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The Life Cycle of Stars
Formation, Evolution and End States
ASTR 101
4/19/2017
1
Hertzsprung–Russell Diagram
O
B
A
F
G
K
M
Blue giants
107years
10000
Luminosity
(solar)
100
Red super giants
10M☉
108years
Red giants
A
main sequence
1
1010years
Sun
100 R☉
10 R☉
1011years
0.2M☉ 1 R☉
0.01
white dwarfs
0.0001
Spectral class
25000
10000
red dwarfs
8000
6000
4500
surface temperature
3000
0.1 R☉
•
Main Sequence: The majority of stars (~90%), including the Sun, are in a diagonal
band, going from upper left corner (hot, luminous, massive stars) to the lower right
corner (cool, dim, low mass stars).
– Blue giant: (upper left) large and hot stars
– Red dwarfs: (lower right) small low temperature stars
•
White Dwarfs: Stars in lower left, those are hot but faint stars so must have smaller
surface ⇒ must be very small.
•
Red Giants: : Stars in upper right, those are colder (3000K) but very bright, so they
should have large surface area ⇒ must be very big.
2
Star formation and Evolution
Brown
dwarf
•
A star begins its life as a collapsing interstellar gas cloud under its own gravity
– As the gas cloud contracts, its center heats up and nuclear reactions begins.
•
A star shines by the energy from those nuclear reactions going on in its core.
– In most stars hydrogen is converted into helium.
•
Because stars are powered by nuclear reactions which consume star material,
they have a finite life span.
•
The theory of stellar evolution describes how stars form and change during that
process.
3
Gas/dust clouds in the Milky way
Carina Nebula
size 300 ly.
A young cluster of stars surrounded by the
nebula in which they were formed (NGC 3603 )
Orion nebula
size 20 ly.
Interstellar gas and dust clouds (called nebulae) which could evolve into stars
are abundant in galaxies, concentrated in the galactic disk.
– They could span few to hundreds of light years in size and have masses
many thousand times the Sun.
– They are very cold (10-20K) low-density clouds, 102—106 atoms/cm3.
(compare with 2x1019 atoms/cm3 in air, better than best vacuums
4
produced on Earth)
Horse head nebula
•
Eagle nebula (M16), about 6000 ly
away 70x55 ly in size
‘pillars of creation’ taken by
the Hubble space telescope,
a star forming in M16
Interstellar Gas clouds are mostly hydrogen and helium, primordial mix of
elements:
– about 92% hydrogen atoms and 8% helium atoms by number (or about 75%
H and 25% He by mass), matter formed right after the Big bang.
– There could be 1-2% heavier elements in gas clouds now.
• Heavier elements were produced in stars and ejected into the interstellar
space as a star blows off its outer layers during the final stages of its life.
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•
Some disturbance, (like a shock wave from a nearby exploding star…)
triggers the initial collapse of the gas cloud.
•
A small over density in the cloud exert an extra gravitational pull on the
surrounding lower density gas.
•
As the cloud contracts the over density grows. A cloud may have few such
over dense regions, which pull surrounding matter and grow in size and
density.
•
Contraction subdivides it into smaller pieces, each piece will contact
become a star, thus forming a cluster
–
and
stars are not born in isolation, isolated stars (like our Sun) were born in a
cluster and later ejected from it due to gravitational interactions.
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A young cluster of stars
embedded in the nebula in
which they formed.
Pleiades star
cluster 100M
years old
•
•
Any initial movements in the gas cloud results in
some initial rotation.
As the gas cloud contracts its rotational speed
increases.
– Due to the conservation of angular momentum
The same reason when a skater spins slow when her arms
and legs are extended and faster when she pulls her arms in
Angular momentum = 𝑚𝑎𝑠𝑠 × 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 × 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑜 𝑎𝑥𝑖𝑠
A protostar
7
8
few light years
100 AU
protostar
slowly rotating
interstellar cloud
As the interstellar cloud collapses under self-gravity the small rotation of the
cloud is amplified and flattens out the cloud forming a protostar disk.
• The rotating ball collapses into a thin disk with most of the mass
concentrated near the center forming a protostar (no nuclear burning yet).
• The disk may evolve into a planetary system.
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Protostar stage
Protostar HL-Tauri
www.eso.org/public/videos/eso1436a
•
Proto stars in the Orion nebula
During contraction, Collisions between infalling gas particles dissipate energy
and heat up the protostar and it starts to glow.
– A protostar with a one solar mass can reach a surface temperature of a few
thousand Kelvins.
– and 100 times brighter than the Sun due to its large size.
– (no nuclear reactions yet, it is emitting energy from gravitational collapse)
10
+
+
Electrical repulsion between two
positively charged hydrogen nuclei keep
them apart at lower temperatures.
+
+
At temperature above 10 MK, H nuclei
have enough thermal energy (speed) to
overcome the electrical repulsion and
come sufficiently close to each other to
undergo nuclear interactions.
•
The protostar continues to collapse under gravity, further heating it up in the
process.
•
It becomes hottest near the center (core) where much of the mass is
collected.
•
When the core reaches 10 million K, hydrogen nuclei (protons) are moving
fast enough to overcome the electrical repulsion between them and combine
together.
•
Nuclear fusion of hydrogen begins. The protostar becomes a star.
Hydrogen Nuclear Fusion
e+
+
•
+
++
ν
+ n
2H
1
There is no stable nucleus with just two protons, so when two protons
combine, they rearrange to form a deuterium nucleus 2H1 (the hydrogen
isotope which has a proton and a neutron)
1H
1
or in simple terms:
+ 1H1 → 2H1 + e+ + neutrino
1H
1
+ 1H1 → 2H1 + energy
12
The Proton-Proton Chain
•
1H
•
Deuterium thus produced combine with a hydrogen
nuclei to from a helium 3 isotope 3He,
(3He2 = two protons+ a neutron)
and releasing some energy
2H1 + 1H1 → 3He + energy
2
•
1
+ 1H1 → 2H1 + energy
+ 3He2 → 4He2 + 1H1 + 1H1 + energy
Overall, net result is four hydrogen nuclei
combine to form a Helium nucleus releasing some
energy.
1H
•
E
Two 3He2 nuclei combine to form helium 4
(4He2 = 2 protons+2 neutrons)
3He
•
109 years
4He
2
E
1 second
2
1
+ 1H1 + 1H1 + 1H1
E
E
106 years
→ 4He2 + energy
nucleus is slightly lighter than four
1H
1
nuclei
mass of 4 hydrogen nuclei
= 6.693x10-27 kg,
mass of a Helium (4He) nucleus = 6.645x10-27 kg
Difference in mass = 0.048x10-27 kg, 0.7%,
E
●proton, ●neutron , E energy
this lost mass is converted to energy according to E=mc2
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Main Sequence Stars
pressure
gravity
H→He
+energy
•
Once nuclear fusion sets in, energy production is maintained by hydrogen
fusion, further gravitational contraction stops and the protostar becomes a
star.
•
The star has reached a hydrogen burning stable state of a ‘main sequence’
star and will remain there as long as it has hydrogen to burn.
•
A hydrogen burning main sequence star is in a stable equilibrium state, its
temperature (and pressure) is maintained by the energy produced by
hydrogen fusion.
•
A star spends most of its life as a main sequence star ⇒ they are the most
common type of stars in the galaxy(90%).
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Life time of a star
pressure
gravity
larger gravitational force
balanced by a larger pressure
⇒ higher core temperature
massive star
•
Mass of a star determines its evolutionally path and rate of nuclear
burning.
– A massive a star has a higher gravitational pull towards the center
– To stay balance it has to have a larger internal pressure ,which requires a
higher internal temperature.
– At a higher temperature star burns hydrogen faster, and releases more
energy.
⇒ hydrogen consumption and brightness of a star goes up with the mass
of star
•
Luminosity (energy output) of a star is approximately proportional to the
third power of its mass
𝐿 ∝ 𝑀3
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.
Massive stars are short lived!
A massive star has more hydrogen, but burns its hydrogen fast, so has
a shorter life time
– The Sun will stay ~10 billion as a main sequence star.
– a 100 solar mass star has a luminosity of 106L☉ (million times the sun)
and will stay there less than a million years.
– a star with tenth of a solar mass (0.1 M☉) has a luminosity of 10-3L☉ (one
thousandth of the Sun) and a lifetime of 100 billion years.
•
Massive stars are brighter, but has a shorter life.
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Brown Dwarfs
Brown dwarf Gliese 229B, about
20-50 times the mass of Jupiter
•
If the mass of a star is lower than 0.08 Solar mass (80 times the mass of
Jupiter), its core won’t get hot enough to begin hydrogen fusion.
•
Initially they glow in infrared due to the gravitational energy, and gradually cools
off and become brown dwarfs.
•
Brown dwarfs are difficult to observe directly, as they are very dim.
•
On the other end, if the protostar is more than about 100 solar masses, nuclear
reactions happens so fast, the star blows apart.
•
Mass of a hydrogen burning main sequence star is between 0.08M☉ and 100M☉
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Red Giant stage
•
As the star ages it gradually runs out of hydrogen, and
helium accumulates at the center.
•
Helium (4He) cannot undergo nuclear fusion until the
core temperature is extremely high, of the order of 100
million K.
•
So core cools down and the pressure decreases.The
inert He core contracts, and gets hotter.
•
Hydrogen nuclear burning continues in a shell around
the core.
•
Higher temperatures around the core increase the
hydrogen fusion rate and the energy production.
•
This increased outflow of energy expands the outer
layers of the star. The star extends over 100 times
becoming a red giant star.
– Expanding outer layers cools down, lower temperature
of the outer layers makes it appear red.
– Despite its cooler temperature, its luminosity increases
enormously due to the large size.
– When the Sun becomes a red giant in about 7 billion
years its will extend beyond the orbit of Mercury.
H burning
shell
inert He
core
cool extended
envelope
++
++
A helium nucleus has two
protons, a larger electrical
charge. So mutual repulsion
is greater, a higher energy
(temperature) is needed to
bring them closer to undergo
nuclear reactions.
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Helium Fusion
•
Meanwhile, the He core continue to contract until it reaches a
temperature of about 100 million K.
•
Once the core temperature has risen to 100M K, helium in
the core starts to fuse through the triple-alpha process:
•
4He
He burning
shell
inert C
core
+ 4He + 4He → 12C + energy
(4He nucleus is often called an alpha particle, hence the name)
•
This reaction produces carbon in the core of the star.
•
Now there is hydrogen burning outer shell and Helium
burning inner shell and an inert carbon core.
•
Higher energy production results in a further expansion of
the outer layers.
•
Low escape velocity of the expanded outer layers of the giant
star causes the outer layers to escape the star.
•
The ejected envelope expands into interstellar space, forming
a planetary nebula.
•
Light from the central star makes the planetary nebula
shine.
–
H burning
shell
Early astronomers thoughts they were newly forming
planetary systems, so they called them planetary nebulae,
actually they are the opposite, dying stars.
cool extended
envelope
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Degenerate Pressure
•
•
•
The star now has two parts:
•
•
•
A small, extremely dense carbon core
An envelope about the size of the solar system
As the dead core of the star cools, the nebula continues to expand
and dissipate into the surroundings.
If the star has a mass less than 8 solar masses the
core cannot get hot enough for carbon fusion (due to
higher nuclear repulsion).
Without pressure generated by nuclear reactions, the
core contracts up to the point where electrons resist
further packing.
– Like electrons have no more room to move around,
all available energy states elections can occupy are
filled, which creates an ‘electron degenerate
pressure’.
– No further contraction of the core is possible.
– This state of a star core, which had contacted to
about the size of Earth and held by the electron
degenerate pressure is called a white dwarf.
– Densities of 108 to 1011 kg/m3
e-
e-
+ e+
+
eee
+
e+ +
+ +
+
+
+
eeee+ +
+ +
ee- +
+
e
e+
+ +
eeee+
e-
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White Dwarfs
Comparison of a
White Dwarf Star
and the Earth.
Sirius and its white
dwarf companion
Serius B
•
Initially a white dwarf is very hot, and it cools down slowly (in billion year
time scale). White dwarf surface temperatures extend from over 150,000 K
to barely under 4,000 K.
•
Densities of ~ 109 kg/m3 (a teaspoon of white dwarf matter would weight
over a ton).
•
Due to their small size, white dwarfs have low luminosities even when they
are in the initial hot stage. (down to 10-4 solar luminosities).
•
The Sun will become a red giant, then lose about 40% of its mass and
finally become a white dwarf of 0.6 solar masses.
•
Maximum mass of a white dwarf is about 1.4 solar masses ( called the
Chandrasekhar limit). Beyond that the electron degenerate pressure is
not sufficient to counter gravity, and the core further collapses.
•
A white dwarf is the end product for low and intermediate mass stars (0.08
21
to 8 solar masses).
Evolution of massive stars (more than 8 solar masses )
•
Core temperature of massive stars reach higher
temperatures required for the fusion of heavier
elements, before the electron degenerate pressure
sets in.
•
Heavier elements are formed in their cores
–
–
–
12C
+ 4He → 16O + photon (for 500M K)
16O + 4He → 20Ne + photon (for 500M K)
16O + 16O → 32S + photon (for 1 billion K)
•
and so on until iron is produced
•
Reactions to heavier elements go faster and faster :
He burning last less than a million years, creating
oxygen; Neon, Magnesium takes only few years,
production of sulfur takes few days.
•
Fusion reactions undergo like an onion skin structure with more heavy
elements concentrated toward the center of the star where temperature
is highest.
•
Iron has the most stable nuclear structure, it does not release energy
when undergo nuclear fusion, instead absorb energy. So once iron is
formed fusion chain stops.
•
This process is called Steller Nucleosynthesis most elements other
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than hydrogen and helium are produced by this process in stars.
Steller Nucleosynthesis
energy is
consumed
Relative abundance of elements in the solar
system elements with even atomic numbers are
more abundant then odd atomic numbers, a
possible consequence of alpha capture process
alpha capture process
(After 56Ni energy is consumed in alpha
capture and the process stops, 56Ni
decays to iron isotope 56Fe )
23
Supernova explosions
gravity
•
e-
e-
+ e+
+
+
eee
e+
e+ +
+ +
+
+ +
eeeee+ + e+ +
ee- +
+
ee+ + +
ee-
gravity
A star initially more massive than 8 solar masses will have a core heavier
than the Chandrasekhar limit (1.4M☉ ) after the red giant stage.
– electron degenerate pressure can not overcome the crushing force of
gravity. It is too big to be a white dwarf!
– Gravitational crush will overcome the electron degenerate pressure,
further squeezing the core.
– Under the enormous crushing force of gravity protons and electrons in
the core fuse together producing neutrons in a fraction of a second.
proton + electron → neutron + neutrino
( p+e- → n + ν )
releasing a large number of neutrinos in the process
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Supernova explosions
the core of the star is
compressed into neutrons
infalling material
bounce off the
hard neutron core
expelling the stellar
material into space,
forming the supernova.
•
At such high densities neutrinos interact with the matter and push the star
envelope outward at a high velocity (~5% the speed of light).
•
In addition infalling stellar material bounces back off the core creating a
shock wave that propagates out through the star, expelling the stellar
material into space
•
As a result the whole star blows apart in an explosion called supernova.
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Supernova explosions
Supernova 1987 in the large Magellanic cloud
a supernova in the Pinwheel Galaxy
•
A supernova momentarily outshines the Sun by 10 billion times, as bright
as an entire galaxy
•
During supernova explosion heavier elements (nickel, silver, lead….) are
produced and ejected into the interstellar space, along with other elements
produced in the star over its life time.
•
Supernovas are rare :
– once in a century in a galaxy.
– The last one in the Milky way was in 1604 (observed by Kepler).
– There was one in the Large Magellanic Cloud in 1987.
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Crab nebula 1000 years
(size 5.5 light years)
Remnant of Tycho’s 1572 Supernova
Veil nebula 6000 years
(size 50 light years)
Images of few supernova remnants
•
Interstellar material enriched with heavy elements from a supernova
becomes the raw material for a new generation of stars and the cycle of
stellar birth and death begins again.
•
The Sun is a second-generation star. Each of the heavy elements in the solar
system (including planets, people…) had been produced in the core of a star
that blew away in a supernova explosion billions of years ago.
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Neutron stars
•
The core of the star is now mostly composed of closely packed neutrons.
– Without anywhere to move around, they create neutron degenerate
pressure.
– Neutron degenerate pressure stops further collapse of the core and forms a
neutron star.
•
•
It is like a one giant nucleus made of neutrons
If the core is less than 3 solar masses, gravity can be balanced by the
neutron degenerate pressure.
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Neutron stars
earth
white dwarf
neutron star
An artist's rendering of a neutron star
•
Mass of a neutron star is between 1.4-3 solar masses, typical radius ~
10 to 20 km!
•
It is the densest form of matter in the observable universe: 1017 kg/m3
– a teaspoon of neutron star matter would be 100 million tons
Due to conservation of angular momentum, it is spinning very fast, up to
thousands times a second
– angular momentum of the star now carried by a small object, hundreds
of thousand times smaller in size
•
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Pulsars
•
•
A Neutron star has an extremely strong magnetic field.
Material falling to a neutron star are accelerated and taken to the poles by
the magnetic field.
– This results in strong beams of electromagnetic radiation emitted at the magnetic
poles.
•
•
•
•
If the rotation axis is not the same as the magnetic axis, the two beams will
sweep out circular paths.
If the Earth is in one of those paths, the beams of radiation from the jets
sweep around us as the pulsar rotates (just as the light beam from a
lighthouse does.)
So from earth we see regular flashes of radiation from the neutron star, at
the frequency equal to its rotation rate.
Neutron stars for which we see such radiation pulses are called "pulsars"30
•
The first pulsar was discovered in 1967. It emitted extraordinarily regular
radio pulses (every 1.33733 seconds), nothing like it had ever been seen
before.
•
After some initial confusion, it was realized that it was a neutron star
spinning very rapidly (no large object could spin so fast).
– It was discovered by Jocelyn Bell, a gradate student at the time. Her advisor
(Anthony Hewish) shared the 1974 Nobel Prize for the discovery.
(a good article about the discovery by Jocelyn Bell at :
http://www.bigear.org/vol1no1/burnell.htm)
•
Typical period of pulsars very from milliseconds to few seconds.
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Crab nebula and the Pulsar
Crab nebula in visible light,
debris ejected from a
supernova explosion in 1054
•
•
•
X ray image of the crab nebula,
by Chandra X-Ray observatory
Slow motion
video of the
crab pulsar
Chinese astronomers had reported of an extremely bright star appeared
in 1054, which was bright enough to be visible during the day.
Today at that location we see a nebula, with gases in the cloud
expanding outward at about 1,500 km/s.
In 1967 a pulsar was discovered in it.
– period 33 ms (flashes 30 times per second), slowing down by 38 ns/day
– about 20 km in diameter
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Black Holes
Nothing can escape from a black hole
•
If the core has more than 3 solar masses, even the neutron degenerate
pressure cannot stop the gravitational collapse.
– Gravity wins, and the core collapses to a highly dense state, a space-time
singularity called a black hole.
•
Gravitational force of a black hole is so high that even light cannot escape from it.
So it would look “black”.
• Its escape velocity is larger than the speed of light (fastest speed an object can
have)
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Event Horizon
light circles the
black hole at the
the event horizon
Escape velocity =
2GM
c2
Event horizon
Escape velocity = speed of light.
r
2𝐺𝑀
𝑟
Black
hole
(M is the mass within a sphere of radius r)
•
If an object is squeezed to an extremely small size, at some point escape
velocity from it exceeds the speed of light.
•
It is called event horizon, when an object is squeezed beyond the size of its
event horizon, it will become a black hole.
– Nothing come closer than the event horizon can escape the black hole.
• For the Earth, that happens when it is compressed beyond 1 cm.
• For the Sun 3km, for a star core of 3 solar masses it is 9 km.
•
Only gravitational force can crush matter to such extreme densities.
– Object has to be massive enough to create the needed gravitational crush
– which eventually overcome all mechanisms preventing gravitational collapse.
Thermal pressure, electron degeneracy, neutron degeneracy
34
Observing Black holes
An Illustration of
Cygnus X-1
A composite image
(visible, Xray, IR) of
the active galaxy M82
•
No light is coming out of a black hole as their gravitational fields will cause light
to bend around them.
•
•
But when matter fall into a black hole, before they enter the event horizon due
to gravitational acceleration they emit radiation, in the form of strong X-rays.
Few such X-ray sources have been found and are likely black hole candidates.
•
Cygnus X-1 a strong X resource is the first suspected black hole candidate.
– It is a binary system, visible partner is about 25 solar masses.
– The system’s total mass is about 35 solar masses, so the X-ray source must
be about 10 solar masses, too large for a neutron star.
– Hot gas appears to be flowing from the visible star to an unseen companion,
with X-ray emission that flickers in hundredths of a second.
•
There are strong evidence that the centers of most galaxies contain
supermassive black holes—about 1-1000 million solar masses.
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The Supermassive black hole at the center of Milky way Galaxy
visible
infrared
radio
center of Milky way in different wavelength light
•
Galactic center is a very strong radio source (called Sagittarius A)
•
Infrared studies of the region done over last two decades have discovered
stars orbiting around a very massive object at the location of Sagittarius A,
sometimes reaching 4% speed of light.
•
Very likely a supermassive black hole of 4.1 million solar masses.
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Star formation and Evolution
Life Cycle of a Star
nebula
Brown
dwarf
< 0.08M☉
< 8M☉
protostar
> 8M☉
Core < 3M☉
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Hertzsprung–Russell Diagram
O
B
A
F
G
K
M
Blue giants
107years
10000
Luminosity
(solar)
100
Red super giants
10M☉
108years
Red giants
A
main sequence
1
1010years
Sun
100 R☉
10 R☉
1011years
0.2M☉ 1 R☉
0.01
white dwarfs
0.0001
Spectral class
25000
10000
red dwarfs
8000
6000
4500
surface temperature
3000
0.1 R☉
•
Main Sequence: The majority of stars (~90%), including the Sun, are in a diagonal
band, going from upper left corner (hot, luminous, massive stars) to the lower right
corner (cool, dim, low mass stars).
– Blue giant: (upper left) large and hot stars
– Red dwarfs: (lower right) small low temperature stars
•
White Dwarfs: Stars in lower left, those are hot but faint stars so must have smaller
surface ⇒ must be very small.
•
Red Giants: : Stars in upper right, those are colder (3000K) but very bright, so they
should have large surface area ⇒ must be very big.
38
Evolutionary path of a star like Sun in the HR diagram
10000L☉
1000 R☉
planetary nebula forms
500 million years
helium burning
red giant stage
100L☉
Proto star
contraction
50 million years
main sequence
1L☉
10 billion years
helium core 10 R☉
contracting
Sun
1 R☉
0.01L☉
white dwarf
0.1 R☉
0.0001L☉
10000K
6000K
3000K
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Evolution of Stars in a Cluster
When an interstellar gas cloud collapsed under gravity, a large
number of stars, a cluster, with stars of different masses are
formed.
Massive stars evolve and go through their life cycles faster than
less massive stars.
The open cluster Hyades: Its H-R diagram shows a high man sequence
cut off, and few white dwarfs. Probably it is 600 million years old.
Globular cluster 47 Tucana: Main sequence turnoff has reached sun
like stars, well developed red giant and white dwarf branches. So
it has to be more than 10 billion years old.
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Cepheid Variable Stars
•
Later in their evolution, at the end of the red giant phase most stars
undergo unstable oscillations.
– The star becomes a variable star, its brightness fluctuates periodically.
•
There are many types of variable stars, and many reason why they change their
luminosity periodically.
•
One type of variable stars, called Cepheid Variables shows a direct relationship
between their luminosity and the period of variation.
•
In 1912 Henrietta Leavitt working at the Harvard College Observatory was looking
for variable stars in the Small Magellanic Cloud.
•
She noticed that one type of variable stars, Cepheids (named after delta
Cepheus, first star of that type) had a longer period when they are brighter.
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apparent brightness
log(period)
A Cepheid in the Andromeda galaxy.
•
All stars in the Small Magellanic cloud are at about the same distance,
•
Thus if its period is known, its luminosity can be estimated.
•
Cepheids are bright supergiant stars (~1000 times brighter than the Sun),
so they can be identified even they are outside the Milky way, in other
galaxies.
•
In 1924 Edwin Hubble showed that Andromeda galaxy was an object outside the
Miky way by identifying few Cepheid variables in the Andromeda nebula (as it was
called then) and estimating its distance.
⇒ the brighter ones have longer periods suggested that period and luminosity
were related.
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Review Questions
What are the interstellar gas clouds made of?
Where does the interstellar dust come from?
What cause an interstellar gas cloud to collapse and form a star?
What is the source of energy of a protostar?
Why does proto stars smaller than 0.08 solar masses never become stars?
Why don’t we see stars heavier than 100 solar masses?
What is a main sequence star?
What is a planetary nebula?
Why are most stars we see are main sequence stars?
What keeps a main sequence star from collapsing under gravity?
Why does a massive star have a shorter life span while less massive stars lives longer?
Why isn’t hydrogen fusion occur at temperatures below 10 million K.
Why does Helium fusion need a higher temperature than for hydrogen fusion?
What keeps a white dwarf star collapsing under gravity?
What is a neutron star? How is it different from a white dwarf?
Why does a neutron star spins so fast?
What is the difference between a neutron star and a pulsar?
What is the main factor which determine the course of a star’s evolution?
Why isn’t elements heavier than iron produced by nuclear fusion in stars?
How are the elements heavier than iron produced?
What would be the end state of a star like the Sun?
What is the end state of a massive star?
What are the evidence that black holes exist?
How is it possible to estimate the age of a star cluster from its HR diagram.
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Why do Cepheid variable stars important in astronomy?