Introduction to AstronomyIntroduction to Astronomy

• Announcements

White DwarfsWhite Dwarfs&&

Neutron StarsNeutron Stars

White DwarfsWhite Dwarfs

Structure

Observations

• Background: the discovery of Sirius B– Companion star to large Sirius A

– First time White Dwarf observed as part of binary system

Sirius A: A-type mainsequence star

Sirius B: white dwarf

StructureStructure

• Generally,– Hot, compact stars w/ mass comparable to

the Sun & size comparable to the Earth

– Shines from residual (left-over) heat produced in core during normal lifetime

– 25,000 K < T < 4,500 K

– Tavg = 10,000 K

• A white dwarf is nothing more than the left-over core of a low-mass star (few x MSun)– During red giant phase, outer layers are

blown off• Radiation pressure

– These layers are mostly H & He• Fusion products of early star’s life• Blown off by Helium Flash

– Not enough mass to ignite C/O fusion by contraction

• Low mass

• So the core just cools off, radiating heat into space– Over ~ 10 million years, cools down to 20,000

K• So still emits some observable light

– Would take much longer than the age of the Universe to cool off to the point where it doesn’t emit any visible light

• “BLACK DWARF” (still just theoretical)

Interior of a White DwarfInterior of a White Dwarf

• In hydrostatic equilibrium– But no fusion pressure, so how is this

possible????

– Electron Degeneracy PressureElectron Degeneracy Pressure• Atoms squeezed incredibly close together• Electron orbits overlap

– Electrons easily move around, from atom to atom– Like a “sea” of electrons flowing over atomic nuclei

• The Pauli Exclusion Principle– A fundamental limit to the number of electrons

that can be squeezed into a given volume– When this limit is reached, there appears a

“pressure” that keeps any more electrons from entering the volume

– This “electron pressure” supports the white dwarf against its own gravity

• This leads to behavior that seems to defy common sense…– The more mass you pile onto a white dwarf,

the smaller it gets!

• In a normal gas, pressure depends on temperature and density

• This “sea” of electrons is what is called a degenerate gas– In a degenerate gas, pressure does not

depend on temperature, only density (of electrons)

• A small chunk of “white dwarf material” would weigh ~ couple dozen tons on Earth!

• Gravitational Redshift– Like Doppler shift

due to star’s motion– As light escapes from

the star’s surface, gravity pulls on it, “stretching” the light wave

– So light from a high-mass object appears redder than it actually is

But photons have no mass, so how doesGravity work on it?! General Relativity!

• White Dwarfs in binary systems

– White dwarf – red giant system is potentially very dangerous

– Outer layers of the red giant are not very bound to the star itself

• White dwarf can “strip away” the outer layers (feeding)

• Usually H-rich, so white dwarf gets a new fusion fuel source

• Two options

– A Nova

– A “Type I” Supernova

• Nova (or Nova Stella)– The white dwarf’s compression of H pulled

from the red giant heats it to fusion temperatures

– H explodes off the surface of the white dwarf• “Nova”

– White dwarf may begin to strip Hydrogen from the red giant again

• Repeated novae

Time, t Time, t + 7.5 months

HST observation of a Nova

• Supernovae– Different from novae– When fusion in a high-mass star stops, core

continues contracting, but no pressure to stop it

– Temperature increases so much the Iron nuclei start to break apart

– Collapse pushes electrons into protons, creating bulk of neutrons at nuclear density

– Rapid implosion causes material to “bounce” off high-density core, creates massive shock wave that rips the star apart

“Type II Supernova”

• Type I Supernova– Only occurs if white dwarf strips off too much

material– White dwarf passes the Chandrasekhar Limit

• If mass of white dwarf grows beyond ~ 1.44MSun, it compresses violently

• Squeezes Carbon & Oxygen together hard enough to fuse into 28Si

• 2 28Si smashed together into an isotope of Nickel

• Fusion releases tremendous amount of energy from white dwarf’s core, blows the dwarf apart (completely)

• Leaves behind no remnant– Just an expanding cloud of heavy elements

(C, O, Si, Ni, Co, Fe, etc)

• Type II Supernova– Collapse of massive star (not white dwarf)

White Dwarf ObservationsWhite Dwarf Observations

A team at the University of Arizonalocated a star cluster made almostcompletely of white dwarfs.

This cluster had to be extremely oldsince every member had used upit’s nuclear fuel.

From this, they figured the age ofthis particular cluster would givea lower bound to the age of the Universe.

They found

τuniverse > 13 billion years

M4 Globular Cluster (7,000 ly distant) White dwarfs

Sun-like stars

Red dwarfs(very coolMS stars)

• Old White Dwarfs– Recall interiors are C, O, Si– When cool enough, the Carbon atoms can

lock in to a crystalline structure– What is crystalline carbon?

• A diamond!

– Old white dwarfs may be giant diamonds floating around in space!!!

Neutron StarsNeutron Stars

Structure

Observations

• One step beyond white dwarfs– Requires a star of higher initial mass

• More intense collapse converts white dwarf’s “sea of electrons” to something else entirely…

History of Neutron StarsHistory of Neutron Stars

• 1934, Astronomers Walter Baade & Fritz Zwicky– Proposed that gravity could crush the core of a

star so much that the electrons are pushed into the nucleus

– Positive protons combine with negative electrons to form neutrons

– This would occur when core compressed to diameter of ~ 6 miles

• Smaller than many asteroids!

– Maximum possible mass 2 – 3MSun

This ultrahigh density meansa single cm3 of neutronstar material would weighbillions of tons here on Earth!

electron

neutronproton

Electrons pushed into nucleus

Ultradense ballof neutrons

Size of a Neutron Star…

Neutron Star ObservationsNeutron Star Observations

1997, first visible image (HST) of a lone Neutron star.

Mysterious X-ray source foundhere in 1992, but astronomerscouldn’t see anything.

HST determined temperature~ 1.2 million F

Diameter < 17 miles

Therefore, must be Neutron starbecause nothing else can bethis hot, small, and dim.

X-Ray image of wandering Neutron StarX-Ray image of wandering Neutron Star

PulsarsPulsars

• The hunt for neutron stars was on!

• But none found…

• Until a student discovered an extraterrestrial radio signal that pulsed precisely once every 1.33 seconds– Initially thought to be a sign of civilization,

dubbed LGM-1 (“little green men #1”)

• Other similar discoveries soon followed– Thought maybe they were pulsating stars

• Radius expands & contracts periodically, making the star alternately brighter & dimmer

– But periods were too short (impossibly small for the size-change required to change the brightness that much)

• Not pulsating, but spinning !– Like a cosmic lighthouse, see a pulse when

some “beam” of radiation points toward Earth

• But why so fast?!– Pulsar in Crab Nebula rotates 30 times a

second!

OFF ON

• Conservation of Angular Momentum– A.k.a. the Ice-Skater Effect– Bringing mass closer to the axis of rotation

makes the rotation speed increase– Same principle during formation of solar

system from slowly rotating IS cloud

– If Sun shrank down to 10 km:• Current rotational period = 30 days• New rotational period = 0.5 millisecond !

Pulsar Emission: The Lighthouse Pulsar Emission: The Lighthouse EffectEffect

• Intense electric and magnetic fields strip charged particles off star, accelerate them along magnetic poles

• Fact: accelerating charges emit EM radiation– Like a radio transmitter/antenna

• Charges travel along field lines, which form a tight core at the poles– Creates a tight cone of emission coming from each

pole = the lighthouse beam!

Protons &Electrons acceleratedslowly here

Protons & Electrons acceleratedquickly here

So a lot of emission near poles

Synchrotron Radiation: created by accelerating particles

• Non-thermal radiation– Depends on the strength of the charge, the

speed of the particle, and the strength of the magnetic field

– DOES NOT depend on the temperature

• Most “pulsar lighthouses” emit radio waves, but some emit more broadband…– Crab Nebula pulsar emits flashes of visible

light 30x a second

Pulsar Spin-DownPulsar Spin-Down

• Pulsar is constantly losing energy– Magnetic field exerts force on charged

particles, so particles exert equal & opposite force on magnetic field (“back-reaction”)

– Magnetic “friction” or “drag”• Slows rotation very slightly

• Very lengthy measurements of pulsar periods show that time between pulses is slowly increasing

• If it slows down enough, strength of EM radiation decreases until the “lighthouse” beams become invisible

Hand-crank generator

• Structure (kind of like a balloon)– Thin, gaseous atmosphere ( ~ 1 mm thick )

• Source of particles accelerated by magnetic field

– Solid iron crust ( ~ few hundred meters thick )

– Liquid sea of neutrons (bulk)

Why don’t the surface layers become neutrons?

Pulsar CuriositiesPulsar Curiosities

• X-Ray Bursters– Infalling gas violently heated as it flows down

magnetic field to surface– Creates thermonuclear explosion like a nova

• X-Ray Pulsars– Infalling gas doesn’t explode, but still heated enough

to create a “hot-spot” on the surface of the neutron star

– Rotates in and out of view, creating pulses of X-rays

X-Ray Pulsar

What We See…

• Millisecond Pulsars– Rotate 1000x per second– Most have companion stars– Gravity attracts companion material into an

accretion disk around the neutron star• This rotating disk transfers its angular momentum

(rotation) to the neutron star, speeding it up

• But some millisecond pulsars have been observed without companions…where did they go?– Another star passed by, gravitationally pulled

companion away?

– Companion evaporated by intense heat from pulsar

• The “black widow” pulsar theory• Neutron star cannibalizes the companion for it’s

mass, which generates heat that “boils away” the remains of the companion

NEXT TIMENEXT TIME

• Black Holes!– The most exotic/fascinating/misunderstood

objects in the universe