Download Booklet 5 – Stellar Processes and Evolution

Teachers Notes Booklet 5: Stellar Processes and Evolution
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The European Space Agency
The European Space Agency (ESA) was formed on 31 May 1975. It currently has 17
Member States: Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland,
Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland &
United Kingdom.
The ESA Science Programme currently contains the following active missions:
Venus Express – an exploration of our
Cluster – a four spacecraft mission to
sister planet.
investigate
Rosetta – first mission to fly alongside
Sun and the Earth's magnetosphere
and land on a comet
XMM-Newton – an X-ray telescope
Double Star – joint mission with the
helping to solve cosmic mysteries
Chinese to study the effect of the Sun
Cassini-Huygens – a joint ESA/NASA
on the Earth’s environment
mission to investigate Saturn and its
SMART-1 – Europe’s first mission to
moon Titan, with ESA's Huygens probe
the Moon, which will test solar-electric
SOHO
propulsion in flight, a key technology for
atmosphere and interior
future deep-space missions
Hubble Space Telescope – world's
Mars Express - Europe's first mission
most important and successful orbital
to Mars consisting of an orbital platform
observatory
searching for water and life on the
Ulysses
planet
investigate the polar regions around the
INTEGRAL – first space observatory to
Sun
-
interactions
new
–
views
the
first
between
of
the
the
Sun's
spacecraft
to
simultaneously observe celestial objects
in gamma rays, X-rays and visible light
Details on all these missions and others can be found at - http://sci.esa.int.
Prepared by
Anne Brumfitt
Content Advisor
Chris Lawton
Science Editor, Content Advisor, Web Integration & Booklet Design
Karen O'Flaherty
Science Editor & Content Advisor
Jo Turner
Content Writer
© 2005 European Space Agency
Teachers Notes Booklet 5: Stellar Processes and Evolution
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Booklet 5 – Stellar Processes and Evolution
Contents
5.1
Stellar Fusion ................................................................ 4
5.2
Nucleosynthesis ............................................................. 5
5.3
Evolution ...................................................................... 8
5.4
Paths on HR Diagram ..................................................... 9
5.5
Solar Mass Stars .......................................................... 10
5.6
Medium Mass Stars ...................................................... 12
5.7
High Mass Stars........................................................... 14
5.8
Other Materials............................................................ 14
Tables
5.1
Proton-Proton Chain ....................................................... 5
5.2
CNO Cycle .................................................................... 7
5.3
Dates of Primary Meteor Showers..................................... 8
Figures
5.1
Proton-Proton Chain ....................................................... 6
5.2
CNO Cycle .................................................................... 7
5.3
Evolutionary Paths Off the Main Sequence ......................... 9
Teachers Notes Booklet 5: Stellar Processes and Evolution
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5.1 Stellar Fusion
Stars form out of nebulae – giant clouds of gas found in the spiral arms of galaxies. The
gas (predominantly hydrogen) can exist in a cloud for many millions of years, but if it is
somehow disturbed (by the blast from a nearby supernova, or through intercloud
collision for example) the cloud may collapse in on itself.
As the density of the core region increases the collapse accelerates due to the everincreasing gravitational attraction. The central region will start to collapse faster than
the outer regions and so a cloud of gas shrouds the core. If the cloud is particularly
large it may produce multiple stars. This process of cloud collapse signifies the start of
star formation.
Under continuing collapse, the gas in the cloud begins to warm up and gradually
brightens. Eventually the core region reaches a critical temperature such that nuclear
reactions can begin and the body evolves from a protostar into a true star.
The remaining gas and dust from the original interstellar cloud starts to condense and
planets can form. A strong stellar wind, however, drives away some of the matter. Such
stars are called as T Tauri stars (from the first star of this type to be observed).
Teachers Notes Booklet 5: Stellar Processes and Evolution
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5.2 Nucleosynthesis
Nuclear fusion, where nuclei combine to make a nucleus with a larger number of
protons and neutrons, occurs in main sequence stars once they reach a critical
temperature. As the core temperature rises, due to ongoing core contractions and
resulting increase in pressure, the atomic nuclei move faster thereby increasing the
probability of two nuclei colliding with each other and fusing into heavier elements.
Proton-Proton Chain
The simplest reaction that occurs in stars is the conversion of hydrogen into helium - a
process known as the proton-proton chain and requires a core temperature of at least
10 million K. In this process six hydrogen atoms are needed to create one helium
nucleus of two protons and two neutrons.
1
2
3
Process
proton + proton
proton + deuterium
helium-3 + helium-3
Result
deuterium
helium-3
helium-4
Extras
positron, neutrino
gamma-ray
2 protons
Table 5.1: Proton-Proton Chain Reactions
In Step 1 two protons come together to form deuterium (a nucleus of one proton and
one neutron). This interaction also involves the liberation of a positron (a positively
charged electron) and a neutrino. This process occurs 1038 times per second in the Sun.
In Step 2 the resulting deuterium nucleus combines with a proton to make the rare
isotope helium-3 consisting of one neutron and two protons. This reaction also produces
a gamma-ray.
In Step 3 the helium-3 nucleus becomes a helium-4 nucleus by the addition of a
neutron. This step can occur in several ways, but the combination of two helium-3
nuclei is the most common way.
Steps 1 and 2 must occur twice before step 3 can occur. Six protons go into the cycle,
but two come back out. The overall process results in four protons (hydrogen nuclei)
becoming a helium-4 nucleus, two positrons, two neutrinos and two gamma-rays.
Teachers Notes Booklet 5: Stellar Processes and Evolution
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Figure 5.1: Proton-Proton Chain
Each helium-4 nucleus has a mass that is about 99.3% of the mass of four protons. In
the Sun, for example, 600 million tons of hydrogen are converted into 596 million tons
of helium every second. The missing four million tons of matter is released as energy in
accordance with Einstein's equation E=mc2.
CNO Cycle
In more massive stars (at least 4 solar masses) the Carbon-Nitrogen-Oxygen cycle
dominates as the main process for proton burning proton. The heavier elements are
already present in the star, therefore largely restricting this process to younger
Population I stars.
The CNO cycle occurs only in more massive stars due to the necessity of a convective
core with a temperature of at least 20 million K. In the process a
12
C nucleus and four
protons combine in various stages ultimately resulting in the creation of another
12
C
nucleus, a helium nucleus and the liberation of a significant amount of energy. The
carbon is acting as a catalyst in the process of converting hydrogen into helium. There
is no net creation of the heavier elements through this process.
Teachers Notes Booklet 5: Stellar Processes and Evolution
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Figure 5.2: CNO Cycle
1
2
3
4
5
6
Process
carbon-12 + proton
nitrogen-13 decay
carbon-13 + proton
nitrogen-14 + proton
oxygen 15 decay
nitrogen-15 + proton
Result
nitrogen-13
carbon-13
nitrogen-14
oxygen-15
nitrogen-15
carbon-12
Extras
gamma-ray
neutrino, positron
gamma-ray
gamma-ray
neutrino, positron
helium nucleus
Table 5.2: CNO Cycle Reactions
The Future?
Once the stellar core has used up the majority of its hydrogen source the core region is
dominated by helium. The future evolution of the star now depends very firmly on its
mass and whether there is sufficient gravitational pressure to induce helium burning
and the burning of successively heavier elements.
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5.3 Evolution
All stars, irrespective of their initial mass, spend the majority of their life consuming
hydrogen. This period, which can last tens of billions of years, is the most stable time in
the stellar lifecycle.
The lifetime of a star depends largely on its mass. Somewhat counter intuitively, the
more massive a star is the shorter its lifetime! Although massive stars contain a larger
reservoir of fuel, the increased gravitational pressure in the core means an accelerated
consumption rate.
Star Type
O5
B0
A0
F0
G0
K0
M0
Temp (K)
40 000
28 000
9 900
7 400
6 600
4 900
3 500
Mass
40
18
3.5
1.7
1.1
0.8
0.5
Age
1 My
11 My
440 My
3 Gy
8 Gy
17 Gy
56 Gy
Table 5.3: Stellar Properties and Lifetime
As hydrogen converts into helium, so the composition of the stellar core changes. The
initial ball of hydrogen becomes a ball of helium surrounded by a shell of hydrogen.
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5.4 Paths on HR Diagram
Figure 5.3: Evolutionary Paths Off the Main Sequence
The HR diagram can be used to plot the changes in a stars brightness and temperature
over time. All stars, irrespective of their mass spend most of their lifetime on the Main
Sequence - the diagonal band spanning from top left to lower right.
Stars of differing mass exist at different points along the main sequence. The more
massive a star the further up and left it appears. As all stars enter a giant phase their
brightness remains constant, but the effective surface temperature cools. This reflects a
change in the process at work in the star. The outer layers have expanded and are no
longer places of nuclear burning. As these layers cool, the star drifts towards the right
side of the HR diagram.
The subsequent evolution depends on the mass of the star. Low mass stars undergo an
increase in luminosity, while more massive stars retain the same luminosity but have an
oscillating surface temperature.
Teachers Notes Booklet 5: Stellar Processes and Evolution
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5.5 Solar Mass Stars
A star, mass comparable to the Sun, with a core of pure helium and surrounded by a
hydrogen shell, will continue evolve through helium burning.
Since helium has two protons in its nucleus, compared to the one in hydrogen, a higher
core temperature is necessary to overcome the electrostatic potential. As hydrogen
burning comes to an end the core temperature is not sufficient for helium burning to
occur.
With no source of energy production in the core there is no longer any outward
radiative pressure to resist gravitational collapse. As the outer regions start to collapse
the temperature increases. This heat raises the temperature in hydrogen shell such that
hydrogen fusion can occur.
The core continues to collapse and the temperature in the hydrogen shell keeps on
increasing and so the luminosity also increases. The core collapse is now fuelled by two
sources – gravity extends radiation pressure and the hydrogen burning shell also exerts
a pressure.
The burning shell also provides pressure on the outer layers of the star and causes
them to expand. As the layers expand they cool and the star appears to become redder.
After just a few million years the hydrogen shell will eventually run out of fuel.
Once again the star will contract under its own weight. The compact core may flash into
life for a short period and helium be fused into carbon. As the energy released in the
helium flash reaches the outer layers the star becomes a red giant again and up to half
its mass is thrown out into space and seen as a planetary nebula leaving a core behind.
The core is a curious object. It weighs around half the mass the star had during its
lifetime yet it is smaller in size than Uranus or Neptune. The core is also very hot,
hotter again than when on the main sequence, yet produces no energy and will
eventually cool down.
The surface gravity can be in excess of 100 000 times gravity on Earth. The average
density is over 1000 kg per cubic centimetre causing the atoms to be packed so closely
making the star electron degenerate. This star is known as a white dwarf.
Teachers Notes Booklet 5: Stellar Processes and Evolution
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Electron-degeneracy theory predicts that the uppermost mass a white dwarf can attain
is about 1.44 times the mass of the Sun, called the Chandrasekhar Limit. Any heavier,
and the tremendous pressure on the innermost atoms would squeeze their electrons
into the nuclei they orbit, turning all the protons and electrons in the star into neutrons.
The low surface area and high specific heat means that such an object would take
longer than the currently estimated age of the Universe to cool.
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5.6 Medium Mass Stars
Stars with a mass of between 8 and 20 solar masses have a more complex evolution.
Initially, they evolve in the same way as low mass stars, turning into red giants and
undergoing a core helium burning phase.
In medium mass stars, however, the burning of helium into carbon is no longer the end
phase of stellar evolution. When the core helium supply is exhausted, the additional
mass allows stellar collapse to take place and the outer layers to reignite. A cross
section through the star at this point would show an outer shell of hydrogen burning, an
inner shell of helium burning and the core, where there is now sufficient energy for the
carbon to fuse with helium into oxygen.
Once the carbon supply is exhausted so oxygen fuses into neon, the helium shell
becomes a carbon burning shell, the hydrogen shell a helium shell and a new outer
layer of hydrogen burning forms. Neon can then fuse into magnesium, into silicon, and
so on to chromium into iron. Each of these stages produces less energy than the
previous stage and lasts for less time. During these final stages the star expands to
thousands of times the diameter of the Sun, becoming a red supergiant like Betelgeuse.
The star finally hits a problem. To fuse iron into heavier elements requires an input of
energy. Separating into the lighter elements again requires an input of energy. So as
iron burning stars the core cools down – it draws in heat from its surroundings to power
the fusion. Suddenly the outward radiative pressure, which has supported the star for
many millions of years, ceases and the star undergoes a free fall gravitational collapse.
The core, which represents a large percentage of the stellar mass, now exceeds the
1.44 Chandrasekhar mass limit for a white dwarf. The protons and electrons in the core
are compressed into a ball of neutrons, the size of a large city and the density of an
atomic nucleus, held up by neutron degeneracy pressure. Such an object is a neutron
star.
As a result of the core collapse a shock wave forms and blasts out through the star
releasing an enormous amount of energy in a few seconds. All the outer layers of the
star become superheated plasmas, with temperatures high enough to fuse iron and
heavier elements, like gold and uranium. These outer layers brighten rapidly and are
ejected into the interstellar medium at speeds approaching the speed of light. Such
events are witnessed as Type II Supernovae.
Teachers Notes Booklet 5: Stellar Processes and Evolution
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Pulsars
A pulsar is a neutron star that produces pulsed emissions across the entire
electromagnetic spectrum. Neutron stars spin rapidly and have large magnetic fields.
Radiation channelled down the field lines is beamed out across the Universe like the
light from a lighthouse. If the Earth happens to be in the light of sight then the emission
is seen. Many pulsars spin rapidly with pulse durations of the order of fractions of a
second.
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5.7 High Mass Stars
Stars with over 20 solar masses have an even more catastrophic end. They evolve in
the same way as their slightly less massive companions dying in a supernova explosion,
but the core evolves in a very different way.
A neutron star can have up to around three solar masses. After this point neutron
degeneracy pressure is no longer sufficient to prevent core collapse. With nothing left to
resist collapse the core condenses into an infinitely small, infinitely dense point called a
singularity.
Nothing can escape from the singularity. Nothing that comes within three kilometres
times the mass of the singularity in solar mass (the Schwarzchild Radius) can escape.
The whole stellar core swallows itself, leaving behind a gravitational potential well. It is
known as a black hole.
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5.8 Other Materials
This is booklet five in a series of six booklets currently available. The full range of titles
is:
Booklet 1
Introduction to the Universe
Booklet 2
Stellar Radiation and Stellar Types
Booklet 3
Stellar Distances
Booklet 4
Cosmology
Booklet 5
Stellar Processes and Evolution
Booklet 6
Galaxies and the Expanding Universe
Each booklet can be used to cover a topic on its own, or as part of a series. Booklets 5
and 6 expand on the material covered in the other booklets and there is, therefore,
some overlap in content.
All the booklets can be accessed via the ESA Science and Technology at:
http://sci.esa.int/teachernotes
For other educational resources visit the ESA Science and Technology Educational
Support website at:
http://sci.esa.int/education
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Teachers Notes Booklet 5: Stellar Processes and Evolution
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