Chapter 13Chapter 13The Bizarre Stellar GraveyardThe Bizarre Stellar Graveyard

White Dwarfs...White Dwarfs...

...are stellar remnants for ...are stellar remnants for low-masslow-mass stars. stars.

...are found in the centers of ...are found in the centers of planetary nebulaplanetary nebula..

...have diameters about the same as the Earth’s....have diameters about the same as the Earth’s.

...have masses less than the ...have masses less than the Chandrasekhar massChandrasekhar mass..

Sirius B is a white dwarf starSirius B is a white dwarf star

Sirius A And Sirius B In X-ray

Sirius B

Sirius A

Novas and SupernovasNovas and Supernovas NovaNova - a stellar explosion - a stellar explosion

SupernovaSupernova - a stellar explosion that marks the end - a stellar explosion that marks the end of a star’s evolutionof a star’s evolution

White Dwarf SupernovaWhite Dwarf Supernova (Type I supernova)- (Type I supernova)- occur in binary systems in which one is a white occur in binary systems in which one is a white dwarfdwarf

Massive Star Supernova (Type II Supernova)Massive Star Supernova (Type II Supernova) - - occur when a massive star’s iron core collapsesoccur when a massive star’s iron core collapses

Close Binary Systems and Mass TransferClose Binary Systems and Mass Transfer

March 1935 May 1935

Nova HerculisNova Herculis

Diagram of nova process

A nova occurs when hydrogen fusion ignites on the surface of a white dwarf star system

Nova T Pyxidis(HST)

Light Curve of typical NovaLight Curve of typical Nova

Semidetached Binary System With White Dwarf Star Semidetached Binary System With White Dwarf Star (may result in a white dwarf (type I ) supernova)(may result in a white dwarf (type I ) supernova)

Type II SupernovaType II Supernova The star releases more energy in a just a few minutes The star releases more energy in a just a few minutes

than it did during its entire lifetime.than it did during its entire lifetime.» Example: SN 1987AExample: SN 1987A

After the explosion of a massive star, a huge glowing After the explosion of a massive star, a huge glowing cloud of stellar debris - a cloud of stellar debris - a supernova remnantsupernova remnant - steadily - steadily expands.expands.

» Example: Crab NebulaExample: Crab Nebula

After a supernova the exposed core is seen as a After a supernova the exposed core is seen as a neutron neutron starstar - or if the star is more than 3 solar masses the core - or if the star is more than 3 solar masses the core becomes a becomes a black holeblack hole..

The remnant of this explosion is The remnant of this explosion is The Crab Nebula The Crab Nebula

On July 4, 1054 astronomers in China On July 4, 1054 astronomers in China witnessed a supernova within our own galaxy.witnessed a supernova within our own galaxy.

Supernova 1987aSupernova 1987a

Type I and Type II SupernovaType I and Type II Supernova

Supernova Light CurvesSupernova Light Curves

Hydrogen and Helium BurningHydrogen and Helium Burning

positronsneutrinosenergyHeH 22)(4 41

energyCHe 124 )(3

Carbon Burning and Helium CaptureCarbon Burning and Helium Capture

energyOHeC 16412

energyMgCC 241212

Still heavier elements are created in the Still heavier elements are created in the final stages of life of massive starsfinal stages of life of massive stars

energyNiHeSi 56428 )(7

Alpha ProcessAlpha Process – – Helium Capture Helium Capture produces heavier produces heavier elements up to elements up to Fe and Ni.Fe and Ni.

Elements beyond Fe and Ni involve neutron capture. Elements beyond Fe and Ni involve neutron capture.

FenFe 5857

FenFe 5958

Formation of Elements beyond Iron occurs very Formation of Elements beyond Iron occurs very rapidly as the star approaches supernova.rapidly as the star approaches supernova.

This forms unstable nuclei which then decay into This forms unstable nuclei which then decay into stable nuclei of other elementsstable nuclei of other elements

The supernova explosion then distributes The supernova explosion then distributes the newly formed matter throughout the the newly formed matter throughout the interstellar space interstellar space (space between the stars). (space between the stars).

This new matter goes into the formation This new matter goes into the formation of interstellar debris.of interstellar debris.

The remnant core is a dense solid core of The remnant core is a dense solid core of neutrons – a neutron star! neutrons – a neutron star!

Neutron StarsNeutron Stars ...are stellar remnants for high-mass stars....are stellar remnants for high-mass stars.

...are found in the centers of some type II ...are found in the centers of some type II supernova remnants.supernova remnants.

...have diameters of about 6 miles....have diameters of about 6 miles.

...have masses greater than the ...have masses greater than the Chandrasekhar Chandrasekhar massmass. (1.4M. (1.4M)

Relative SizesRelative Sizes

Earth White Dwarf Neutron Star

PulsarsPulsars The first pulsar observed was originally thought to be signals The first pulsar observed was originally thought to be signals

from extraterrestrials. from extraterrestrials. (LGM-Little Green Men was their first designation)(LGM-Little Green Men was their first designation)

~ 20 seconds of Jocelyn Bell’s data- the first pulsar discovered

Period = 1.337301 seconds exact!

It was later shown to be unlikely that the It was later shown to be unlikely that the pulsar signal originated from pulsar signal originated from extraterrestrial intelligence after many extraterrestrial intelligence after many other pulsars were found all over the sky. other pulsars were found all over the sky.

PulsarsPulsars

The pulsing star inside the Crab Nebula The pulsing star inside the Crab Nebula was a pulsar.was a pulsar.

Pulsars are rotating, magnetized neutron Pulsars are rotating, magnetized neutron stars.stars.

The Crab NebulaThe Crab Nebula

The Crab PulsarThe Crab Pulsar

Period = 0.033 seconds = 33 milliseconds

Light House ModelLight House Model

– Beams of radiation emanate from the Beams of radiation emanate from the magnetic poles.magnetic poles.

– As the neutron star rotates, the beams As the neutron star rotates, the beams sweep around the sky.sweep around the sky.

– If the Earth happens to lie in the path of If the Earth happens to lie in the path of the beams, we see a pulsar.the beams, we see a pulsar.

Rotating Rotating Neutron StarNeutron Star

Light House model of neutron star emission accounts for many properties of observed Pulsars

Artistic rendering of the light house model

Rotation Rates of PulsarsRotation Rates of Pulsars The neutron stars that appear to us as pulsars The neutron stars that appear to us as pulsars

rotate about once every second or less. rotate about once every second or less.

Before a star collapses to a neutron star it Before a star collapses to a neutron star it probably rotates about once every 25 days.probably rotates about once every 25 days.

Why is there such a big change in rotation rate?Why is there such a big change in rotation rate?

Answer: Answer: Conservation of Angular MomentumConservation of Angular Momentum

Neutron –Star Binaries

Mass LimitsMass Limits

Low mass starsLow mass stars– Less than 8 MLess than 8 M on Main Sequence on Main Sequence

– Become White Dwarf (< 1.4 MBecome White Dwarf (< 1.4 M))» Electron Degeneracy PressureElectron Degeneracy Pressure

High Mass StarsHigh Mass Stars– Less than 100 MLess than 100 M on Main Sequence on Main Sequence

– Become Neutron Stars (1.4MBecome Neutron Stars (1.4M < M < 3M < M < 3M))» Neutron Degeneracy Pressure Neutron Degeneracy Pressure

Black HolesBlack Holes

...are stellar remnants for ...are stellar remnants for high-masshigh-mass stars. stars.– i.e. remnant cores with masses greater than 3 solar massesi.e. remnant cores with masses greater than 3 solar masses

……have a gravitational attraction that is so strong that have a gravitational attraction that is so strong that light cannot escapelight cannot escape from it. from it.

……are found in some are found in some binary star systemsbinary star systems and there may and there may be super-massive black holes in the centers of some be super-massive black holes in the centers of some galaxiesgalaxies..

Supermassive StarsSupermassive Stars

If the stellar core has more than three solar If the stellar core has more than three solar masses after supernova, then no known masses after supernova, then no known force can halt the collapseforce can halt the collapse

Black Hole

Black holes were first predicted by the General Theory of Relativity, which is theory of gravity that corrects for some of the short-falls of Newton’s Theory of Gravity.

In general Relativity, space, time In general Relativity, space, time and mass are all interconnectedand mass are all interconnected

Space-TimeSpace-Time

No mass

Distortion caused by

mass

Predictions of General RelativityPredictions of General Relativity

Advance of Mercury’s perihelionAdvance of Mercury’s perihelion Bending of starlightBending of starlight

Advance of Mercury’s PerihelionAdvance of Mercury’s Perihelion

43” per century not due to perturbations from other planets

Bending of StarlightBending of Starlight

Sun

Light from star bent by the gravity of the Sun

Apparent position of the star

1.75”

Schwarzschild Black HoleSchwarzschild Black Hole

Rs

Singularity+

Event Horizon

Rs = 3(Mass)

Mass Rs

3 M 9 km

5 15

10 30

Near a Black HoleNear a Black Hole

What Can We Know?What Can We Know?

MassMass– gravitygravity

ChargeCharge– Electric FieldsElectric Fields

Rotation RateRotation Rate– Co-rotationCo-rotation

How Can We Find Them?How Can We Find Them?

Look for X-ray sourcesLook for X-ray sources– Must come from compact sourceMust come from compact source

» White DwarfWhite Dwarf

» Neutron StarNeutron Star

» Black HoleBlack Hole

– Differentiate by MassDifferentiate by Mass» WD - < 1.4 MWD - < 1.4 M

» NS - between 1.4 and 3 MNS - between 1.4 and 3 M

» BH - > 3 MBH - > 3 M

Cygnus X-1Cygnus X-1

End of ChaptersEnd of Chapters

End of Section.End of Section.

NucleosynthesisNucleosynthesis

Fused Products Time TemperatureH 4He 107 yrs. 4 X 106 K

4He 12C Few X 106 yrs 1 X 108 K12C 16O, 20Ne,

24Mg, 4He1000 yrs. 6 X 108 K

20Ne + 16O, 24Mg Few yrs. 1 X 109 K16O 28Si, 32S One year 2 X 109 K

28Si + 56Fe Days 3 X 109 K56Fe Neutrons < 1 second > 3 X 109 K

Evolutionary Time Scales for a 15 M Star

Energy BudgetEnergy Budget

Ene

rgy

Fusion Stages

H He C Fe

Anazasi PictographsAnazasi Pictographs

Supernova 1998S inSupernova 1998S inNGC 3877NGC 3877

Supernova RemnantsSupernova Remnants

Tycho’s SNR - 1572

PSR 0628-28PSR 0628-28

LGM?LGM?

Several more found at widely different Several more found at widely different places in the galaxyplaces in the galaxy

Power of a power equals total power Power of a power equals total power potential output of the Earthpotential output of the Earth

No Doppler shiftsNo Doppler shifts

PULSARS

Light Time ArgumentLight Time Argument

An object which varies its light can be no An object which varies its light can be no larger than the distance light can travel in larger than the distance light can travel in the shortest period of variation.the shortest period of variation.

To Darken the SunTo Darken the Sun

Time Delay = Radius/c

500,000 km/300,000 km/s = 1.67 sec

Only candidates: White Dwarfs, Neutron Stars

Pulse MechanismsPulse Mechanisms

Binary Stars - How quickly can two stars orbit?Binary Stars - How quickly can two stars orbit? Two WD about 1Two WD about 1mm

Two NS about 1Two NS about 1s.s.

Neutron Stars in orbit should emit gravity waves which should Neutron Stars in orbit should emit gravity waves which should be detectable.be detectable.

Oscillations - Depends only on densityOscillations - Depends only on density WD about ten secondsWD about ten seconds NS about .001NS about .001ss Little variation permitted. Little variation permitted.

Rotation - Until the object begins to break up.Rotation - Until the object begins to break up. WD about 1WD about 1ss

NS about .001NS about .001ss with large variation. with large variation.

SS 433SS 433

Synchrotron RadiationSynchrotron Radiation

Magnetic lines of force

Electron

Radiation

GlitchesGlitches