• They follow very
different paths when
leaving the main
sequence
• The temperatures
and densities in the
cores are much
different => different
reactions
HRD – massive stars
Evolution – massive stars
• They leave the main sequence when the
hydrogen is depleted in their cores
• The first few events are similar to those in lowermass
stars:
1. hydrogen shell
2. core burning helium to carbon, surrounded by
helium- and hydrogen-burning shells
• Difference: stars with masses > 2.5 M have no
helium flash but it starts gradually burning
• It becomes a supergiant
Evolution – massive stars
• They undergo an extended sequence of
thermonuclear reactions in its core and shells
• These include carbon (12C) fusion, neon (20Ne)
fusion, oxygen (16O) fusion, and silicon (28Si)
fusion
• See the slides in the lecture “Fusion – Fission”
Evolution – core burning
Evolution – massive stars
Stage Core temperature
(K)
Core density
(kg/m3)
Duration of stage
Hydrogen fusion 4 x 107 5 x 103 7 x 106 years
Helium fusion 2 x 108 7 x 105 7 x 105 years
Carbon fusion 6 x 108 2 x 108 600 years
Neon fusion 1.2 x 109 4 x 109 1 year
Oxygen fusion 1.5 x 109 1010 6 months
Silicon fusion 2.7 x 109 3 x 1010 1 day
Core collapse 5.4 x 109 3 x 1012 ¼ second
Core bounce 2.3 x 1010 4 x 1015 milliseconds
Explosive
(Supernova)
about 109 varies 10 seconds
Stages for a star with 25 M
• The last stage: an iron-rich core surrounded by concentric
shells hosting the various thermonuclear reactions
• The sequence of thermonuclear reactions stops here, because
the formation of elements heavier than iron requires an input
of energy rather than causing energy to be released
• Once the iron core is formed, the core contracts very rapidly and the
temperature rises dramatically
• Photodisintegration: high energy photons break the iron nuclei to
helium nuclei
• Electron combines with proton to form neutrons, neutrinos transport
the energy
• The core ends up as all neutron, with nuclear density (1017 kg/m3)
• The degenerate neutron pressure suddenly halts the core contract
• The outer core bounce back and sends a powerful wave of pressure
• The pressure wave becomes a powerful shock wave as it go
outwards, and expel most stellar material outward
• Shock wave produces a series of nuclear reaction, the only place
elements heavier than iron are produced in the universe
Evolution – Supernova
Supernova remnants
X-ray
Different Types of Supernovae
• Type 1 supernova: no hydrogen lines
– Type 1a supernova: explosion of white dwarf in a
closed binary system; mass accumulation exceeds
the critical mass and ignites the carbon fusion at the
core
– Type 1b supernova: core collapse of massive star
with hydrogen shell lost before
– Type 1c supernova: core collapse of massive star
with both hydrogen and helium shells lost before
• Type 2 supernova: strong hydrogen lines
– core collapse of massive star with hydrogen shell
largely intact
Supernovae – classification scheme
Type Ia supernovae are those produced by
accreting white dwarfs in close binaries
Type Ib supernovae occur when the star has lost a
substantial part of its hydrogen shell
Type Ic supernovae occur when the star has lost a
substantial part of both its hydrogen shell and helium
shell
Type II supernovae are created by the deaths of
massive stars
Supernovae – light curves
• Absolute magnitude is strongly correlated with rate
of decline (faster = fainter)
• Apply “stretch factor” to compensate for this
Type Ia Supernovae as Standard
Candles
SN 1987A - the best observed SN
• Occurred on 23.02.1987
• In the Large Magellanic Cloud reached V = 2.9 mag
SN 1987A - the best observed SN
• Progenitor star: blue B3 I supergiant
SN 1987A: Neutrinos were detected
• More than 99% of the energy from such a
supernova is emitted in the form of neutrinos
from the collapsing core
• Neutrino energy 1046 Joules, 100 times as
much energy as the Sun has emitted in its
entire history
• Neutrinos arrived 3 hours before the first SN
light was seen
• The 3-hour delay was due to the propagation
time of the shock wave from the core to the
surface of the supergiant
SN 1987 A - the best observed SN
SN 1987A - the best observed SN
HST in 2017
SN 1987A - the best observed SN
SN 1987A - the best observed SN