Astro 1050 Wed. Nov. 5, 2003

Today: Ch. 11: Neutron Stars & Black Holes

The Crab Nebula – Supernova from 1050 AD

Can see expansion between 1973 and 2001

Kitt Peak National Observatory Images

What happens to the collapsing core?

Neutron star

Quantum rules also resist neutron packing

Densities much higher than white dwarfs allowed

R ~ 5 km r ~ 10¹⁴ gm/cm³ (similar to nucleus)

M limit uncertain, ~2 or ~3 M_Sun before it collapses

Spins very fast (by conservation of angular momentum)

Trapped spinning magnetic field makes it:

Act like a “lighthouse” beaming out E-M radiation (radio, light)

pulsars

Accelerates nearby charged particles

Spinning pulsar powers the
Crab nebula

Red: Ha

Blue: “Synchrotron” emission from high speed electrons trapped in magnetic field

Another pic of the Crab, Pulsar

Why a “pulsar?”

“Lighthouse” Model for Pulsars

Another Neutron Star in a SNR

Other cool stuff about Neutron Stars

Novel Dragon’s Egg by Robert L. Forward

Short Story “Neutron Star” by Larry Niven

Binary Pulsars

Gamma Ray Bursts?

Pulsar Planets

Black Holes -- basics

Nothing can stop collapse after neutron pressure fails

Escape velocity from a surface at radius R:

As R shrinks (but M is fixed), V_escape gets larger and larger

At some point V_Escape= c (speed of light)

Happens at Schwarzschild radius:

Not even light can escape from within this radius

Examples:

The Schwarzschild Radius:

Mass in solar masses R_s (km)

10

3

2

1

0.000003 (Earth)

Examples:

The Schwarzschild Radius:

Mass in solar masses R_s (km)

10 30

3 9

2 6

1 3

0.000003 (Earth) 0.9 cm

Black Holes -- details

Remember – gravity is same as before, away from mass

Black holes do NOT necessarily pull all nearby material in

A planet orbiting a new black hole would just keep on orbiting as before (assuming the ejected material or radiated energy didn’t have an effect)

Any mass can potentially be made into a black hole – if you can compress it to a size smaller than R_S = 2GM/c²

1 M_Sun: 3.0 km 10⁶ M_Sun 3´10⁶ km 1 M_Earth 8.9 mm

Black Holes -- details

If you do make material fall into a black hole, material will be falling at close to the speed of light when it reaches R_S

If that falling gas collides with and heats other gas before it reaches R_S, then light from that hot material (outside R_S) can escape (important in quasars!).

Black Holes – detection

By definition – can’t see light from black hole itself

Can see large amounts of energy released by falling material just before it crosses R_S

Can see motion of nearby objects caused by gravity of black hole

Black Holes – detection

Example: Like White Dwarf accretion disk but w/ black hole instead

Gas from red giant companion spills over towards black hole

Gas spirals in toward black hole, through accretion disk

Gas will be much hotter because it falls further, to very small R_S

Gas will be moving at very high velocity

Much faster than with white dwarf since much closer (P² µ a³)

Signature of black hole: Very high energy release, very high velocity

We find MASSIVE black holes in centers of most galaxies

Cygnus X-1

More Cool Stuff About Black Holes

Time Dilation – originally “Frozen Stars”

Gravitational Redshift

Wicked Tidal Forces

Hawking Radiation


	Can see expansion between 1973 and 2001
		Kitt Peak National Observatory Images


Neutron star
	Quantum rules also resist neutron packing
		Densities much higher than white dwarfs allowed
			R ~ 5 km r ~ 10¹⁴ gm/cm³ (similar to nucleus)
		M limit uncertain, ~2 or ~3 M_Sun before it collapses

	Spins very fast (by conservation of angular momentum)

	Trapped spinning magnetic field makes it:
		Act like a “lighthouse” beaming out E-M radiation (radio, light)
			pulsars
		Accelerates nearby charged particles


	Red: Ha

	Blue: “Synchrotron” emission from high speed electrons trapped in magnetic field


	Novel Dragon’s Egg by Robert L. Forward
	Short Story “Neutron Star” by Larry Niven

	Binary Pulsars
		Gamma Ray Bursts?
	Pulsar Planets


	Nothing can stop collapse after neutron pressure fails

	Escape velocity from a surface at radius R:

	As R shrinks (but M is fixed), V_escape gets larger and larger

	At some point V_Escape= c (speed of light)

		Happens at Schwarzschild radius:

		Not even light can escape from within this radius


The Schwarzschild Radius:
	Mass in solar masses R_s (km)
		10
		3
		2
		1
		0.000003 (Earth)


The Schwarzschild Radius:
	Mass in solar masses R_s (km)
		10 30
		3 9
		2 6
		1 3
		0.000003 (Earth) 0.9 cm



	Remember – gravity is same as before, away from mass
		Black holes do NOT necessarily pull all nearby material in
		A planet orbiting a new black hole would just keep on orbiting as before (assuming the ejected material or radiated energy didn’t have an effect)

	Any mass can potentially be made into a black hole – if you can compress it to a size smaller than R_S = 2GM/c²
		1 M_Sun: 3.0 km 10⁶ M_Sun 3´10⁶ km 1 M_Earth 8.9 mm




	If you do make material fall into a black hole, material will be falling at close to the speed of light when it reaches R_S
		If that falling gas collides with and heats other gas before it reaches R_S, then light from that hot material (outside R_S) can escape (important in quasars!).



	By definition – can’t see light from black hole itself

	Can see large amounts of energy released by falling material just before it crosses R_S

	Can see motion of nearby objects caused by gravity of black hole




Example: Like White Dwarf accretion disk but w/ black hole instead
	Gas from red giant companion spills over towards black hole
	Gas spirals in toward black hole, through accretion disk
		Gas will be much hotter because it falls further, to very small R_S
	Gas will be moving at very high velocity
		Much faster than with white dwarf since much closer (P² µ a³)

Signature of black hole: Very high energy release, very high velocity

We find MASSIVE black holes in centers of most galaxies


	Time Dilation – originally “Frozen Stars”

	Gravitational Redshift

	Wicked Tidal Forces

	Hawking Radiation