life and evolution of a massive star

Life and Evolution of aMassive 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• 12C + 1H 13N + γ• 13N 13C + e+ + νe

• 13C + 1H 14N + γ• 14N + 1H 15O + γ• 15O 15N + e+ + νe

• 15N + 1H 12C + 4He + γ

• Overall reaction:4 1H 4He

• 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=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

• Black holes have infinitely deep potential wells

• Schwarzchild radius – the point in the gravitational well where vesc = c

r

GMvesc

2

kmM

M

c

GMR

Suns 0.32

2

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