Life and Evolution of a
Massive Star
M ~ 25 MSun
• Birth in a Giant Molecular Cloud
• Main Sequence
• Post-Main Sequence
• Death
The Main Sequence
• Stars burn H in their cores via the
CNO cycle
• About 90% of a star’s lifetime is
spent on the Main Sequence
The CNO Cycle
• Hotter core temp
allows H to fuse with
C, N, and O
• More possibilities
means faster reaction
rate
• Faster reaction rate
means higher
luminosity and a
shorter life
•
•
•
•
•
•
+ 1H  13N + γ
13N  13C + e+ + ν
e
13C + 1H  14N + γ
14N + 1H  15O + γ
15O  15N + e+ + ν
e
15N + 1H  12C + 4He + γ
12C
• Overall reaction:
4 1H  4He
Supergiants
• H burning in core stops
• He core contracts
• T high enough for He
fusion, no need for
degeneracy pressure
• No He Flash
Supergiants
• Low mass stars cannot
burn past He
– Degeneracy pressure
prevents core from
contracting enough
• High mass stars have
more mass, so they can
burn heavier elements
– Degeneracy pressure
never plays a part
• As core burning
element runs out, core
contracts
• Shell burning rate
increases, star expands
• Eventually core
contracts enough for
fusion of heavier
element to begin
• Shell burning slows and
star contracts
The Most Important Graph
in the Whole Course
E=mc2
Supernova: Type II
• Fe core temporarily supported by electron
degeneracy pressure
• Gravity is stronger in high mass stars, crushes star
further
• Electrons combine with protons to form neutrons
and neutrinos
• Core collapses until neutron degeneracy pressure
causes core to rebound
• Tons of neutrinos push material out with a ton of
energy (10,000 km/s)
• Extra energy can create heavier elements than Fe
Neutron Stars
• Supported by neutron
degeneracy pressure
• M ~ 1-2 Msun
• R ~ 10 km
• ρ ~ 1014 g/cm3
• Vesc ~ 0.5c
• Rotational periods
range from msec – sec
– Angular momentum
Pulsars
• Rapidly spinning
neutron star
• Tightly bunched
magnetic field lines
direct radiation out
from poles
• Magnetic axis not
aligned with rotation
axis; lighthouse effect
• All pulsars are neutron stars, not all neutron stars are pulsars
Pulsars
• Nearly perfect clocks
• Radiation takes angular
momentum away,
slowing down rotation
• Pulsar dies when
rotation gets too slow
Millisecond pulsars/X-ray binaries
• Gain angular
momentum from
material accreted from
companion
• Can be recycled pulsars
• Very strong gravity
makes disk very hot and
bright in x-rays
• X-rays pulse due to
rotation
• VIDEO
X-Ray Bursters
• Accreted H builds up
into layer
• Pressure below H layer
is high enough for
fusion, which makes He
• If T reaches 108 K, He
fusion can ignite,
releasing tons of energy
– P ~ 100,000 Lsun
• Bursts last few seconds
General Relativity
Black Holes
• Escape velocity – the speed
necessary to climb out of a
gravitational potential
vesc
2GM

r
• Black holes have infinitely deep
potential wells
• Schwarzchild radius – the point in
the gravitational well where vesc = c
2GM
M
Rs  2  3.0 
km
c
M Sun
Black Hole Formation
• Neutron star can hold itself up against gravity
with neutron degeneracy pressure
• A star that is so massive that it collapses past
the neutron degeneracy limit will become a
black hole
• The result is a singularity
Cygnus X-1
• X-ray binary system
• 18 Msun star orbiting
with an unseen 10 Msun
object
• 10 Msun is much more
massive than neutron
degeneracy pressure
can support
• It must be a black hole
Gamma Ray Bursts
• First discovered in the 60s by US spy satellites
looking for nuclear bomb tests
• Astronomers first thought GRBs were just
more energetic versions of X-ray binaries
– X-ray binaries are concentrated in the disk of the
Milky Way
– GRBs are not, so they must be extragalactic
Gamma Ray Bursts
• Extremely short, luminous burst followed by
long afterglow of lower-energy radiation
• Most energetic outbursts in the Universe
– Brighter than 1,000,000 Milky Ways
Long GRBs
• Appear to be correlated
with core-collapse SN or
galaxies with active star
formation
– Suggests that
progenitors are SN from
super massive stars
– Formation of black hole
• Burst lasts > 2 sec
• Afterglow lasts several
days to a month
Short GRBs
• Do not appear to come
from SN explosions
• Theory suggests that a
double neutron star
binary or neutron star
and black hole binary
collision would produce
the energies necessary
to make a short GRB
• Burst lasts < 2 sec
• No afterglow
• Not well understood