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Death of Stars
11 August 2005
AST 2010: Chapter 22
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Questions of Interest
What happens to old stars?
How does death differ for small and
large stars?
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Death of Low-Mass Stars
Toward the end of life of a star whose final mass
just before death is like the Sun’s mass,
the star sheds some of its outer layers to form a
planetary nebula
the star’s core continues shrinking until it reaches a
density equal to nearly a million times the density
of water!
What remains of the star becomes an object
called a white dwarf
Since white dwarfs are far more dense than any
substance on Earth, the matter inside them
behaves in a very strange way
unlike anything we know from experience
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White Dwarfs
In the extremely hot, dense gas inside a white dwarf,
the electrons resist being squeezed closer together
and set up a powerful pressure
Such a gas is said to be degenerate
White dwarfs, then, are stars with degenerate electron
cores that cannot contract any further
Theoretical calculations show that stars with final
masses (just before death) less than 1.4 MSun end up
as white dwarfs
This number is called the Chandrasekhar limit, after the
scientist who first calculated it
Low-mass stars, with initial masses up to 10 MSun, can
lose enough mass (during their dying process) to
become white dwarfs
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Masses & Radii of White Dwarfs
Theoretical calculations show that the more
massive a white dwarf is, the smaller its radius
This is contrary to the behavior of regular matter,
which gets bigger
in size when its
mass is greater
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Sun-Like Star: From Death to White Dwarf
After the star becomes a giant again (A), it will lose
more and more mass as its core begins to collapse
The continued mass
loss exposes the hot
inner core, making
the remaining core’s
surface temperature
increase (B)
At first the luminosity
remains nearly the
same, but as the star
begins to cool off, it
becomes less and
less bright (C)
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Ultimate Fate of White Dwarfs
Since a white dwarf can no longer produce
energy by gravitational contraction (or by
nuclear fusion), it shines simply from the
release of the (substantial) energy left over
inside its core
Gradually the white dwarf radiates away all its
heat into space
The white dwarf will eventually stop shining
and become a black dwarf
This is a cold, dense stellar corpse with the
mass of a star and the size of a planet
It is composed mainly of carbon and oxygen
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Death of Massive Stars
High-mass stars, with initial masses larger than 10
solar masses, end their lives in other ways
In a massive star, after helium fusion in its core has
ended, the weight of the star’s outer layers is
sufficient to force the carbon and oxygen core to
contract until it becomes hot enough to fuse carbon
into oxygen, neon, and magnesium
This cycle of contraction, heating, and fusion repeats
several more times
Each cycle produces heavier elements
These cycles cause a massive star, near the end of its
life, to develop a structure resembling an onion
Depending on the mass of the star, the cycles
continue until it has exhausted all of its sources of
energy
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Old Massive Star: Stellar Onion
As we get farther from the center, we find shells of
decreasing temperature, in which nuclear reactions
involve nuclei of progressively lower mass (silicon,
sulfur, oxygen,
neon, carbon,
helium, hydrogen)
The cycles stop
after all the silicon
has fused into iron
because iron is the
most stable
Nuclear reactions
involving iron
require
instead of
energy
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energy
producing
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Collapse into a Ball of Neutrons
No longer able to generate energy, the star now faces
catastrophe
The iron core starts to collapse, overcoming the
resistance of the degenerate electron gas
As a result, the electrons are squeezed into the atomic
nuclei, where they combine with protons to form
neutrons
As is true for electrons, the neutrons in the collapsing
core eventually become degenerate and resist being
squeeze further
Theoretical calculations show that a star with a final
mass (just before death) less than 3 MSun can end up
as a crushed ball made mainly of neutrons, which is
called a neutron star
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Supernova
When the collapse is stopped by the degenerate
neutrons, the core is saved from further destruction,
but the rest of the star is literally blown apart
The core collapse occurs very rapidly
Its size changes from that similar to Earth’s to that of a
midsize town in less than a second!
When the collapse is abruptly halted by the
degenerate neutrons, the shock of the jolt initiates a
powerful wave that propagates outward and is quickly
absorbed by the outer layers of the star
This huge, sudden input of energy reverses the infall
of these layers and drives them explosively outward
The resulting explosion is called a supernova
This type of stellar explosion, which signals the death of
a massive star, is also called Type II supernova
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Supernova Explosion
When a supernova occurs, heavy elements up to iron,
produced in stellar nucleosynthesis, are ejected into
space
During a supernova, elements heavier than iron, such
as gold, silver, uranium, can also be produced
They are built up from the ejected neutrons and the
protons produced when the neutrons are absorbed by
the ejected iron and other nuclei
Supernovae are believed to play a major role in the
creation of chemical elements in the universe
Supernovae are also thought to be the source of
cosmic rays (high-energy charged particles)
The ejected gas from a supernova crashes into the
surrounding interstellar gas at thousands of km/s
The shock wave heats up the interstellar gas to very
high temperatures, making it glow
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The Crab Nebula
A famous
supernova
remnant is the
Crab Nebula
Chinese
astronomers
recorded the
explosion in
1054
Anasazi Indians
painted a
picture of it
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Remnant of Cygnus Loop Supernova
Neutrinos from Supernovae
According to nuclear physics, each time an
electron and a proton merge to form a
neutron, a neutrino is released
When a neutron star’s dense core forms, an
enormous number of neutrinos are released
from the core
They carry most of the energy of the explosion
The detection of such neutrinos provides a way
to measure the core temperature of the star
In the first second of the explosion, the power
carried by the neutrinos is greater than the
power put out by all the stars in all the
galaxies that we can see!
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Supernova 1987a
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AST 2010: Chapter 22
A supernova occurred in
a satellite galaxy of the
Milky Way at the star of
1987
It was called SN 1987a
The Kamiokande
neutrino detector saw a
burst of neutrinos
Studies of SN 1987a
confirmed models of
supernovae
2002
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Supernova Rate in the Universe
One average, one supernova occurs somewhere in our
Milky Way Galaxy every 25 to 100 years
Unfortunately, none has been detected in our Galaxy
since the invention of the telescope
Supernovae are very rare because the massive stars
that produce them are rare
But there are billions of galaxies in the universe
Simple probability suggests that there should be a few
supernovae happening somewhere in the universe
during a year
And that is what is seen!
Since supernovae are so luminous and the energy is
concentrated in a small area, they stand out and can
be seen from hundreds of millions of light years away
Supernova images from the Chandra X-Ray Observatory
Pulsars
In the late 1960s, astronomers discovered sources of
radio waves that emit rapid pulses of radiation at
very regular intervals
Their periods range from 0.001 to 10 seconds
The great degree of regularity of the signals led
some scientists to speculate that they were picking
up signals from extra-terrestrial intelligent
civilizations
But the discovery of several other similar sources
discounted that idea
Now more than a thousand of them have been found
They are now called pulsars, short for “pulsating
radio sources”
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Discovery of Neutron Stars
By combining theory and observation, astronomers
concluded that pulsars must be spinning neutron stars
Neutron stars are the ideal candidates because the core
collapse has made them so small that they can turn very
rapidly
The rotating neutron star acts like a lighthouse,
sweeping its beam in a circle and giving us a pulse of
radio radiation when the beam sweeps over the Earth
The magnetic poles are located in different places from the rotation poles
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Pulsar in the Crab Nebula
Pulsars lose energy as they age, the rotation slows,
and their periods increase
A pulsar has been
discovered at the
center of the Crab
Nebula
Animation
Novae
An isolated white dwarf has a boring future: it simply cools
off, dimming to invisibility
A white dwarf in a binary-star system, where the
companion is still a main sequence or red- giant star, can
have a more interesting future
If the white dwarf is close enough to its red-giant or mainsequence companion, gas expelled by the star can fall onto
the white dwarf
The hydrogen-rich gas from the companion star's outer layers
builds up on the white dwarf's surface and gets compressed
As more and more hydrogen gradually accumulates and
heats up on the white dwarf's surface, the new layer
eventually reaches a temperature that causes fusion to
begin in a sudden explosive way, blasting away much of
the new material and creating what is called a nova (“new”
in Latin)
To early astronomers, a nova was a new star that suddenly
appeared and faded away after a few months or years
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Type Ia Supernovae
If the white dwarf accumulates matter
from a companion star rapidly, the
dwarf’s mass may exceed 1.4 MSun (the
Chandrasekhar limit)
If this limit is exceeded, the dwarf can no
longer support itself as a white dwarf and
begins to collapse
As a result, it heats up and new nuclear
reactions begin in its core
In less than a second, an huge amount of
fusion occurs that causes the dwarf to
explode completely
This kind of explosion is called a Type Ia
supernova
to be distinguished the Type II supernova,
which signals the death of a massive star
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Summary of Ultimate Fates of Stars
Stars with final
masses < 1.4 MSun
end their lives as
white dwarfs
Stars with final
masses between 1.4
and 3 solar masses
become neutron stars
Stars with final
masses > 3 MSun
become black holes
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