR diagram of the cluster,
namely at the evolutionary phase of stars of the
same age but with different mass
The Hertzsprung-Russell Diagram
More mass,
more fuel,
very fast burning.
Shorter
Lifetime
of Star
Less mass,
less fuel,
slow, steady burning.
Longer
How do we know the age of a star?
Our First Measurement of Age
Star Clusters
Open cluster: 103 stars, up to 30 pc
in size, found in disk of galaxy.
All have mostly young stars
Globular cluster: up to 106
stars and 150 pc in size, in
disk and halo of galaxy. All
have old stars
Why are clusters useful to astronomers?
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All stars in a cluster are at about same distance
from Earth.
All stars in a cluster are of about the same age.
Clusters therefore are natural laboratory in
which mass, rather than age, of stars is only
significant variable.
The Hertzsprung-Russell Diagram
We can date
a cluster by
observing its
population of
stars.
Turn-Off point:
Age indicator
The oldest clusters
known have been
measured to be
~14 billion years old.
All these stars in the
cluster have burned
themselves out!
Variable Stars
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Chepeids
RR-Lyrae
Variability due to Instability
Variability is PERIODIC
Instability caused by presence of ionized He
More luminous variable stars have large Period
Variability is EXTREMELY USEFUL,
because it is an absolute distance indicator
Cepheid Variable Stars
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Cepheid variable stars have
variable brightness that is
very regular.
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The period of the variation
can be from days to weeks
Pulsation due to instability:
He ionization layer acts a
energy sponge
it seems to be a reliable
indication of the star’s
luminosity!
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Cepheids: the Period-Luminosity Relation
Henrietta Leavitt
Henrietta Leavitt
(1868-1921).
Luminosity=4D2B
Standard Candles
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If we know an object’s
true luminosity, we can
measure its distance by
Luminosity
measuring its apparent Brightness 
2
4  distance
brightness.
An object that has a known luminosity
is called a standard candle.
What is burning in stars?
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Gasoline
Nuclear fission
Nuclear fusion
Natural gas
Review Questions
1.
How can one estimate the age of a stellar
cluster?
Survey Questions
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4.
How is the helium core of a star
supported?
What causes the expansion of a star to
become a red giant?
What is a supernova?
When does a massive star explode as a
supernova?.