Download Stellar Evolution: The Live and Death of a Star

Stellar Evolution: The
Live and Death of a Star
Star ch. 20
Standards
• Understand the scale and contents of the
universe, including stars
• Describe how stars are powered by fusion,
how luminosity and temperature indicate
their age, and how stellar processes
create heavier and stable elements that
are found throughout the universe.
• As a star begins to run out of fuel & die, its
properties change greatly.
• They travel evolutionary paths that take
them far from the main sequence.
• Their ultimate fate depends on their mass.
Leaving the Main Sequence
• Most stars spend most of their life on the
main sequence.
The coolest M – type stars burn so slowly
not one has yet left the main sequence.
The most massive O & B – type stars
evolve from main sequence after only a
few tens of millions of years
 Most high mass stars that ever existed
perished long ago
Structural Change
• As hydrogen is consumed, balance between
gravity and pressure begins to shift, both
internal structure and outward appearance
begin to change, and the star leaves the
main sequence.
• The end of a star’s life depends critically on
its mass.
Low mass stars die gently
High mass stars die catastrophically
The dividing line between the two is about 8
times the mass of the sun
Evolution of a Sun-like Star
• A solar mass star does not experience
sudden, large-scale changes in properties.
Its average surface temperature remains
constant, while luminosity increases very
slowly over time
After about 10 billion years of steady core
hydrogen burning, a sun-like star begins to
run out of fuel (like a car cruising down the
highway at a constant 70 mph for many
hours, only to have engine suddenly cough &
sputter as the gas gauge reaches empty).
The Sub-Giant Branch
• Composition of the star’s interior changes:
It has increased helium and decreased
hydrogen.
The helium content increases fastest in
the center
When hydrogen becomes depleted in the
center fusion moves to higher layers in the
core
The Sub-Giant Branch
An inner core of non-burning helium starts
to grow
The gas pressure weakens in the helium
core and gravity causes the inner core to
begin to contract
Hydrogen Shell-Burning Stage
• Hydrogen burns at a furious rate in a shell
surrounding the non-burning inner core of
helium “ash”
Hydrogen Shell-Burning Stage
• The hydrogen shell generates energy
faster than the original main sequence
fusion, & energy production continues to
increase as the helium core continues to
shrink
• The star’s response is to get brighter
Hydrogen Shell-Burning Stage
• After a lengthy stay on the
main sequence, the star’s
temperature and
luminosity begin to change
• The star evolves to the right
on the H-R diagram to the
subgiant branch
The Red Giant Branch
• The star is now far from the main
sequence and no longer in stable
equilibrium
The helium core is unbalanced and
shrinking
The rest of the core is unbalanced &
fusing at an increased rate
The Red Giant Branch
Gas pressure exerted by enhanced
hydrogen burning forces star’s nonburning outer layers to increase in radius,
and the overlying layers are expanding
and cooling
Star is on its way to becoming a red giant
This change takes around 100 million
years
• The red giant has a luminosity many
hundreds of times the luminosity of the
sun and its radius is around 100 solar radii
Helium Fusion
• A few hundred million years after a solarmass star leaves the main sequence
helium begins to burn in the core
• The helium fuses into carbon and the
central fires reignite
Helium Flash
• At the highest densities in the core, gas
enters a new state of matter governed by
the laws of quantum mechanics (deals
with behavior of matter on subatomic
scales)
In this state, the Pauli exclusion principle
prohibits electrons in the core from being
squeezed too close together, known as
electron degeneracy
The pressure associated with the contact
of electrons is called electron degeneracy
pressure
Helium Flash
• In the core’s degenerate state, helium
burning becomes unstable with explosive
consequences
When burning starts and temperature
increases, there is no corresponding rise in
pressure, no expansion of gas & no
stabilization of core
The rapid temperature rise results in
runaway explosion called the helium flash
The helium burns ferociously for a few hours,
then equilibrium is eventually reached and
stable core fuses helium into carbon
Back to the Giant Branch
• Whatever helium exists in the core is
rapidly consumed (lasts a few tens of
millions of years after helium flash)
• As helium fuses to form carbon, a new
carbon-rich inner core forms, surrounded
by helium burning, hydrogen burning and
non-burning shells
• The non-burning layer expands and star
becomes red giant or red supergiant
Core of Carbon Ash
Death of a Low-Mass Star
• The inner carbon core becomes too cool for
further nuclear burning and continues to
contract
• The fires go out
Before the core attains the temperature
necessary to fuse carbon, its density
reaches a point where core can no longer be
compressed
At this density, a cubic centimeter of core
matter would weigh 1000 kg on Earth: a ton
of matter compressed into a volume the size
of a grape
Planetary Nebulae
• Driven by increasing radiation and
instabilities in the core and outer layers,
all of the star’s outer envelope is ejected
into space in less than a few million years
at a speed of a few 10’s of km/s
Planetary Nebulae
• The star now has two distinct parts: a core
of carbon ash (a.k.a. white dwarf) and an
expanding cloud of dust and cool gas
spread over a volume roughly the size of
our solar system
This is a planetary nebula (they have no
association with planets)
Planetary Nebulae
It continues to spread out over time, and
eventually disperses into interstellar
space, enriching it with atoms of helium,
carbon, oxygen & heavier elements
• These elements eventually get swept up into
nebulae (see ch. 18) and formed into new
stars and planets
White and Black Dwarfs
• The carbon core at the center of the
planetary nebula continues to evolve
The core is very small, size of Earth or
smaller
Its mass is about half the mass of the sun
It shines by stored heat, not nuclear
reactions
The core’s temperature & size give it the
name of white dwarf
White and Black Dwarfs
• Once a star becomes a white dwarf, its
evolution is over
It eventually becomes a black dwarf – a
cold, dense, burned-out ember in space
that remains about the size of Earth
Evolution of Stars More
Massive than the Sun
• High-mass stars evolve much faster than
low-mass stars.
Its ravenous fuel consumption shortens its
main sequence lifetime.
• A solar mass star spends 10 billion years on
the main sequence
• A 5 solar mass B-type star is on main
sequence for about a 100 million years
• A 10 solar mass O-type star is on main
sequence for about 20 million years
Evolution of Stars More
Massive than the Sun
• At 8 solar masses and larger, stars can
fuse carbon, oxygen and even heavier
elements. These stars die in violent
explosions soon after leaving main
sequence (next chapter!!!)