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 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 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

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 ~ 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 Black holes have infinitely deep potential wells Schwarzchild radius – the point in the gravitational well where v esc = c

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

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