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Introduction to Astrophysics
Lecture 7: Stellar life and death
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Aims of the lecture
• To explain the properties of stars on different parts of the HR
diagram:
Main sequence
Giant branch
White dwarf branch
• To briefly describe energy generation in stars.
• To describe the evolution of stars of different mass, how they end
their lives, and their end-states.
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General ignorance
• What makes stars hot?
• Which of these is the odd-one out?
White dwarfs
Red dwarfs
Blue dwarfs
Brown dwarfs
Black dwarfs
• How have most stars ended their lives?
• How will most stars end their lives?
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The Main Sequence
Most stars reside in a broad band
stretching from the top left (hot and
luminous) to the bottom right (cold and
faint) of the HR diagram.
The Sun lies pretty close to the centre of
this main sequence.
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Stellar physics and evolution
• The main sequence consists of stars whose
principal source of energy is the nuclear
fusion of hydrogen to form helium in the
star’s core.
4 p+ ➞ He2+ + 2 e- + 2 νe + 2 γ + heat
• The nuclear reactions take place deep in the
star, where the temperatures are extremely
high, a few million degrees.
• The energy slowly leaks out, because theenvironment is so dense. It is estimated that
a photon of light experiences so many
collisions that it take 10 million years to
esca e the Sun.
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The surface of the Sun
The Chromosphere in X-rays
Sunspot close-up
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Zones in the Sun
• The core: in the inner one third of its radius, nuclear fusion is taking
place, generating energy which heats the core to between five and
fifteen million degrees.
• The radiation zone: for the next one third energy transport is mostly
by radiation, bringing the temperature down to around one million
degrees.
• The convection zone: energy transport is primarily by convection,
with the temperature falling to just 5800K at the Sun’s surface.
• The photosphere: this is the surface where light escapes from.
• The chromosphere: this is the region above the visible surface of the
Sun, visible mainly during eclipses. It is heated to very high
temperatures by magnetic activity.
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The lifetime of stars
The main sequence has a relation between mass and luminosity of
approximately
L ∝ M 4.
The rate at which fuel is used up is proportional to the luminosity, withthe amount of fuel proportional to the mass. This gives the crucial
relation
Main sequence lifetime ∝Fuel/Power ∝ M/L ∝1/ M 3
The more massive stars are more short lived!
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Some sample main sequence lifetimes
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Evolutionary stages
When a star’s hydrogen runs
out it becomes a red giant,
burning helium in the core.
Later on it goes through cycles
as it is forced to burn heavierand heavier elements. The
ultimate fate of the star depends
upon its mass.
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Giants and supergiants
These lie in the upper right of the HR
diagram, meaning that they are cool but
luminous.
Their luminosity is high because they are very
large, and so have a big surface area toradiate from. Typically they may have a radius
one hundred times that of the Sun.
The most luminous are known as
supergiants.
The giants and supergiants are stars which
have exhausted their supply of hydrogen fuel
in their cores, and which produce energy by
burning heavier nuclei such as helium.
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Low mass stars
For stars with mass up to about eight times the solar mass, the
outer layers of the star are eventually blown away as a
planetary nebula exposing the core of the star.
Computer simulation of a red giant star
The core has too little mass to
overcome the support fromelectron degeneracy pressure
and cannot collapse any further.
Nuclear reactions cease.
This core is known as a whitedwarf . It is initially very hot,
but cools and fades.
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Planetary nebulae
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White Dwarfs
These lie in the lower left of the HR
diagram, meaning that they are hot but
faint.
There are probably very large numbers of
these, but they are not easy to detect.
White dwarfs are remnants of stars which
have completely exhausted their core
nuclear fuel and which have too little
gravity to contract further. They have nonew source of energy and are cooling into
obscurity.
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• They are extremely dense, perhaps up to a million times the density
of water. Despite having a mass comparable to the Sun, their size
can be comparable to the Earth!
• They are prevented from total collapse because of electron
degeneracy pressure. The Pauli Exclusion Principle does not allowelectrons to be compressed into a smaller volume.
• The more massive they are, the smaller their radius.
• The highest mass they can have is just over 1.4 solar masses,
known as the Chandrasekhar limit .
White dwarf properties
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High-mass stars
A high mass star can burnheavier and heavier elements,
until it creates Iron at its core.
Iron is the most stable
element there is; it cannot beburned to create anything else
unless it absorbs energy.
Deprived of energy to support
it, the core collapses and the
star explodes!
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Supernova!!
Close up of supernova 1987a
A supernova explosion is one of the Universe’s most
spectacular events. Briefly, the explosion of a single star can
be as bright as all the stars in a galaxy put together.
The outer layers of the star are ejected at speeds of up to
10,000 km s-1.
In a typical galaxy there are a few supernovae every century.
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We are all made from supernovae remnants!
A supernova is the main way in which the heavy elements, such as
oxygen, carbon and iron, escape the stars in which they are created
and are returned to the interstellar dust.
Without supernovae, the elements from which we are made would
not exist outside the cores of stars.
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What’s left behind?
Chandra satellite X-ray image of Cassiopeia A
The supernova explosion
throws off the outer shell of the
star.
What’s left behind depends onthe initial mass. Either
a neutron star, or
a black hole
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Neutron stars
• Towards the lower end of the mass range, what’s left is a neutron
star. Neutron stars are ...
• Composed of neutrons. The intense force of gravity is so strong that
it forces electrons and protons together to form neutrons. Being
much more massive that electrons, these allow the star to becomeeven more dense.
• A neutron star is in effect a giant atomic nucleus!
• They spin quickly. Some emit radio waves and are known as
pulsars.• They have masses up to about three times the Sun’s mass.
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Computer animation of a pulsar in action
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Black holes
• If the mass of the core that remains is more than about three solar
masses, even neutrons are not able to survive the gravitational
attraction. Gravitational collapse is so powerful that nothing, not
even light, can escape.
• Black holes can therefore only be identified by their gravitationaleffects on nearby objects.
• We’ll explore the astrophysics of black holes in a later lecture.
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Things to remember
• Stars on the main sequence are fusing Hydrogen in their cores to
make Helium.
• Once the core hydrogen is exhausted, the core contracts and the
outer layers of the stars swell to for a giant star. These stars are
burning Helium and higher elements.
• Low-mass stars shed their outer layers as planetary nebulae and
leave behind a hot, dense core supported by electron degeneracy
pressure — a white dwarf.
• High-mass stars explode as supernovae, seeding the interstellarmedium with metals. They leave behind a neutron star or black
hole.
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Quick quiz
• The evolution of a star depends primarily upon:
its chemical composition
its location in the Galaxy
its massits radius
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ExerciseIn its lifetime the Sun will consume approximately 10% of its Helium
(the core mass). How long will the Sun remain on the main-
sequence?
• The mass of the Sun is 2 x 1030 kg.
• The luminosity of the Sun is 4 x 1026 W.