Transcript Lecture20

The Lifetime of Stars
• Once a star has reached the main sequence stage
of it life, it derives its energy from the fusion of
hydrogen to helium
• Stars remain on the main sequence for a long time
and most of their lifetime
• The left hand edge of the H-R diagram is called
the zero-age main sequence
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Zero age is the time when the star matures enough to
reach the main sequence
• As the hydrogen is used up, the mass of the star
does not change but its luminosity and
temperature do change
ISP 205 - Astronomy Gary D. Westfall
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Lifetime on the Main Sequence
• The amount of time a star spends on the main
sequence depends on its mass
• Large star are much more luminous and have
shorter lifetimes
• Small stars are much less luminous and live much
longer
Spectral Type
Mass (Sun = 1)
Lifetime on Main Sequence
O5
40
1 million years
B0
16
10 million years
A0
3.3
500 million years
F0
1.7
2.7 billion years
G0
1.1
9 billion years
K0
0.8
14 billion years
M0
0.4
200 billion years
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From Main Sequence to Red Giant
• When all the hydrogen in the core is fused, the
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star must contract
Hydrogen falling into the core releases
gravitational energy heating the hydrogen just
outside the core and causes it to ignite
The helium begins to burn providing even more
heat
The outer layers of the star heat up and expand
Rapid departure from main sequence and creation
of a red giant
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Comparing a Supergiant to the Sun
Property
Sun
Betelgeuse
Mass (2 x 1033
g)
1
16
Radius (km)
700,000
500,000,000
Surface
Temperature
(K)
5,800
3,600
Core
Temperature
(K)
15,000,000
160,000,000
Luminosity (4 x
1026 W)
1
46,000
Density (g/cm3)
1.4
1.3 x 10-7
Age (years)
4.5 billion
10 million
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Models for the Evolution of Red Giants
• Computer
models for the
evolution of red
giants with
different masses
and
compositions
on the H-R
Diagram
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Star Clusters
• It is reasonable to assume that stars located very
close together were formed at about the same time
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Star clusters
• Globular clusters
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Old stars
• Open Clusters
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Young stars
• Stellar associations
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Young stars
Globular cluster M15
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Globular Clusters
• Globular clusters were given their name because of their
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appearance
They have 104 to 106 stars
The brightest stars are giants that are pale yellow in color
About 150 globular clusters
are known in our galaxy
located in a spherical halo
surrounding the flat disk of
the galaxy
They are found very far
away from the Sun at
distance of 65,000 LY from
the galactic plane
ISP 205 - Astronomy Gary D. Westfall
Omega Centauri, the largest globular
cluster in the Milky Way
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Open Clusters
• Open clusters are found in the disk of our galaxy
associate with interstellar matter
Open cluster contain far fewer stars than globular clusters
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• There are many
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thousands of
open clusters but
many are
invisible because
of dust clouds
Open clusters
often have a few
brilliant stars
Open star cluster M6, the Butterfly Cluster
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Stellar Associations
• Stellar associations
have very young stars
and are often obscured
by interstellar dust
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Most are hidden from
our view
• The association
typically has 5 to 50
hot, bright O and B
spectral class stars less
than a million years
old
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Model for Young Clusters
Model predictions for young
clusters
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Measurements for young cluster
NGC 2264
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Model for Older Clusters
Model for older clusters
ISP 205 - Astronomy Gary D. Westfall
Measurements for globular
cluster 47 Tucane
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Helium Burning
• When the star uses up its hydrogen, it burn helium
through the triple alpha process to form carbon
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Helium has 2 p, 2 n, stable
Carbon has 6 p, 6 n (3 He), stable
Beryllium-8 is not stable (4 p, 4 n)
• When the helium burning process starts in low
mass stars, the entire core ignites in a helium flash
• Following the helium flash, the star readjusts
more toward the main sequence
• When the helium is consumed, carbon burning
can only take place in large stars
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In stars like our Sun, death is near
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Mass Loss
• When stars become giants, they begin to lose mass into
space
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By the time a star reaches the helium flash, it will have lost
25% of its mass
• The outer layers of the star are stripped and planetary
nebula are formed
Animation of the
formation of the
planetary nebula called
the Helix Nebula
ISP 205 - Astronomy Gary D. Westfall
QuickTime™ and a
decompressor
are needed to see this picture.
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Nucleosynthesis
• All elements heavier than helium are made in
stars
• The elements in globular clusters are depleted in
heavier elements
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They were formed from material that had not been
processed in stars as much as younger stars
Newer stars have 1% - 4% heavier elements
 Old stars have 1/10 to 1/100 as many heavier elements as the
Sun
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• The first generation of stars could not have
formed planets like Earth that are rich in silicon
and iron
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The Death of Low Mass Stars
• Let’s start with stars smaller than 1.4 Msun
• After the small star burns all its hydrogen and then all its
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helium, it has an energy crisis
Because the star is small, it cannot ignite the remaining
material in its core and so the star collapses and forms a
white dwarf
White dwarfs are so dense that the electrons are
degenerate
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So compressed that they are nearly on top of each other
Electrons cannot be on top of each other
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Fermions, Pauli exclusion principle
Cannot collapse further
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White Dwarfs
• White dwarfs are stabilized against further collapse
• Calculations show the larger the mass the smaller the radius
• Stars with mass greater
than 1.4 Msun have zero
radius
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Chandrasekhar limit
• For larger stars, the
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force of degeneracy
cannot prevent the
collapse of the star
Black hole!!
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Evolutionary Track of a Sun-like Star
• Start the calculation
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when the star becomes a
red giant (A)
The star loses mass and
its core begins to
collapse
The star heats up (B) as it
collapses
• The luminosity remains constant until it begins to shrink
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significantly and then it begins to dim
The star is now a white dwarf (c) and will continue to
radiate its energy into space
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The Ultimate Fate of White Dwarfs
• The onset of hydrogen fusion constitutes the birth
of a star and the exhaustion of of all fusion fuel
mean the death of the star
• The only energy source of the white dwarf is
residual heat which radiates into space leaving a
black dwarf
• This black dwarf is composed mostly of carbon
and oxygen
• In its final stages the black dwarf becomes a
monumental diamond!
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Evolution of Massive Stars
• Stars with masses less that 7.5 Msun will lose mass
as they age and end up as white dwarfs
• Stars are known with masses as large as 150 Msun
• Massive stars continue to fuse elements after the
hydrogen and helium are fused
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Helium to carbon
Carbon to neon
Neon to silicon
Silicon to iron
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It all stops at iron because iron is the most well bound
nucleus
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Collapse into a Ball of Neutrons
• A massive star builds up a white dwarf in its
center where no nuclear reactions are taking place
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For a large star this center is made of iron
• Fusion takes place outside this core producing
more heavy nuclei that fall into the core
• A higher mass means smaller radius so the core
contracts
• The core density surpasses that supported by
degenerate electrons and passes over to
degenerate neutrons
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Neutron star!
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Collapse and Explosion
• The collapse of the core to a neutron star takes place
catastrophically
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The core goes from Earth size to 20 km in 1 second
• When the core reaches nuclear density the collapse is
halted abruptly and a shock wave bounces back through
the star blowing off the outer layers of the star
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Supernova!
• Supernovae have been observed in history
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1006
1054, Crab nebula
1181
1572, Tycho Brahe
1605, Johannes Kepler
1987, Supernova 1987A
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Material Ejected by Supernova
• Most of the material of the star is ejected into space
• This material is very important because the processed nuclear
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material are recycled into space
In addition, the supernova produces a flood of neutrons that can be
absorbed by iron and other nuclei to build up all the heavier
elements
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Iron is Z=26 and elements up to Z=92 occur naturally
• Supernova are the source of high energy cosmic rays that have
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contributed to mutation and evolution
However, you would not want to be near a supernova when it
explodes
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Anything within 100 LY would be disastrous
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Supernova 1987A
• First observed February 24, 1987 in the Large Magellanic Cloud
• Thought to be 10 million years old with a mass of 20 Msun 160,000
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LY away
When it became a red giant, material was ejected
Helium fusion last about 1 million years forming a core of carbon
and oxygen
When the helium was exhausted, carbon and oxygen burning
began and the star became a blue supergiant
When the carbon was exhausted, burning to heavier elements only
lasted for a few years
Once iron was created, collapse occurred
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Collapse of SN 1987A
• Collapse occurred in a few 0.1 of a second blowing off
the outer layers of the star
Picture
taken in
1994
Picture
taken in
1997
• Pictures show collision of material from supernova
colliding with material ejected from red giant stage
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SN 1987A
• Up until 40 days after the explosion, the light
produced originated from the explosion
• After that time, the radioactive decay of the
produced heavy elements kept the star bright
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56Ni
to 56Co to 56Fe, 6 days and 77 days
• Most of the energy emitted by SN 1987A was in
the form of neutrinos
• 11 neutrinos were seen in Japan and 8 in the US
over a span of 13 seconds
• These neutrinos were detected after passing
through the Earth
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The Discovery of Neutron Stars
• Neutron stars are giant nuclei, 1057 nucleons
• The radius of a neutron star is about 10 km
• They were first discovered in 1967 when Bell and
Hewish found an intense, regularly varying radio
source that repeated every 1.33728 seconds
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Pulsars
• Soon after several more were found
• A pulsar was found in the center of the Crab
Nebula
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Supernova of 1054
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Model of Pulsar
• Pulsars are spinning neutron stars
• Their collapse has made them spin very rapidly
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Conservation of angular momentum
• Radiation comes from the north and south
magnetic poles
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
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The Evolution of Pulsars
• The pulsar gives off immense amounts of energy
and must slow down over time
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Old pulsars are slower than new pulsars
The pulsar in the Crab Nebular has been observed to
be slowing down over the past 34 years
• Pulsars are not always observed in the center of a
nebular left over from a supernova
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Pulsars live 100 times longer than the time it takes for
a nebula resulting from a supernova to disperse
• Pulsars may also be ejected from the region of the
supernova by the explosion
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Binary Systems
• As many as half of all stars exist in binary
systems
• If one of the stars is white dwarf, material from
other star can accumulate on that star causing a
reignition of the hydrogen
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Nova
Thousands have been observed, all in binary systems
The white dwarf survives
• If the white dwarf is large, the explosion may
destroy the white dwarf
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Supernova type I
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Neutron Stars with Companions
• A binary system can survive the explosion of one
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of the stars
An ordinary star can be paired with a neutron star
It is possible for the neutron star to accrete
material from its partner and become a gammaray emitter or an x-ray burster
These bursters can rotate 1000 times a second
(compared with pulsars of 30 per second)
Another fate of star is the black hole which is the
subject of our next lecture
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