Download Lecture 13

Properties of Stars II
• The Hurtzprung-Russell Diagram
• How long do stars live?
• Star clusters.
How stars age (evolve)
• Suppose we want to study how people change
over their life time.
• How could we do this.
• We could follow a person from birth to death.
• That would take a long time.
• But there are a lot of people in the world so …we
could study people of different ages so we don’t
have to wait a lifetime for our research to finish.
• For obvious reasons we cannot wait for stars to
change .
The Hertzsprung-Russell Diagram
Luminosity
An H-R diagram
plots the
luminosities and
temperatures of
stars.
THIS IS
PROBABLY THE
MOST
IMPORTANT
DIAGRAM IN
ASTRONOMY.
Temperature
Most stars fall
somewhere on
the main
sequence of the
H-R diagram.
large radius
Stars with lower T and higher
L than main-sequence stars
must have larger radii:
giants and supergiants
These stars have no H to He
fusion going on in their core
(as they have run out of H
fuel).
They are fusing elements
heavy than H in their cores
(for example He to C). They
are also fusing H to He in a
shell outside their core but not
in the core.
Stars with higher
T and lower L
than mainsequence stars
must have smaller
radii:
white dwarfs
These are dead
stars that have no
nuclear fusion of
any kind.
small radius
H-R diagram
depicts:
Temperature
Luminosity
Color
Spectral type
Luminosity
Radius
Temperature
C
B
D
A
Which
star is the
hottest?
C
B
D
A
Which star
is the most
luminous?
C
B
D
A
Which star is
a mainsequence star?
C
B
D
A
Which star
has the largest
radius?
What is the significance of the main
sequence?
Main-sequence
stars are fusing
hydrogen into
helium in their
cores, like the
Sun.
Luminous mainsequence stars are
hot (blue).
Less luminous
ones are cooler
(yellow or red).
High-mass stars
Low-mass stars
Mass measurements
of main-sequence
stars show that the
hot, blue stars are
much more massive
than the cool, red
ones.
The mass of a star is its
High-mass stars key property.
A star’s mass determines
where it will rest on the
main sequence. Its
temperature, radius,
luminosity and lifetime on
the main sequence are all
determined by its mass
only.
Low-mass stars
The reason for this is that a
star’s mass determines the
rate of H to He fusion.
The core
temperature of a
higher-mass star
needs to be higher
in order to
balance gravity.
Hydrostatic Equilibrium
A higher core
temperature
boosts the fusion
rate, leading to
greater
luminosity.
Stellar Properties Review
Luminosity: from brightness and distance
10-4LSun– 106LSun
Temperature: from color and spectral type
3000 K – 50,000 K
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08MSun – 100MSun
Radius: from blackbody radiation the L = constant * T4 * R2
(we will not use this formula or discuss this method on the course).
0.1RSun – 10RSun (on the main sequence)
Stellar Properties Review
Luminosity: from brightness and distance
-4L
6L
10
–
10
(0.08MSun)
Sun
Sun
(100MSun)
Temperature: from color and spectral type
(0.08MSun) 3000 K–50,000 K
(100MSun)
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08MSun–100MSun
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of a 10MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years × 10/104
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Life expectancy of a 10MSun star:
Until core hydrogen
(10% of total) is
used up
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years × 10/104
Life expectancy of a 0.1MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years × 0.1/0.01
Given the age of the Universe is 14 billion years no star of
0.1MSun has ever die of old age.
Main-Sequence Star Summary
High-mass:
High luminosity
Short-lived
Large radius
Blue
Low-mass:
Low luminosity
Long-lived
Small radius
Red
What are giants, supergiants, and
white dwarfs?
These are Off the Main Sequence
• Off the main sequence stellar properties depend on
both mass and age.
• These stars have finished fusing H to He in their
cores are no longer on the main sequence.
• They may be fusing He to Carbon in their core or
fusing H to He in shell outside the core … but there is
no H to He fusion in the core.
• All stars become larger and redder after exhausting
their core hydrogen fuel: giants and supergiants.
• Most stars end up small and white after all fusion
has ceased: white dwarfs.
• The white dwarf stage is the final stage for most stars.
C
B
D
A
Which star is
most like our
Sun?
C
B
D
A
Which of
these stars
will have
changed the
least 10
billion years
from now?
C
B
D
A
Which of
these stars can
be no more
than 10
million years
old?
Star Clusters
Our goals for learning:
• What are the two types of star clusters?
• How do we measure the age of a star
cluster?
Open cluster: A few thousand loosely packed stars
Globular cluster: Up to a million or more stars in a dense
ball bound together by gravity
How do we measure the age of a
star cluster?
Massive blue stars die
first, followed by white,
yellow,
orange, and red stars.
To measure a star clusters
age we make one
important assumption,
ALL STARS IN A
CLUSTER ARE BORN
AT THE SAME TIME.
Visual Representation of a Star Cluster Evolving
How do we measure the age of a
star cluster?
Main-sequence
turnoff
Pleiades
now has no
stars with
a life
expectancy
less than
around 100
million
years.
The mainsequence
turnoff
point of a
cluster tells
us its age.
To determine
accurate ages,
we compare
models of stellar
evolution to the
cluster data.
Using the H-R Diagram to Determine the Age of a Star Cluster
Globular cluster are
very old.
Detailed modeling
of the oldest
globular clusters
reveals that they
are about 13
billion years old.
• Now do the lecture tutorial section on HR
Diagram.
• When you have finished do the lecture
tutorial section on Star Formation and
Lifetimes.