Download Stellar Evolution: The Live and Death of a Star

yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Astronomical spectroscopy wikipedia, lookup

P-nuclei wikipedia, lookup

Nucleosynthesis wikipedia, lookup

Stellar evolution wikipedia, lookup

Planetary nebula wikipedia, lookup

Star formation wikipedia, lookup

Main sequence wikipedia, lookup

Standard solar model wikipedia, lookup

Supernova wikipedia, lookup

Hayashi track wikipedia, lookup

White dwarf wikipedia, lookup

Stellar Evolution: The
Live and Death of a Star
Star ch. 20
• 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
The helium content increases fastest in
the center
When hydrogen becomes depleted in the
center fusion moves to higher layers in the
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
• 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
The helium core is unbalanced and
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
• 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
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
Helium Flash
• In the core’s degenerate state, helium
burning becomes unstable with explosive
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
• 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
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
Its mass is about half the mass of the sun
It shines by stored heat, not nuclear
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!!!)