Neutron Stars and Black Holes

PHYS390: Astrophysics

Professor Lee Carkner

Lecture 18

Question

1) Should there be a lower limit for the mass of observed white dwarfs? Why or why not?

Yes, size of white dwarf depends on initial mass of star, very low mass stars have not had time to evolve to white dwarf yet

Neutron Stars

None detected until the 1960s

Principally observed as pulsars rapidly rotating neutron star

producing beamed radio emission

Neutron Degeneracy

Density of neutron star ~ Star is like a big ball of ~ Acceleration of gravity at surface ~ trillion

meters per second2

Mass limit of ~3 Msun

Neutron Star Formation

Start with iron core At high densities electrons become relativistic

and combine with protons to produce neutrons (and neutrinos)

As density increases neutrons “drip” outside of nuclei Can form superconducting superfluid

Neutron Star Structure

inner crust of heavy nuclei and free neutrons

interior mostly neutrons maybe a core of sub-

nuclear particles?

Rotation

Ratio of initial and final periods:

Pf/Pi = (Rf/Ri)2

End up with neutron star rotation periods ~ 1 second

Flux Freezing

Magnetic fields get “frozen” into core material and concentrated as core shrinks

Bf/Bi = (Ri/Rf)2

Again, hard to know initial core magnetic field Typical neutron star B ~108 T

Pulsars

P ~ 1 sec Only something very

small and compact could change that fast

Many pulsars have large space motions

Can be found in the center of SNR

Pulsar Model

Changing magnetic field produces magnetic dipole radiation

If the cone intersects the Earth, we see the radio pulse

Energy is drawn from rotation and the pulsar slows down over time

Black Hole Gravity is so strong that the escape velocity exceeds the

speed of light Point occurs at the Schwarzschild radius

RS = 2GM/c2

Marks the event horizon

At the center is the singularity

Even outside of the event horizon, tidal forces are very strong Material nearing a black hole is violently ripped apart Can heat up material causing it to emit

Rotation

Maximum angular momentum is:

Lmax = GM2/c

May cause frame dragging of local spacetime

Types of Black Holes

3-15 Msun, stellar remnant black holes

100-1000 Msun: intermediate mass black holes possible explanation for superbright X-ray sources

105-109 Msun, supermassive black holes

Create Active Galactic Nuclei (AGN) when active, hard to find if not active

NS and BH Binaries

Called an X-ray binary If the mass of the

compact object is greater than ~3 Msun, it is a black hole

More than anything else but annihilation

Types of Binaries X-ray pulsar

Matter falls onto pulsar, heating it up to X-ray temperatures (107 K)

X-ray hot spot may be eclipsed

Mass transfer may spin-up the pulsar, decreasing the period

X-ray burster If the magnetic field is too

weak the material will build up in a layer on the surface

Next Time

Test 3 Same format as 1 and 2 For Friday:

Read 24.2-24.4 Homework: 24.15, 24.32