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Lecture 21:
Stars
Review from last time:
from observations of nearby stars,
we can determine:
distance to star
apparent brightness  luminosity
spectral type  temperature
(for binary systems: mass)
radius
Spectral Classes
H-R Diagram
Mass-Luminosity Relationship
Main Sequence Lifetime
stars stay on the main sequence as
long as they can convert Hydrogen to
Helium in their core
the hotter/more massive a star, the
faster it burns up all of its Hydrogen
the more massive a star, the shorter
its ‘main sequence lifetime’.
Estimating the lifetime of a Star
Estimate the lifetime of:
the Sun (M = 2.0E30 kg; L = 3.8E26 J/s)
a B star (M = 10 Msun; L = 3000 Lsun)
an M star (M = 0.1 Msun; L = 0.001 Lsun)
Main Sequence Lifetimes
larger
radius
Stellar Luminosity Classes
Class
Description
I
Supergiants
II
Bright giants
III
Giants
IV
Subgiants
V
Main Sequence
The Stellar Luminosity Function
there are many
more low-luminosity
stars than highluminosity stars
there are many
more low-mass
stars than highmass stars
Selection Effects
Bright stars are easier to see.
Most observed samples contain a bias:
bright objects are over-represented
compared to fainter objects.
How does this affect our ‘census’ of
other stellar properties?
Variable Stars
Some special stars change their
luminosity with a regular pattern.
For example, Cepheid Variable stars
have a relationship between their
luminosity and the period of variation.
this can be used to determine the
distance to the star.
Cepheid Variable in M100
Light Curve for a Cepheid
Variable Star
Why do Cepheids Pulsate?
He+ + light  He++
atmosphere becomes more opaque
radiation pressure pushes the
atmosphere out
atmosphere cools, He++  He+
pressure decreases, star contracts.
We can use Cepheids to find the
distance to far-away galaxies…
measure apparent brightness (flux)
of Cepheid
measure period of variation (t)
find luminosity L using LuminosityPeriod relationship
f = L/(4p d2)
 d = (4p L/f)1/2
Star Clusters
Open clusters are loose, irregular
groups of young stars, found mainly in
the disk of the Galaxy.
Globular clusters are round, regular
balls of old stars, found mainly in the
halo of our Galaxy.
Star Clusters
Open Cluster
Globular Cluster
Star Clusters are useful because:
All the stars in the cluster are at
about the same distance from us
All the stars in the cluster formed at
about the same time (so they are
about the same age)
Pleiades
Luminosity
main sequence
turnoff
Temperature
What is the
age of this
cluster?
Palomar 3
The Age of the Universe
Stars in the oldest clusters have ages
of 10-15 billion years
From the expansion rate of the
universe, we can estimate the time
since the Big Bang. Current values are
around 13 billion years.
Are there stars older than the
Universe???