Lecture 19
The fate of massive stars:
supernovae
Massive stars
• Helium burning continues
to add ash to the C-O
core, which continues to
contract and heat up.
• Carbon is ignited, forming
16
8
20
23
24
O, 10
Ne, 11
Na, 1223Mg , 12
Mg
Shell structure
If each reaction has time to reach equilibrium,
the stellar interior will consist of shells of
different composition and reactions
• Oxygen is ignited next producing a Silicon core.
Silicon burning
Silicon burning produces
numerous elements near
the iron peak of
stability
The most abundant:
54
26
Fe
56
26
Fe
56
28
Ni
Further reactions are
endothermic and thus do not
provide stellar luminosity.
Timescales
As the iron peak is approached, the energy released per
unit mass of reactant decreases. Thus the timescale
becomes shorter and shorter
Core
burning
Lifetime
H
10 million
years
1 million
years
He
C
O
Si
300 years
200 days
2 days
Photodisintegration
• During Silicon burning the core has
reached extremely high temperatures
and densities:
• The photons produced are so
energetic they can destroy heavy
nuclei, reversing the process of
fusion. In particular:
56
26
4
2
Fe    1324 He  4n
He    2 p   2n
Tc  8 109 K
 c  1013 kg / m3
Core collapse
• The inner core collapses, leaving the
surrounding material suspended above it,
and in supersonic free-fall at velocities
of ~100,000 km/s.
• The core density increases to 3x the
density of an atomic nucleus and becomes
supported by neutron degeneracy
pressure.
• The core rebounds somewhat, sending
pressure waves into the infalling material
Stalled shocks
As the shock wave propagates outward and encounters
the infalling core, the high temperatures result in
further photodisintegration.
 This removes a lot of energy from the shock: it loses
1.7x1044 J of energy for every 0.1MSun of iron it breaks down.
 If the iron core is too large, the shock becomes a stationary
accretion shock, with matter accreting onto it.
Instability growth
The rapid growth of long-wavelength mode
instabilities may play a role
Explosion
As the shock moves toward the surface, it drives the hydrogen-rich
envelope in front of it.
When the expanding shell becomes optically thin, the radiation can
escape, in a burst of luminosity that peaks at about 1036 W
Break
Light curves
After the initial burst of luminosity, the supernova slowly
fades away over a period of several hundred days.
• As the shock wave
propagates through
the star, it creates a
large amount of
heavy, radioactive
elements.
• Each species decays
exponentially with a
unique timescale
Radioactive decay
For example, the following beta-decay reaction
occurs:
56
56

Co

Fe

e
 e  
27
26
This decay is a statistical process: the rate of decay must be
proportional to the number of atoms in the gas:
dN
 N
dt
where  is the decay constant, and is characteristic of each
radioactive element.
Example: radioactive decay
The energy released by the decay of one cobalt-56 atom
is 3.72 MeV. Given 0.075 MSun of this isotope (this is
how much was estimated to have been produced in
SN1987A) how much energy does the decay release?
L  9.8 1034 e 3.26tW
(for t measured in years)
The initial luminosity is 2.5x108 LSun. After one year it has decreased to
9.9x106 LSun.
Remnants
• If the star is relatively low
mass, roughly M<25MSun, it
can be supported by
neutron degeneracy and
becomes a neutron star.
• For more massive stars, the
gravitational attraction
overcomes neutron
degeneracy, and the core
collapses to form a black
hole.
Supernova remnants
Crab nebula: believed to
be the remnant of the
supernova that went off
in 1054 A.D.
 Nebula is still expanding,
at ~1450 km/s
 The source of the
luminosity and electrons
is a pulsar in the centre
of the nebula.
The Crab nebula is ~2 kpc away, with an angular size of 4x2
arcminutes. The expansion velocity is measured from the Doppler
shift to be 1450 km/s. Estimate the age of the nebula. How
bright would the supernova that gave rise to the Crab nebula have
been?
Supernova remnants
Cygnus loop: this is a
~15,000 year old
remnant.
 The filaments are
caused by shocks
encountering the
interstellar medium.
These shocks excite the
gas which then emits
emission lines.
A small part of the remnant,
expanding left to right
SN1987A
• Occurred in the Large
Magellanic cloud, a small
galaxy near the Milky
Way.
SN1987A progenitor
• Progenitor was a much smaller star than usually
responsible for Type II explosions.
• Smaller stars are denser, so more energy was required
to lift the atmosphere, and this resulted in a slower
brightening and fainter peak luminosity.
SN1987A light curve
• The initial decay mostly tracks Co-56,
followed by Co-57
56
Co26
Fe  e   e  
56
27
• This reaction produces high energy
gamma rays which were detected for the
first time, confirming the presence of
this isotope.
Neutrinos
56
Co26
Fe  e   e  
56
27
• Neutrinos produced in part by this decay were also detected: this was the
first time neutrinos were detected from an astronomical source other
than the Sun.
SN1987A: the rings
• The central ring is due to
ejection by a stellar wind
prior to the explosion.
• Lies in the plane that
contains the centre of
explosion
 Glows due to [OIII] emission,
excited by radiation from the
explosion
SN1987A: the rings
• The central ring is due to ejection
by a stellar wind prior to the
explosion.
 When the shock wave from the
explosion reached this ring, in
2004, it excited the gas causing
it to glow brightly.
SN1987A: the rings
• The two other rings are
not in the plane of the
explosion, but in front of
and behind the star
 The explanation of these
rings is still unknown