1 Chapter 14: Neutron Stars and Black Holes Neutron Stars • Form from a 8-20 M Sun star • Leftover 1.4-3 M Sun core after supernova • Neutron Stars consist entirely of neutrons (no protons) Neutron Star (tennis ball) and Washington D.C. Neutron Stars • About the size of a large city (5-10 miles), Several times the mass of the Sun • So they are incredibly dense! • One teaspoon of a neutron star would weigh 100 million tons! Neutron Star (tennis ball) and Washington D.C. •Held up by degeneracy pressure: the neutrons don’t like to be squished close together! What’s holding it up? Electron energy White Dwarfs and Neutron Stars are made of degenerate matter. Degenerate matter cannot be compressed….the electrons (or neutrons) are already as close as possible. White dwarfs and neutron stars are held up by degeneracy pressure Pulsars: Stellar Beacons • Spinning neutron stars • Strong magnetic field emits a beam radio waves along the magnetic poles • These are not aligned with the axis of rotation. • So the beam of radio waves sweeps through the sky as the Neutron Star spins. Model of a Pulsar (a rotating Neutron Star) The Lighthouse Model of Pulsars If the beam shines on Earth, then we see a Pulse of energy (radio waves) Neutron star’s magnetic field A pulsar is a rotating neutron star. A pulsar’s beam is like a lighthouse
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Chapter 14: Neutron Starsand Black Holes
Neutron Stars• Form from a 8-20 MSun star
• Leftover 1.4-3 MSun core aftersupernova
• Neutron Stars consist entirelyof neutrons (no protons)
Neutron Star (tennis ball) andWashington D.C.
Neutron Stars• About the size of a large city(5-10 miles), Several times themass of the Sun• So they are incredibly dense!• One teaspoon of a neutronstar would weigh 100 milliontons!
Neutron Star (tennis ball) andWashington D.C.
•Held up by degeneracy pressure: theneutrons don’t like to be squished closetogether!
What’s holding it up?
Ele
ctro
n en
ergy
White Dwarfs and NeutronStars are made of degeneratematter.
Degenerate matter cannot becompressed….the electrons (orneutrons) are already as closeas possible.
White dwarfs and neutron stars are held up bydegeneracy pressure
Pulsars: Stellar Beacons• Spinning neutron stars
• Strong magnetic field emits a beamradio waves along the magneticpoles
• These are not aligned with the axisof rotation.
• So the beam of radio wavessweeps through the sky as theNeutron Star spins. Model of a Pulsar
(a rotating Neutron Star)
The Lighthouse Model of Pulsars
If the beam shines on Earth, then we seea Pulse of energy (radio waves)
Neutron star’s magnetic field
A pulsar is a rotatingneutron star.
A pulsar’s beam is like a lighthouse
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The Crab Pulsar
Inside the Crab Supernova Remnant, a Pulsar has been found
A massive star dies in a Supernova explosion.
Most of the star is blasted into space.
The core that remains can be a neutron star.However…
Neutron stars can not existwith masses M > 3 Msun
If the core has more than 3 solar masses…
It will collapse completely to single point –
=> A black hole!
Degenerate MatterIf a White Dwarf gets tooheavy it will collapse… into aNeutron Star (this triggers asecond type of Supernovaexplosion)
White dwarfs cannot be moremassive than 1.4 Msun
Similarly, Neutron stars cannotbe larger than about 3 M Sun
They will collapse completelyand turn into a black hole!
Black Holes: Overview
•A total victory for gravity.
•Collapsed down to a single point.
•This would mean that they have infinitedensity
•Their gravity is so strong, not even lightcan escape!
Escape VelocityEscape Velocity (vesc) is the speed requiredto escape gravity’s pull.
On Earth vesc ≈ 11.6 km/s.
vesc
If you launch a spaceshipat v= 11.6 km/s or faster, it
will escape the Earth
But vesc depends on themass of the planet or star…
Why Are Black Holes Black?On planets with more gravity than Earth,Vesc would be larger.
On a small body like an asteroid, Vescwould be so small you could jump intospace.
A Black Hole is so massive thatVesc = the speed of light.
Not even light can escape it, so it givesoff no light!
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Black Holes & Relativity• Einstein’s theory of General Relativity says space is
curved by mass• So a star like the Sun should bend space, and light
traveling past it will get thrown off course• This was confirmed during a solar eclipse in 1919
Light Can be Bent by Gravity
Event Horizon
We have no way offinding out what’shappening inside!
Nothing can get outonce it’s inside the
event horizon
The Schwarzschild RadiusIf Vescape > c, then nothing can leave the star, not light,
not matter.
Rs =2GM____
c2
Rs = Schwarzschild radius
If something is compressed smaller than Rs it will turninto a black hole!
G = gravitational constantM = mass
c = speed oflight
We can calculate the radius of such a star:
Vesc = c
Black Holes: Don’t Jump Into One!If you fall into a Black Hole, you
will have a big problem:
Your feet will be pulled withmore gravity than your head.
You would experience “tidalforces” pushing & pulling
Time is also distorted near ablack hole
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How do we know they’re real?
• Black holes:– Kepler’s Laws, Newton’s Laws– Accretion disks
• Pulsars:– Observe radio jets– Strong magnetic fields
Evidence for Black HolesNo light can escape a black hole, so
black holes can not be observed directly.
However, if a blackhole is part of a binarystar system, we canmeasure its mass.
If its mass > 3 Msun thenit’s a black hole!
Evidence for Black Holes• Cygnus X-1 is a source of X rays• It is a binary star system, with an O type supergiant & a compact
object
Cygnus X-1: A black hole
The mass of the compact object ismore than 20 Msun
This is too massive to be a whitedwarf or neutron star.
This object must be a black hole.
Evidence for Black Holes: X-rays
Artists’ drawings ofaccretion disks
Matter falling into a black hole may form an accretion disk.As more matter falls on the disk, it heats up and emits X-rays.If X-rays are emitted outside the event horizon we can seethem.
Supermassive Black Holes
• Stellar black holes come fromthe collapse of a star.
• They have masses of severalMsun
• Bigger mass = bigger BH!
• This happens in the center ofmost galaxies.
A supermassive black holedevours a star, releasing X-rays
Life Cycles of Stars• Low-mass stars: Fade out, stay on Main Sequence• Sun-like stars: White dwarf & planetary nebula• High-mass stars: Supernova -> SN remnant & dense