Neutron Stars and Black Holes

Chapter 14

When stars like the sun die, they leave behind white dwarfs, but more massive stars leave behind the strangest beasts in the cosmic zoo. Now you are ready to meet neutron stars and black holes, and your exploration will answer five essential questions:

• How did scientists predict the existence of neutron stars?

• What is the evidence that neutron stars really exist?

• How did scientists predict the existence of black holes?

• What is the evidence that black holes really exist?

• What happens when matter falls into a neutron star or black hole?

Guidepost

Answering these questions has challenged scientists to create new theories and to test them critically. That raises an important question about science:

• What checks are there against fraud in science?

This chapter ends the story of individual stars, but it does not end the story of stars. In the next chapter, you will begin exploring the giant communities in which stars live – the galaxies.

Guidepost

I. Neutron StarsA. Theoretical Prediction of Neutron StarsB. The Discovery of PulsarsC. A Model PulsarD. Recognizing Neutron StarsE. Binary PulsarsF. The Fastest PulsarsG. Pulsar Planets

II. Black HolesA. Escape VelocityB. Schwarzschild Black HolesC. Black Holes Have No HairD. A Leap into a Black HoleE. The Search for Black Holes

Outline

III. Compact Objects with Disks and JetsA. X-Ray BurstersB. Accretion Disk ObservationsC. Jets of Energy from Compact ObjectsD. Gamma-Ray Bursts

Outline (continued)

Formation of Neutron Stars

Compact objects more massive than the

Chandrasekhar Limit (1.4 Msun) collapse

beyond the formation of a white dwarf.

Pressure becomes so high that electrons and protons

combine to form stable neutrons throughout the object:

p + e- n + e

Neutron Star

A supernova explosion of a M > 8 Msun star blows away its outer layers.

The central core will collapse into a compact object of

~ a few Msun.

Formation of Neutron Stars (2)

Properties of Neutron Stars

Typical size: R ~ 10 km

Mass: M ~ 1.4 – 3 Msun

Density: ~ 1014 g/cm3

a piece of neutron star matter of the size of a

sugar cube has a mass of ~ 100 million tons!!!

A neutron star (more than the

mass of the sun) would

comfortably fit within the

Capital Beltway!

Discovery of Pulsars

=> Collapsing stellar core spins up to periods of ~ a few

milliseconds.

Angular momentum conservation

=> Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars

Magnetic fields are amplified up to B ~ 109 – 1015 G.

(up to 1012 times the average magnetic field of the sun)

Pulsars / Neutron Stars

Wien’s displacement law,

max = 3,000,000 nm / T[K]

gives a maximum wavelength of max = 3 nm, which corresponds to X-rays.

Cas A in X-rays

Neutron star surface has a temperature of~ 1 million K.

Pulsar Periods

Over time, pulsars lose energy and

angular momentum.

=> Pulsar rotation is gradually

slowing down.

Pulsar Winds

These winds carry away about 99.9 % of the energy released from the slowing-down of the

pulsar’s rotation.

Pulsars are emitting winds and jets of highly energetic particles.

Lighthouse Model of Pulsars

A Pulsar’s magnetic field has a dipole structure, just like Earth.

Radiation is emitted

mostly along the magnetic

poles.

Images of Pulsars and Other Neutron Stars

The Vela Pulsar moving through interstellar space

The Crab nebula and

pulsar

The Crab Pulsar

Remnant of a supernova observed in A.D. 1054

Pulsar wind + jets

The Crab Pulsar (2)

Visual image

X-ray image

Light Curves of the Crab Pulsar

Proper Motion of Neutron Stars

Some neutron stars are moving rapidly through

interstellar space.

This might be a result of anisotropies during the supernova explosion,

forming the neutron star.

MagnetarsSome neutron stars have magnetic fields ~ 1000 times stronger even than normal neutron stars.

These care called Magnetars.

Earthquake-like ruptures in the surface crust of Magnetars cause bursts of soft gamma-rays.

Binary PulsarsSome pulsars form binaries with other neutron stars (or black

holes).

Radial velocities resulting from the orbital motion lengthen the pulsar period when the pulsar is moving away from Earth...

…and shorten the pulsar period when it is approaching

Earth.

Neutron Stars in Binary Systems: X-ray Binaries

Example: Her X-1

2 Msun (F-type) star

Neutron star

Accretion disk material heats to several million K => X-ray emission

Star eclipses neutron star and accretion disk periodically

Orbital period = 1.7 days

Pulsar PlanetsSome pulsars have planets orbiting around them.

Just like in binary pulsars, this can be discovered through variations of the pulsar period.

As the planets orbit around the pulsar, they cause it to wobble around, resulting in slight changes of the observed pulsar period.

Black Holes

Just like white dwarfs (Chandrasekhar limit: 1.4 Msun), there is a mass limit for neutron stars:

Neutron stars can not exist with masses > 3 Msun

We know of no mechanism to halt the collapse of a compact object with > 3 Msun.

It will collapse into a single point – a singularity:

=> A Black Hole!

Escape VelocityVelocity needed

to escape Earth’s gravity from the surface: vesc ≈

11.6 km/s.

vesc

Now, gravitational force decreases with distance (~ 1/d2) => Starting

out high above the surface => lower escape velocity.

vesc

vesc

If you could compress Earth to a smaller radius => higher escape velocity from the surface

The Schwarzschild Radius

=> There is a limiting radius where the escape velocity

reaches the speed of light, c:

Vesc = cRs = 2GM ____ c2

Rs is called the Schwarzschild Radius.

G = Universal const. of gravity

M = Mass

Schwarzschild Radius and Event Horizon

No object can travel faster than the speed of light

• We have no way of finding out what’s

happening inside the Schwarzschild

radius.

=> nothing (not even light) can

escape from inside the Schwarzschild

radius

“Event horizon”

Black Holes in Supernova Remnants

Some supernova

remnants with no pulsar /

neutron star in the center may contain black

holes.

Schwarzschild Radii

“Black Holes Have No Hair”

Matter forming a black hole is losing almost all of its properties.

Black Holes are completely determined by 3 quantities:

Mass

Angular Momentum

(Electric Charge)

General Relativity Effects Near Black Holes (1)

At a distance, the gravitational fields of a black hole and a star of the same mass are virtually identical.

At small distances, the much deeper gravitational potential

will become noticeable.

General Relativity Effects Near Black Holes (2)

An astronaut descending down towards the event horizon of the BH will be stretched vertically (tidal effects) and squeezed

laterally.

This effect is called “spaghettification.”

General Relativity Effects Near Black Holes (3)

Time dilation

Event Horizon

Clocks starting at 12:00 at each point.

After 3 hours (for an observer far away

from the BH):Clocks closer to the BH run more slowly.

Time dilation becomes infinite at the event horizon.

General Relativity Effects Near Black Holes (4)

Gravitational Red Shift

Event Horizon

All wavelengths of emissions from near the event horizon are stretched (red shifted).

Frequencies are lowered

Observing Black HolesNo light can escape a black hole

=> Black holes can not be observed directly.

If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and

radial velocity.

Mass > 3 Msun

=> Black hole!

Candidates for Black Hole

Compact object with > 3 Msun must be a

black hole!

Compact Objects with Disks and Jets

Black holes and neutron stars can be part of a binary system.

=> Strong X-ray source!

Matter gets pulled off from the companion star, forming an accretion

disk.

Heats up to a few million K

X-Ray Bursters

Several bursting X-ray sources have been

observed:

Repeated outbursts: The longer the interval before the burst, the

stronger the burst.

Rapid outburst followed by

gradual decay

The X-Ray Burster 4U 1820-30

In the cluster NGC 6624

Optical Ultraviolet

Black-Hole vs. Neutron-Star Binaries

Black Holes: Accreted matter disappears beyond the event

horizon without a trace.

Neutron Stars: Accreted matter produces an X-ray flash as it impacts on the neutron star surface.

Black Hole X-Ray Binaries

Strong X-ray sources

Rapidly, erratically variable (with flickering on time scales of less than a second)

Sometimes: Quasi-periodic oscillations (QPOs)

Sometimes: Radio-emitting jets

Accretion disks around black holes

Gamma-Ray Bursts (GRBs)

Short (~ a few s), bright bursts of gamma-rays

Later discovered with X-ray and optical afterglows lasting several

hours – a few days

GRB a few hours after the GRB

Same field,

13 years earlier

Many have now been associated with host galaxies at large (cosmological) distances.

A model for Gamma-Ray Bursts

At least some GRBs are probably related to the deaths of very massive (> 25 Msun) stars.

In a supernova-like explosion of stars this massive, the core

might collapse not to a neutron star, but directly to a black hole.

Such stellar explosions are termed

“hypernovae”