Chapter 12

Collapsing, Exploding, Interacting Stars, and Stellar Black Holes

Deaths of High Mass Stars:

10 - 100 solar masses

Successive fusion of heavier elements in core, then shells

Layered shells of fusing elements

Silicon burning (fusion) produces iron

Iron core - fusion stops

Iron fusion is endothermic

Core contracts, heats

Shell fusion continues

Calculations suggest core collapse at iron mass ~ 2 Msun

Core more massive than Chandrasekhar limit

Electron degeneracy cannot stop collapse

Electrons + protons => neutrons + neutrinos

Neutron degeneracy pressure stops collapse if remaining mass less than 2-3 solar masses

Neutron star - radius ~ 10 km

density ~ 4 x 1017 kg/m3

Outer layers fall inward

"Bounce" on hot core

Star's outer layers ejected

Dramatic increase in optical luminosity

Most of energy released as neutrinos

Fusion in outer layers - elements heavier than iron produced

Type I supernova - weak or absent H lines in spectra

Type II supernova - strong H lines in spectrum

Massive star core collapse supernova: Type Ib, Type Ic, Type II

Type Ia - brighter than Type II

Explosion of white dwarf at Chandrasekhar limit due to mass transfer in a binary system

Models predict star completely destroyed

Type Ia used in cosmology studies

Supernova Remnant

Expanding shell of gas + surrounding material

Visible ~104 years

Synchrotron radiation - emitted by electrons spiraling around magnetic field lines in shell

Examples - Crab nebula, Vela remnant

SN 1987A - Type II

First modern, nearby supernova
(Large Magellanic Cloud)

Neutrinos observed before visible light - as predicted

Neutron star predicted

Neutron Stars and Pulsars

Predicted 1932 - Baade, Zwicky, Landau

Discovered 1967 - Bell, Hewish

Radio sources emitting short, regular pulses (pulsars)

From conservation of angular momentum, neutron stars rotate ~ 1 rotation/sec

Magnetic field strength also conserved => very strong magnetic fields on neutron stars

Tsurface ~ 106 K

Lighthouse theory:

Electric field accelerates electrons.

Electrons emit beamed photons along magnetic axis (synchrotron radiation).

Magnetic, rotation axes not aligned.

Photon beam rotates across line-of-sight.

Pulses in gamma rays to radio, depends on electron energy and magnetic field strength (Crab and Vela pulsars)

Slowing rotation, sudden period changes support lighthouse theory.

Millisecond pulsars - old neutron stars in close binaries

Spun up by mass transfer from evolving companion

Collapsed Objects in Close Binaries

Nova - luminosity increases ~106,
returns to normal

Binary companion transfers mass to accretion disk around white dwarf

Matter accumulates on white dwarf.

Explodes as nova.

Shell of 0.0001 Msun ejected.

Recurs 10,000 - 100,000 years.

Neutron stars in binary systems

Mass transfer to neutron star directly or via accretion disk

T ~ 10 - 100 x 106 K

Strong x-ray emission

X-ray bursters - bursts of x-rays, due to repeated ignition of accumulating He on neutron star

Binary Pulsars

2 neutron stars

Orbit shrinking.

Gravitational waves - disturbance in space-time due to motion of mass.

Travel at c

Energy lost due to gravitational waves

First observation of effect of gravity waves

Black Holes

Region of space where escape velocity greater than speed of light.

Receive no information from inside

Produced by large mass in small volume

Schwarzschild radius - characteristic size of black hole

Rs = 2 x G x M/c2

Example: 1 Msun black hole, R = 3 km.

Generally: R (km) = 3 x M (solar masses)

Mass of collapsing core greater than maximum mass of neutron star, ~3Msun

Degenerate electrons, neutrons can't support

Collapses to singularity

Infinite density, gravity

Zero radius.


Singularity would be INSIDE black hole.

The Event Horizon

Theoretical black hole properties: mass, angular momentum, electric charge

Schwarzschild black holes only have mass

Event horizon - sphere with Schwarzschild radius

Nothing escapes from within event horizon

Material can fall in to black hole

Tidal effects strong at event horizon of stellar mass black holes

Time dilation - clocks run more slowly in strong gravitational fields, as observed remotely

To remote observer, in principle, infalling material appears to linger at edge 'forever' while material in fact falls in.

But due to large gravitational redshift, material disappears anyway.

Kerr black holes rotate

Theory suggests:

Two event horizons

Ergosphere - volume between

Part of an object can escape from ergosphere - with added energy

Can't escape from within inner event horizon

Matter falling into black holes emits large amounts of energy - x-rays

Detecting black holes

Detected by impact on nearby matter

E.g. binary star with massive, invisible companion

In close binary, accretion disk emitting x-rays suggests compact object, possibly a black hole, if mass > 3 solar masses.

Look for binary stars emitting X-rays

Mass estimates difficult

Several stellar mass black hole candidates

Prof. Donna Weistrop

University of Nevada, Las Vegas