Chapter 22 The Death of Stars

1
Chapter 22The Death of Stars

• Will a star die with a Bang or a whimper ?

2
Death of Low-mass stars

• Low mass stars go through two red-giant stages.
• First when the core hydrogen is depleted, the
  core shrinks and shell hydrogen burning starts.
  This causes the outer layers to expand and become
  a red-giants.

3
Death of Low-mass stars

• Once the core gets heated to about 100 million
  Kelvin core helium burning starts, the core
  expands and the outer layers shrink - No longer a
  red-giant.
• When the core helium is depleted, once again the
  core shrinks, shell helium burning starts, and
  the outer layers expand, and the star is once
  again a red-giant.

4
(No Transcript)
5
Post-main-sequence evolution of a low mass star

• Red-Giant Branch
• After the star leaves the main-sequence, the core
  shrinks and the outer layers expand.
• Luminosity increases and the surface temperature
  drops.
• The star moves up and to the right in the H-R
  diagram

6
Post-main-sequence evolution of a low-mass star

• Horizontal Branch
• Core helium burning and shell hydrogen burning.
• outer layers shrink.
• The surface temp. goes up, the luminosity goes
  down slightly.
• Star moves to the left slightly down.
• Remains in the branch approximately 100 mill.
  yrs.

7
Post-main-sequence evolution of a low-mass star

• Asymptotic Giant Branch (AGB)
• Core helium depleted. Shell Helium burning.
• Outer layers expand and cool.
• The surface temp. goes down, the luminosity
  goes up due to increasing size
• Ascends to the red-giant region in the H-R
  diagram for the second time.

8
The structure of an AGB star near the end of its
life.
Asymptotic Giant Branch Star
9
Planetary Nebulae

• Dying low-mass stars gently eject their outer
  layers.
• In the Sun, convection is responsible for energy
  transport only in the outer layers.
• This involves the up-and -down movement of gasses
• During the final stages the convection zone can
  reach all the way down to the core.

10
Planetary Nebulae

• During this time the convection currents can
  dredge-up the heavy elements (carbon) produced
  in and around the core to the surface.
• During the last stage of an AGB star it ejects
  shells of mater in to space in a series of
  bursts.

11
Planetary Nebulae

• An aging 1M? star looses as much as 40 of its
  mass.
• As the outer layers are ejected, the hot core
  (about 100,000 K) of the dying star is exposed.
• This hot core emits UV radiation and that
  excites and ionizes the ejected gas.
• The gas then glows, producing planetary nebulae.

12
Planetary Nebulae

• Helix Nebula the closest planetary nebula to
  us.
• Planetary nebulae are very common - there are
  estimated 20,000 - 50,000 in our Galaxy.

13
Planetary Nebulae

• Planetary nebula NGC 7027.

14
White Dwarf Stars

• Sirius and its white dwarf companion.

15
(No Transcript)
16
White dwarfs

• Final state of low mass stars
• Mass less than 1.4 M? - Chandrasekhar limit
• Size about same as Earth
• Temperature typically 25,000 K (after cooling
  from 100,000K)
• No energy source - glows from residual heat
• Cools to about 20,000 K in 10 million years
• Eventually becomes a black dwarf

17
(No Transcript)
18
Evolution from Giants to White Dwarfs
Mass in M?
19
White dwarfs (cont.)

• Density 106 g/cm3 - extremely dense
• Structure of white dwarfs Gravity not balanced
  by heat pressure, but by degeneracy of its
  electrons - due to Pauli exclusion principle.
• Adding mass causes radius to shrink!

20
Degeneracy the Chandrasekhar limit

• Degeneracy means electrons are free to move about
  the whole white dwarf (not just stuck in single
  atom)
• Caused by quantum mechanical Pauli exclusion
  principle
• The gravity is balanced by degeneracy electron
  pressure

21
Degeneracy the Chandrasekhar limit

• Adding mass causes radius to shrink. Eventually
  forces electrons to join with protons to become
  neutrons. White dwarf can collapse into neutron
  star.
• Max. mass of white dwarf Chandrasekhar limit
  1.4M?

22
White dwarf binary systems

• A white dwarf can be a companion star in a binary
  system. When the other star evolves into red
  giant and if the stars are close enough, matter
  from the other star can fall into the white dwarf
  and increase its mass.
• Example Sirius A and B

23
White dwarf binary systems
Companion
24
White dwarfs (cont.)
The Mass-Radius Relationship for white
dwarfs. Higher the mass, smaller the radius due
to gravitational collapse.
25
Nova and Type I supernova

• Adding mass to white dwarf results in
• Nova (new star) hydrogen ignites on surface in
  large explosion that does NOT destroy white
  dwarf. Can be recurring.
• Type I supernova White dwarf increases mass
  over Chandrasekhar limit (perhaps after many
  novae) and collapses into neutron star and may
  explode in supernova - runaway carbon burning
  occurs.
• Results in formation of large amount of
  radioactive Ni-56, which eventually decays to
  Fe-56.

26
(No Transcript)
27
Characteristics of Type I supernova

• Type I
• precursor white dwarf
• cause mass increases beyond Chandrasekhar limit
• result destruction of star, formation of large
  amount of iron
• identified by gamma ray spectrum caused by
  radioactive Ni-56 decay and lack of hydrogen
  emission or absorption lines.
• lack of hydrogen lines in the spectrum because
  most of the hydrogen in the outer layers of the
  star has already been expelled into space in the
  form planetary nebula.

28
Death of massive stars

• In massive stars (M 4 M? ), once the core
  helium burning is over, the degeneracy electron
  pressure is not great enough to stop the core
  from collapsing and heating.
• When the core temp. reaches 600 million Kelvin,
  core carbon burning begins.
• This scenario will proceed through neon burning,
  oxygen burning and finally silicon burning.
• Between each stage, the star will go through
  red-giant phases and the H-R diagram track will
  make several back and forth gyrations.

29
Death of massive stars
At each stage the outer layers expand more and
more. - The final result is a supergiant star
Betelgeuse and Rigel in Orion constellation.
30
The structure of an high-mass star near the end
of its life.

31
Death of massive stars

• Stars whose mass is less then 8 M? eject most of
  their mass in the form of planetary nebulae.
• For stars with M 8 M? , the end comes with a
  spectacular explosion.
• Once such a star gets to the iron core stage,
  (and since iron cannot fuse), the core contracts
  rapidly and the temperature jumps to a whopping 5
  billion Kelvin.
• This will disintegrate the iron into helium ions
  in a fraction of second.

32
Death of massive stars

• The pressure becomes so high that the electrons
  are forced to combine with protons to form
  neutrons. This process takes only another tenth
  of a second.
• e- p ? n ?
• Core becomes very stiff, and the collapse
  suddenly stops.
• The plunging outer layers bounces off the
  extremely stiff core back and into to the
  surrounding space.

33
Death of massive stars

• The energy released in this catastrophic event is
  more than all the energy emitted by our Sun in
  the past 4.6 billion yrs.
• The Star has become a Supernova.
• This type of a Supernova is called a Type II
  supernova.
• The material being ejected by such processes are
  so compressed that there are waves of
  thermonuclear processes that take place and these
  create all the elements heavier than iron.

34
(No Transcript)
35
Death of massive stars
36
Supernova of 1987 SN 1987A

• On February 23, 1987, a supernova was discovered
  in our companion galaxy - the Large Magellanic
  Cloud(LMC) - 50,000 pc from Earth.
• Peak luminosity was so large (108 L? ) that it
  could be seen with the naked eye.
• SN 1987A was the first supernova after 400 years
  that was visible to the naked eye.

37
Supernova of 1987 SN 1987A

Before and after. Progenitor star was a B3 I
supergiant.
38
Supernova of 1987
True color view of Hubble space telescope.
39
Supernova of 1987
Possible origin of the ring.
40
Characteristics of type II supernova

• Type II
• precursor massive star
• cause nuclear fuel spent, iron core no longer
  able to hold up weight of star, collapses, rest
  of star bounces off of shrunken neutron core.
• result destruction of star, formation of
  elements heavier than iron, neutron star or black
  hole remains
• identified by presence of hydrogen emission or
  absorption lines.

41
Type I
Type II
42
The final fate of stars