Pulsars• A pulsar is a

neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis

Pulsars• The radiation beams

sweep through space like lighthouse beams as the neutron star rotates

Why Pulsars must be Neutron Stars

Circumference of NS = 2π (radius) ~ 60 km

Spin Rate of Fast Pulsars ~ 1000 cycles per second

Surface Rotation Velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS

Anything else would be torn to pieces!

What can happen to a neutron star in a close binary system?

Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary

Accreting matter adds angular momentum to a neutron star, increasing its spin

Episodes of fusion on the surface lead to X-ray bursts

A black hole is an object whose gravity is so powerful that not even light can escape it.

Supernovae/Supernova Remnants

• Massive stars fuse heavier elements, up to Iron (Fe)

• Core is billions of Kelvin and greater than Chandrasekhar Limit (1.4 Msun)

• Rapid collapse to neutron star

• Rebound of core results in expulsion of outer layers Supernova Remnant

Before/After!

Tycho SNR (1572)

Supernova 1987A (light took 170,000 years to get here!)

Black HolesThe Science Behind The Science Fiction

A black hole is an object whose gravity is so powerful that not even light can escape it.

Thought Question

What happens to the escape velocity from an object if you shrink it?

A. It increasesB. It decreasesC. It stays the same

Hint:

Escape Velocity

Initial KineticEnergy

Final Gravitational Potential Energy

=

=

(escape velocity)2 G x (mass)

2 (radius)

Eluding Gravity’s Grasp

Earth: Vesc = 27,000 miles/hour (11 km/s)Sun: Vesc = 1.4 million miles/hour (600 km/s)

€

Vesc =2GM

R

Mass MRadius R

Escape VelocityEscape Velocity

Speed Needed ToEscape An Object’s

Gravitational Pull

“Dark Stars” Rev. John Michell (1783) & Pierre-Simon Laplace (1796)

Speed of light 1 billion miles/hour (3x105 km/s)

What if a star were so small, escape speed > speed of light?

A star we couldn’t see!A star we couldn’t see!

Earth massEarth mass: : R R 1 inch 1 inchSolar massSolar mass: : R R 2 miles 2 miles

Vesc = speedof light

“Surface” of a Black Hole

• The “surface” of a black hole is the radius at which the escape velocity equals the speed of light.

• This spherical surface is known as the event horizon.

• The radius of the event horizon is known as the Schwarzschild radius.

3 MSun Black Hole

The event horizon of a 3 MSun black hole is also about as big as a small city

Neutron star

No Escape

• Nothing can escape from within the event horizon because nothing can go faster than light.

• No escape means there is no more contact with something that falls in.

Mass versus radius for a neutron star

Objects too heavy to be neutron stars collapse to black holes

Neutron Star Limit

• Neutron pressure can no longer support a neutron star against gravity if its mass exceeds about 3 Msun

• Some massive star supernovae can make black hole if enough mass falls onto core

Singularity

• Beyond the neutron star limit, no known force can resist the crush of gravity.

• As far as we know, gravity crushes all the matter into a single point known as a singularity.

Singularity•The shrunken star too small to be measured but with indefinite density

If the Sun shrank into a black hole, its gravity would be different only near the event horizon

Black holes don’t suck!

Einstein’s theory of gravity is built on the principle that

1. The speed of light is constant.

2. As an object speeds up its clock runs faster.

3. The effects of gravity cannot be distinguished from the effects of acceleration.

4. Motion is a relative state.

How about if there is wind?

Speed of light is constant

Our conceptions of space and time has to be changed.

• Facts:• Regardless of speed or direction, observers always

measure the speed of light to be the same value.• Speed of light is maximum possible speed.

• Consequences:– The length of an object decreases as its speed

increases– Clocks passing by you run more slowly than do clocks

at rest (example: solar wind particles)

Time dilation

Special Relativity: Length Contraction

Equivalence principle

Gravitational redshift

Gravity deforms space-time

Precession of Mercury’s orbit

Gravity bends the path of light

Geodesics in curved spacetime

Gravity bends the path of light

Light waves are stretched out leading to a gravitational redshift

Tidal forces near the event horizon of a 3 MSun black hole would be lethal to humans

Tidal forces would be gentler near a supermassive black hole because its radius is much bigger

Falling into a black hole

Falling into a black hole gravitational tidal forces pull spacetime in such a way that time becomes infinitely long (as viewed by distant observer).

The falling observer sees ordinary free fall in a finite time.

Falling into a black holes

• With a sufficiently large black hole, a freely falling observer would pass right through the event horizon in a finite time, would be not feel the event horizon.

• A distant observer watching the freely falling observer would never see her fall through the event horizon (takes an infinite time).

• Falling into smaller black hole, the freely falling observer would be ripped apart by tidal effects.

Falling into a black hole

• Signals sent from the freely falling observer would be time dilated and redshifted.

• Once inside the event horizon, no communication with the universe outside the event horizon is possible.

• But incoming signals from external world can enter.

• Time travel and other fairy tales…

Seeing black holes

Seeing black holes

How do we know it’s a BH?• Nature is tricky: couldn’t it be another “small star”

like a neutron star or a white dwarf?

• Measure mass of “X-ray star” by motion of its companion (a star like the sun)

Mass > 3 solar masses BH!

• Roughly a dozen BHs found this way (tip of the iceberg)

Chandrasekhar

Black Hole Verification

• Need to measure mass— Use orbital properties of companion— Measure velocity and distance of orbiting gas

• It’s a black hole if it’s not a star and its mass exceeds the neutron star limit (~3 MSun)

One famous X-ray binary with a likely black hole is in the constellation Cygnus

Gamma-Ray Bursts• Brief bursts of

gamma-rays coming from space were first detected in the 1960s

• Observations in the 1990s showed that many gamma-ray bursts were coming from very distant galaxies

• They must be among the most powerful explosions in the universe—could be the formation of a black hole

Supernovae and Gamma-Ray Bursts

• Observations show that at least some gamma-ray bursts may be produced by supernova explosions

• Some others may come from collisions between neutron stars

Quasars•Small, powerful source of energy thought to be cores of distant spiral galaxies

• Active galaxies are galaxies with exceptionally bright and compact nuclear regions, called Active Galactic

Nuclei (AGN).

• The energy source of AGN is ultimately gravity, in the form of accretion of gas onto a super- massive black hole, one the most efficient engines in the Universe.

Quasars and Active Galaxies

THE AGN ZOO: jet-powered radio lobes

What is the energy source of AGN?

The one characteristic that all AGN share is fast variability, from which astronomers infer the size of the ‘central engine’.

A Unified Model for AGN

Are the different classes of AGN truly different ‘beasts’?

In the Unified Model for AGN, the apparent differences are mainly due to inclination effects.

The ingredients are: the hole, the disk, the jet, some orbiting clouds of gas, plus a dusty torus that surrounds the inner disk.

A Unified Model for AGN

A Unified Model for AGN: observational confirmations