Life and Evolution of a Massive Star M ~ 25 M Sun
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 12 C + 1 H 13 N + γ 13 N 13 C + e + + ν e 13 C + 1 H 14 N + γ 14 N + 1 H 15 O + γ 15 O 15 N + e + + ν e 15 N + 1 H 12 C + 4 He + γ Overall reaction: 4 1 H 4 He
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=mc 2
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 M sun R ~ 10 km ρ ~ 10 14 g/cm 3 V esc ~ 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 10 8 K, He fusion can ignite, releasing tons of energy P ~ 100,000 L sun Bursts last few seconds
General Relativity
Black Holes Escape velocity the speed necessary to climb out of a gravitational potential 2GM v esc r Black holes have infinitely deep potential wells Schwarzchild radius the point in the gravitational well where v esc = c R s 2GM 3. 0 2 c M M Sun km
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 M sun star orbiting with an unseen 10 M sun object 10 M sun 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
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 Short GRBs