lecture07 astro

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Introduction to Astrophysics

Lecture 7: Stellar life and death



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.



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?



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.



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.



The surface of the Sun

The Chromosphere in X-rays

Sunspot close-up



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.



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!



Some sample main sequence lifetimes



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.



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.



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.



Planetary nebulae



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.



• 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



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!



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.



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.



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



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.



Computer animation of a pulsar in action



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.



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.



Quick quiz

• The evolution of a star depends primarily upon:

its chemical composition

its location in the Galaxy

its massits radius



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.

lecture07 astro

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