Download Ch 11c and 12 ( clusters 3-31-11)

Outline of Ch 11: The H-R Diagram
(cont.)
IV.
Star Clusters: Confirmation of Stellar
Evolution
•
Open and Globular Clusters
•
Ages of Clusters
Star Clusters: Confirmation of Stellar Evolution:
1. What is special about star clusters?

All stars formed at same time, so plotting
clusters with different ages on H-R diagrams
we can see how stars evolve

This confirms our theories of stellar evolution
without having to wait billions of years
observing how a single star evolves
2. Two types of clusters: Open and Globular
3. Ages of Clusters
Open cluster: A few thousand loosely packed stars
Globular cluster: Up to a million stars in a dense ball bound together
by gravity
Two types of star clusters
Open clusters: young,
contain up to several
thousand stars and are
found in the disk of the
galaxy.
Globular clusters: old,
contain hundreds of
thousands of stars, all
closely packed together.
They are found mainly in
the halo of the galaxy.
Our Galaxy
Our Galaxy
Which part of our galaxy is older?
How do we measure the age of a
star cluster?
Theoretical
Evolution
of a star
cluster
Massive
blue stars
die first,
followed
by white,
yellow,
orange,
and
red stars
How do we
know that
this
theoretical
evolution is
correct?
How do we
know that
this
theoretical
evolution is
correct?
We plot
observations
of actual
clusters on
the H-R
diagram
Young
Stellar
Cluster
H-R
Diagram
of Young
Stellar
Cluster
Young
Stellar
Cluster
How do we know this
cluster is Young?
H-R
Diagram
of Young
Stellar
Cluster
Old
Stellar
Cluster
H-R
Diagram
of Old
Stellar
Cluster
Old
Stellar
Cluster
How do we know this cluster is
Old?
H-R
Diagram
of Old
Stellar
Cluster
Main-sequence
turnoff
Pleiades
cluster now
has no stars
with life
expectancy
less than
around 100
million years
Mainsequence
turnoff
point of a
cluster tells
us its age
To determine
accurate ages,
we compare
models of
stellar
evolution to
the cluster data
Detailed
modeling of
the oldest
globular
clusters
reveals that
they are about
13 billion
years old
(The universe
is about
13.7billion
years old)
What have we learned?
 How do we measure the
age of a star cluster?
 Because all of a cluster’s
stars we born at the same
time, we can measure a
cluster’s age by finding the
main sequence turnoff
point on an H–R diagram of
its stars. The cluster’s age is
equal to the hydrogenburning lifetime of the
hottest, most luminous stars
that remain on the main
sequence.
Question 1
If the brightest main sequence star in cluster 1
is a B star and the brightest main sequence
star in cluster 2 is an M star. What can we
say about the age of these two clusters?
Question 1
If the brightest main sequence star in cluster 1
is a B star and the brightest main sequence
star in cluster 2 is an M star. What can we
say about the age of these two clusters?
A. Nothing, there is not enough information
B. Cluster 1 is older than cluster 2
C. Cluster 2 is older than cluster 1
D. None of the answers are correct
Chapter 12. Star Stuff (mostly different from book)
I.
Birth of Stars from Interstellar Clouds
•Young stars near clouds of gas and dust
•Contraction and heating of clouds into protostars
• Hydrogen fusion stops collapse
II. Leaving the Main Sequence: Hydrogen fusion stops
1. Low mass stars (M < 0.4 solar masses)
Not enough mass to ever fuse any element heavier than
Hydrogen → white dwarf
2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses,
including our Sun)
He fusion, red giant, ejects outer layers → white dwarf
3.High mass Stars (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster → Core collapses → Supernova
blows up and produces all elements heavier than Fe
Chapter 12. Star Stuff Part I Birth of Stars
I.
Birth of Stars from Interstellar Clouds
•Young stars near clouds of gas and dust
•Contraction and heating of clouds
• Hydrogen fusion stops collapse
12.1 Star Birth
 Our Goals for Learning
• How do stars form?
• How massive are newborn stars?
We are “star stuff ” because the elements necessary for life
were made in stars
How do stars form?
I. Birth of Stars and Interstellar Clouds
•Young stars are always found near clouds of gas and dust
● The
gas and dust between the stars is called the interstellar medium.
•Stars are born in intesrtellar molecular clouds consisting mostly of
hydrogen molecules and dust
• Stars form in places where gravity can make a cloud collapse
Orion Nebula is
one of the
closest starforming clouds
Infrared light from Orion
Summary of Star Birth
• Stars are born in cold, relatively
dense molecular clouds.
• Gravity causes gas cloud to shrink
• Core of shrinking cloud collapses
under gravity and heats up, it
becomes a protostar surrounded
by a spinning disk of gas.
• When core gets hot enough (10
million K), fusion of hydrogen
begins and stops the shrinking
• New star achieves long-lasting
state of balance (main sequence
thermostat)
Hubble
Space
Telescope
Image of
an edge-on
protostar
and its jets
Protostar to Main Sequence (in book)
 Protostar contracts and heats until core
temperature is sufficient for hydrogen fusion.
 Contraction ends when energy released by
hydrogen fusion balances the gravity
 Takes less time for more massive stars to reach the
Main Sequence (more massive stars evolve faster)
I. Birth of Stars and Interstellar Clouds
• Protostar in the H-R diagram
I. Birth of Stars and Interstellar Clouds
• Protostar in the H-R diagram
This is the track
of a collapsing
and heating
protostar but we
do not see most
of them because
they are inside
dense clouds of
gas and dust
I. Birth of Stars and Interstellar Clouds
• Protostar’s T-Tauri phase: a very active phase of
protostars that clears the gas and dust from the
surrounding disk
Question 2
What happens after an interstellar cloud of gas
and dust is compressed and collapses?
Question 2
What happens after an interstellar cloud of gas
and dust is compressed and collapses?
A. It will heat and contract
B. If its core gets hot enough (10 million K) it
can produce energy through hydrogen
fusion
C. It can produce main sequence stars
D. All of the answers are correct
Main Sequence ( Hydrogen Fusion)

Main sequence Thermostat : very stable
phase
How massive are newborn stars?
A cluster of many stars can form out of a single cloud.
•Very
massive
stars are
rare
Luminosity
•Low-mass
stars are
common.
Temperature
•Minimum
mass
needed to
become a
star: 0.08
solar masses
• How massive are newborn stars?
 Low mass stars are more numerous than high mass
stars
 Newborn stars come in a range of masses, but cannot
be less massive than 0.08MSun.
 Below this mass, pressure in the core is not enough
(10 million K) for hydrogen fusion, and the object
becomes a “failed star” known as a brown dwarf.
Equilibrium inside M.S. stars
Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core stays at same temperature
Helium fusion immediately begins
Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core stays at same temperature
Helium fusion immediately begins
Ch. 12 Part II (not like book).
Leaving the Main Sequence: Hydrogen fusion stops
1. Low mass stars (M < 0.4 solar masses)
Not enough mass to ever fuse any element heavier than
Hydrogen  white dwarf
2.Intermediate mass stars (0.4 solar masses
< M < 4 solar masses, including our Sun)
He fusion, red giant, ejects outer layers  white dwarf
3.High mass Stars (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster  Core collapses  Supernova
 Blows up and produces all elements heavier than Fe
Outline of Chapter 12 Part II Evolution
and Death of Stars
I.
Leaving the Main Sequence:
BEWARE THAT THE BOOK DOES NOT USE THE
SAME DEFINITIONS OF LOW, INTERMEDIATE
AND HIGH MASS STARS.
AS MENTIONED, THE EXAM WILL BE BASED ON THE
LECTURES AND NOT ON THE BOOK
Remember: Stellar Masses
Composition inside M.S. stars
Eventually
the core
fills up with
helium and
hydrogen
fusion stops
Leaving the Main Sequence:
Hydrogen fusion stops
1. Low mass stars (M < 0.4 solar masses)
Not enough mass to ever fuse any element heavier than
Hydrogen  white dwarf
White Dwarfs
I. Leaving the Main Sequence:
Hydrogen fusion stops
2. Intermediate mass stars (0.4 solar
masses < M < 4 solar masses, including
our Sun)
He fusion, red giant, ejects outer layers  white
dwarf
Helium fusion requires much higher temperatures than
hydrogen fusion because larger charge leads to greater
repulsion
Stars like our Sun become Red Giants after they
leave the M.S. and eventually White Dwarfs
Most red giants stars eject their outer layers
A star like our
sun dies by
puffing off its
outer layers,
creating a
planetary
nebula.
Only a white
dwarf is left
behind
A star like our
sun dies by
puffing off its
outer layers,
creating a
planetary
nebula.
Only a white
dwarf is left
behind
A star like our
sun dies by
puffing off its
outer layers,
creating a
planetary
nebula.
Only a white
dwarf is left
behind
A star like our
sun dies by
puffing off its
outer layers,
creating a
planetary
nebula.
Only a white
dwarf is left
behind
II. Leaving the Main Sequence:
Hydrogen fusion stops
3.High mass Stars (M > 4 solar masses)
Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster  Core collapses  Supernova
 Produces all elements heavier than Fe and blows up
•
•
Supernovas
3. High mass star (M > 4 solar masses)
•Fusion of He,C,O,…..but not Fe (Iron) fusion
Faster and faster  Core collapses  Supernova
Produces all elements heavier than Fe and blows
envelope apart ejecting to interstellar space most of its
mass
• Supernova Remnants:
Crab nebula and others
An evolved massive star (M > 4 Msun)
An evolved massive star (M > 4 Msun)
before
after
Supernova 1987A in a nearby galaxy is the nearest supernova
observed in the last 400 years
Crab Nebula: Remnant of a supernova observed in 1054 A.D.
Pulsar (a kind
if neutron
star) at center
of Crab
nebula
Older Supernova Remnant