Astronomy 2010Astronomy 2010 11February 21, 2006February 21, 2006

Chapter 22:Chapter 22:The Death of StarsThe Death of Stars

What happens to old What happens to old stars?stars?

How does death differ How does death differ for small and large for small and large

stars?stars?

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Stage 8: Planetary Nebula Stage 8: Planetary Nebula or Supernova or Supernova

The outer layers are ejected as the core The outer layers are ejected as the core shrinks to its most compact state. shrinks to its most compact state. A large amount of mass is lost at this stage as A large amount of mass is lost at this stage as the outer layers are returned to the the outer layers are returned to the interstellar medium. interstellar medium. For the common low-mass stars (I.e with masses For the common low-mass stars (I.e with masses of 0.08 to 5 times the mass of the Sun during of 0.08 to 5 times the mass of the Sun during their main sequence stage), the increased their main sequence stage), the increased number of photons flowing outward from the number of photons flowing outward from the star's hot, compressed core will push on the star's hot, compressed core will push on the carbon and silicon grains that have formed in carbon and silicon grains that have formed in the star's cool outer layers to eject the outer the star's cool outer layers to eject the outer layers and form a layers and form a planetary nebulaplanetary nebula. .

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Stellar NucleosynthesisStellar NucleosynthesisH, He, some Li, Be, B produced during H, He, some Li, Be, B produced during the Big Bang. the Big Bang. Other elements produced in stars Other elements produced in stars through nuclear fusion.through nuclear fusion.When the outer layers of a star are When the outer layers of a star are thrown back into space, the new, heavy thrown back into space, the new, heavy elements can later form stars and elements can later form stars and planets. planets. Source for the stuff our Earth is made of. Source for the stuff our Earth is made of. All of the atoms on the Earth except All of the atoms on the Earth except hydrogen and most of the helium are hydrogen and most of the helium are recycled star material -- they were not recycled star material -- they were not created in the big bang. They were created created in the big bang. They were created in stars.in stars.

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Stellar Nucleosynthesis Stellar Nucleosynthesis (Cont’d)(Cont’d)

Atoms from helium to iron are made in Star cores. Atoms from helium to iron are made in Star cores.

Low mass stars can only synthesize helium. Low mass stars can only synthesize helium.

Stars similar to our Sun can synthesize He, C, O. Stars similar to our Sun can synthesize He, C, O.

Massive stars (Massive stars (MM** > 5 solar masses) can > 5 solar masses) can

synthesize He, C, O, Ne. Mg, Si, S, Ar, Ca, Ti, synthesize He, C, O, Ne. Mg, Si, S, Ar, Ca, Ti, Cr, Fe. Cr, Fe.

Elements heavier than iron are made in supernova Elements heavier than iron are made in supernova explosions from the combination of the abundant explosions from the combination of the abundant neutrons with heavy nuclei. neutrons with heavy nuclei.

Synthesized elements are dispersed into interstellar Synthesized elements are dispersed into interstellar medium by the supernova explosion. medium by the supernova explosion.

Elements later incorporated into giant molecular Elements later incorporated into giant molecular clouds.clouds.

Eventually become part of stars and planets.Eventually become part of stars and planets.

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Degenerate matterDegenerate matterWhen atoms become super-compressed, When atoms become super-compressed, particles bump right up against particles bump right up against each other to produce a kind of each other to produce a kind of gas, called a gas, called a degenerate gasdegenerate gas. . Normal gas exerts higher pressure Normal gas exerts higher pressure when it is heated and expands, but when it is heated and expands, but the pressure in a degenerate gas the pressure in a degenerate gas does not depend on the temperature. does not depend on the temperature. The laws of The laws of quantum mechanicsquantum mechanics must must be used for gases of ultra-high be used for gases of ultra-high densities. densities.

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Degenerate GasDegenerate GasOnly certain energies are permitted in a Only certain energies are permitted in a closely confined space. closely confined space.

The particles are arranged in energy levels like The particles are arranged in energy levels like rungs of an energy ladder. In ordinary gas, most of rungs of an energy ladder. In ordinary gas, most of the energy levels are unfilled and the particles are the energy levels are unfilled and the particles are free to move about. But in a degenerate gas, all of free to move about. But in a degenerate gas, all of the lower energy levels are filled.the lower energy levels are filled.

Only two particles can share the same energy Only two particles can share the same energy level in a given volume at one time. level in a given volume at one time.

For white dwarfs the degenerate particles are the For white dwarfs the degenerate particles are the electrons. For neutron stars the degenerate particles electrons. For neutron stars the degenerate particles are neutrons. are neutrons.

How close particles can be spaced depends How close particles can be spaced depends inverselyinversely on their masses. on their masses.

Electrons are spaced further apart in a degenerate Electrons are spaced further apart in a degenerate electron gas than the neutrons in a degenerate electron gas than the neutrons in a degenerate neutron gas because electrons are much less massive neutron gas because electrons are much less massive than neutrons. than neutrons.

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Consequences (1)Consequences (1)Degenerate gases Degenerate gases strongly resist compressionstrongly resist compression. .

Degenerate particles (electrons or neutrons) Degenerate particles (electrons or neutrons) locked into place because all of the lower locked into place because all of the lower energy shells are filled up. energy shells are filled up.

The only way they can move is to absorb enough The only way they can move is to absorb enough energy to get to the upper energy shells. energy to get to the upper energy shells.

This is This is hardhard to do! to do!

Compressing a degenerate gas requires a change Compressing a degenerate gas requires a change in the motions of the degenerate particle. But in the motions of the degenerate particle. But that requires A LOT of energy. that requires A LOT of energy.

Degenerate particles have no ``elbow room'' Degenerate particles have no ``elbow room'' and their jostling against each other strongly and their jostling against each other strongly resists compression. The degenerate gas is resists compression. The degenerate gas is like hardened steel!like hardened steel!

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Consequences (2)Consequences (2)The pressure in a degenerate gas The pressure in a degenerate gas depends only on the speed of the depends only on the speed of the degenerate particles NOT the degenerate particles NOT the temperature of the gas. temperature of the gas.

But to change the speed of degenerate But to change the speed of degenerate particles requires A LOT of energy particles requires A LOT of energy because they are locked into place because they are locked into place against each other.against each other.

Adding heat only causes the non-Adding heat only causes the non-degenerate particles to move faster, degenerate particles to move faster, but the degenerate ones supplying the but the degenerate ones supplying the pressure are unaffected. pressure are unaffected.

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Consequences (3)Consequences (3)Increasing the mass of the stellar Increasing the mass of the stellar core increases the compression of core increases the compression of the core. the core. The degenerate particles are forced The degenerate particles are forced closer together, but not much closer together, but not much closer together because there is no closer together because there is no room left. room left. A more massive stellar core remnant A more massive stellar core remnant will be will be smallersmaller than a lighter core than a lighter core remnant. remnant. This is the opposite behavior of This is the opposite behavior of regular materials: usually adding mass regular materials: usually adding mass to something makes it bigger! to something makes it bigger!

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White DwarfsWhite DwarfsForm as the outer layers of aForm as the outer layers of alow-mass red giant star pufflow-mass red giant star puffout to make a planetary nebula. out to make a planetary nebula. Since the lower mass stars make the Since the lower mass stars make the white dwarfs, this type of remnant white dwarfs, this type of remnant is the most common endpoint for is the most common endpoint for stellar evolution. stellar evolution. If the remaining mass of the core is If the remaining mass of the core is less than 1.4 solar masses, the less than 1.4 solar masses, the pressure from the degenerate pressure from the degenerate electrons (called electrons (called electron electron degeneracy pressuredegeneracy pressure) is enough to ) is enough to prevent further collapse. prevent further collapse.

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White Dwarfs DensityWhite Dwarfs DensityBecause the core has about the mass of Because the core has about the mass of the Sun compressed to something the size the Sun compressed to something the size of the Earth, the density is tremendous: of the Earth, the density is tremendous: around 10around 1066 times denser than water (one times denser than water (one sugarcube volume's worth of white dwarf sugarcube volume's worth of white dwarf gas has a mass > 1 car)! gas has a mass > 1 car)! A higher mass core is compressed to a A higher mass core is compressed to a smaller radius so the densities are even smaller radius so the densities are even higher. higher. Despite the huge densities and the Despite the huge densities and the ``stiff'' electrons, the neutrons and ``stiff'' electrons, the neutrons and protons have room to move around protons have room to move around freely---they are not degenerate. freely---they are not degenerate.

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Radius of a White DwarfRadius of a White Dwarf

Adding more mass causes the radius to decrease!

At about 1.4 solar masses, the size becomes zero!

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White Dwarf CoolingWhite Dwarf CoolingWhite dwarfs shine simply from the release White dwarfs shine simply from the release of the heat left over from when the star was of the heat left over from when the star was still producing energy from nuclear still producing energy from nuclear reactions. reactions.

There are no more nuclear reactions There are no more nuclear reactions occurring so the white dwarf cools off from occurring so the white dwarf cools off from an initial temperature of about 100,000 K. an initial temperature of about 100,000 K.

The white dwarf loses heat quickly at first The white dwarf loses heat quickly at first cooling off to 20,000 K in only about 100 cooling off to 20,000 K in only about 100 million years, but then the cooling rate million years, but then the cooling rate slows down: it takes about another 800 slows down: it takes about another 800 million years to cool down to 10,000 K and million years to cool down to 10,000 K and another 4 to 5 billion years to cool down to another 4 to 5 billion years to cool down to the Sun's temperature of 5,800 K. the Sun's temperature of 5,800 K.

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From Giant to White DwarfFrom Giant to White Dwarf

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White Dwarf White Dwarf Cooling (2)Cooling (2)

Their rate of cooling and the distribution of their Their rate of cooling and the distribution of their current temperatures can be used to determine the current temperatures can be used to determine the age of our galaxy or old star clusters that have age of our galaxy or old star clusters that have white dwarfs in them. white dwarfs in them.

However, their small size makes them extremely difficult to However, their small size makes them extremely difficult to detect. detect.

The HST can detect these small dead stars in nearby The HST can detect these small dead stars in nearby old star clusters called old star clusters called globular clusters.globular clusters. Analysis of the white dwarfs provides an independent Analysis of the white dwarfs provides an independent way of measuring the ages of the globular clusters way of measuring the ages of the globular clusters and provide a verification of their very old ages and provide a verification of their very old ages derived from main sequence fitting. derived from main sequence fitting.

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Death of Massive StarsDeath of Massive StarsRare high-mass stars (masses of 5 - 50 Rare high-mass stars (masses of 5 - 50 times the Sun's mass in main sequence times the Sun's mass in main sequence stage) end their life in a different way. stage) end their life in a different way.

When a massive star's iron core implodes, When a massive star's iron core implodes, the the protonsprotons and and electronselectrons fuse together fuse together to form to form neutronsneutrons and and neutrinosneutrinos. .

The core, once the size of the Earth, The core, once the size of the Earth, becomes a very stiff neutron star about becomes a very stiff neutron star about the size of a small town in less than a the size of a small town in less than a second. second.

The in falling outer layers hit the core The in falling outer layers hit the core and heat up to billions of degrees from and heat up to billions of degrees from the impact. the impact.

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Death of Massive Stars - Death of Massive Stars - SupernovaSupernova

Enough of the huge number of neutrinos Enough of the huge number of neutrinos produced when the core collapses produced when the core collapses interact with the gas in outer layers, interact with the gas in outer layers, helping to heat it up. helping to heat it up.

During the supernova outburst, elements During the supernova outburst, elements heavier than iron are produced as free heavier than iron are produced as free neutrons produced in the explosion neutrons produced in the explosion rapidly combine with heavy nuclei to rapidly combine with heavy nuclei to produce heavier and very rare nuclei produce heavier and very rare nuclei like gold, platinum, uranium among like gold, platinum, uranium among others. others.

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Supernova ExplosionSupernova ExplosionThe superheated gas is blasted into space The superheated gas is blasted into space carrying a lot of the heavy elements carrying a lot of the heavy elements produced in the stellar nucleosynthesis produced in the stellar nucleosynthesis process. process.

This explosion is a This explosion is a supernovasupernova. .

Expanding gas crashes into the surrounding Expanding gas crashes into the surrounding interstellar gas at thousands of interstellar gas at thousands of kilometers/kilometers/secondsecond, ,

the shock wave heats up the interstellar gas to the shock wave heats up the interstellar gas to very temperatures and it glows. very temperatures and it glows.

Strong emission lines of neutral oxygen and Strong emission lines of neutral oxygen and ionized sulfur distinguish their spectra ionized sulfur distinguish their spectra from planetary nebulae and H II regions. from planetary nebulae and H II regions.

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Supernova Explosion Supernova Explosion (cont’d)(cont’d)

Also, the ratio of the strengths of the Also, the ratio of the strengths of the individual doubly-ionized oxygen is that individual doubly-ionized oxygen is that expected from shock-wave heating.expected from shock-wave heating.Planetary nebulae and H II regions are Planetary nebulae and H II regions are lit up by the action of ultraviolet lit up by the action of ultraviolet light on the gas, while supernova glow light on the gas, while supernova glow from shock-wave heating. from shock-wave heating. Gas from supernova explosions also has Gas from supernova explosions also has strong radio emission with a non-thermal strong radio emission with a non-thermal continuous spectrum that is produced by continuous spectrum that is produced by electrons spiraling around magnetic electrons spiraling around magnetic field lines. field lines. Gas from recent explosions (within a few Gas from recent explosions (within a few thousand years ago) are visible with X-thousand years ago) are visible with X-ray telescopes as well. ray telescopes as well.

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Crab NebulaCrab NebulaA famous A famous supernova supernova remnant is remnant is the Crab the Crab Nebula.Nebula.Chinese Chinese astronomers astronomers recorded the recorded the explosion on explosion on July 4, 1054 July 4, 1054 Anasazi Anasazi Indians Indians painted a painted a picture of picture of it. it.

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Vela Vela SupernovaSupernovaOccurred long before the Occurred long before the

Crab NebulaCrab Nebula

Much more spread out. Much more spread out.

Parts have run into Parts have run into regions regions of the interstellar medium of the interstellar medium

of different densities.of different densities.

For that reason and For that reason and because because of turbulence in expanding of turbulence in expanding

supernova gas, the remnant supernova gas, the remnant

seen today is wispy seen today is wispy strands strands of glowing gas. of glowing gas.

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Supernova OutputSupernova OutputNNeutrinoseutrinos formed when the neutron core formed when the neutron core is created fly away from the stiff is created fly away from the stiff core, carrying most of the energy from core, carrying most of the energy from the core collapse away with them. the core collapse away with them.

Some energy goes into driving the gas Some energy goes into driving the gas envelope outward.envelope outward.

The rest of the energy goes into making The rest of the energy goes into making

the supernova as bright asthe supernova as bright as 10101111 Suns Suns

as bright as an entire as bright as an entire galaxy! galaxy!

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SN 1987aSN 1987a Supernova Supernova occurred in occurred in satellite satellite galaxy of the galaxy of the Milky Way at Milky Way at beginning of beginning of 1987 1987

Called SN1987a.Called SN1987a.

Kamiokande Kamiokande detector detector (Japan) saw a (Japan) saw a burst of burst of neutrinos. neutrinos.

Confirmation of Confirmation of supernova supernova models. models.

after before

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HST Images of SN1987aHST Images of SN1987aThe material from The material from the explosion is the explosion is expanding outward expanding outward at over 9.5 at over 9.5 million km/hr million km/hr preferentially preferentially into two lobes into two lobes that are roughly that are roughly aligned with the aligned with the bright central bright central ring. ring. Central bright Central bright ring and two outer ring and two outer rings are from rings are from material ejected material ejected by the star before by the star before its death. its death.

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Supernova Rate in the Supernova Rate in the UniverseUniverseSupernovae are very rareSupernovae are very rare

about one every hundred years in any given about one every hundred years in any given galaxygalaxybecause the stars that produce them are rare. because the stars that produce them are rare.

But… there are billions of galaxies in But… there are billions of galaxies in the universe, the universe,

simple probability says that there should be simple probability says that there should be a few supernovae happening a few supernovae happening somewhere in the somewhere in the universeuniverse during a year and that is what is during a year and that is what is seen! seen!

Because supernovae are so luminous and Because supernovae are so luminous and the energy is concentrated in a small the energy is concentrated in a small area, they stand out and can be seen from area, they stand out and can be seen from hundreds of millions of light years away. hundreds of millions of light years away.

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Stage 9: Core RemnantStage 9: Core RemnantCore mass < 1.4 solar masses,Core mass < 1.4 solar masses,

Star core shrinks down to a Star core shrinks down to a white dwarfwhite dwarf the the size of the Earth. size of the Earth.

Core 1.4 < mass <3 solar masses,Core 1.4 < mass <3 solar masses,Neutrons bump up against each other to form a Neutrons bump up against each other to form a degenerate gas.degenerate gas.Forms a Forms a neutron starneutron star about the size of small about the size of small city. city. Neutrons prevent further collapse of the core. Neutrons prevent further collapse of the core.

Core > 3 solar masses : Core > 3 solar masses : Complete collapseComplete collapseAs it collapses, it may momentarily create a As it collapses, it may momentarily create a neutron star and the resulting supernova neutron star and the resulting supernova rebound explosion. rebound explosion. Gravity finally wins. Nothing holds it up. Gravity finally wins. Nothing holds it up. Becomes a black holeBecomes a black hole

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Novae and Supernovae Type Novae and Supernovae Type IIAn isolated white dwarf has a boring future: An isolated white dwarf has a boring future:

it simply cools off, dimming to invisibility. it simply cools off, dimming to invisibility.

White dwarfs in binary systems where the White dwarfs in binary systems where the companion is still a main sequence or red companion is still a main sequence or red giant star can have more interesting futures. giant star can have more interesting futures.

If the white dwarf is close enough to its red If the white dwarf is close enough to its red giant or main sequence companion, gas giant or main sequence companion, gas expelled by the star can fall onto the white expelled by the star can fall onto the white dwarf. dwarf.

The hydrogen-rich gas from the star's outer The hydrogen-rich gas from the star's outer layers builds up on the white dwarf's surface layers builds up on the white dwarf's surface and gets compressed and hot by the white and gets compressed and hot by the white dwarf's gravity. dwarf's gravity.

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NovaeNovaeEventually the hydrogen gas gets dense Eventually the hydrogen gas gets dense and hot enough for nuclear reactions to and hot enough for nuclear reactions to start. The reactions occur at an start. The reactions occur at an explosive rate. explosive rate. The hydrogen gas is blasted outward to The hydrogen gas is blasted outward to form an expanding shell of hot gas. form an expanding shell of hot gas. The hot gas shell produces a lot of light The hot gas shell produces a lot of light suddenly. suddenly. From the Earth, it looks like a new star From the Earth, it looks like a new star has appeared in our sky. has appeared in our sky. Early astronomers called them Early astronomers called them novaenovae (``new'' in Latin).(``new'' in Latin).They are now known to be caused by old, They are now known to be caused by old, dead stars.dead stars.

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NovaeNovaeThe spectra of novae show blue-shifted The spectra of novae show blue-shifted absorption lines from hot dense gas expelled absorption lines from hot dense gas expelled towards us at a few thousands of kilometers per towards us at a few thousands of kilometers per second. second.

The continuum is from the hot dense gas and the The continuum is from the hot dense gas and the absorption lines are from the lower-density absorption lines are from the lower-density surface of the expanding cloud. surface of the expanding cloud.

After a few days the gas has expanded and After a few days the gas has expanded and thinned out enough to just produce blue-shifted thinned out enough to just produce blue-shifted emission lines. emission lines.

After a nova burst, gas from the regular star After a nova burst, gas from the regular star begins to build up again on the white dwarf's begins to build up again on the white dwarf's surface. surface.

A binary system can have repeating nova bursts.A binary system can have repeating nova bursts.

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Type Ia SupernovaeType Ia SupernovaeIf enough mass accumulates on the white If enough mass accumulates on the white dwarf to push it over the 1.4 solar mass dwarf to push it over the 1.4 solar mass limit, the degenerate electrons will not limit, the degenerate electrons will not be able to stop gravity from collapsing be able to stop gravity from collapsing the dead core. the dead core.

The collapse is sudden and heats the The collapse is sudden and heats the carbon and oxygen nuclei left from the carbon and oxygen nuclei left from the dead star's red giant phase to dead star's red giant phase to temperatures great enough for nuclear temperatures great enough for nuclear fusion. fusion. The carbon and oxygen quickly fuse to form The carbon and oxygen quickly fuse to form silicon nuclei. silicon nuclei.

The silicon nuclei fuse to create nickel The silicon nuclei fuse to create nickel nuclei. nuclei.

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Type Ia SupernovaeType Ia SupernovaeA huge amount of energy is released very A huge amount of energy is released very quickly with such power that the white dwarf quickly with such power that the white dwarf blows itself apartblows itself apart. .

This explosion is called a This explosion is called a Type Ia supernovaType Ia supernova to distinguish it from the supernovae to distinguish it from the supernovae (called type II supernovae) that occur when (called type II supernovae) that occur when a massive star's iron core implodes to form a massive star's iron core implodes to form a neutron star or black hole. a neutron star or black hole.

Type Ia supernovaeType Ia supernovae are several times are several times brighter than brighter than Type II supernovaeType II supernovae..

Tycho’s supernova was a type Ia.Tycho’s supernova was a type Ia.

Type Ia supernovae are used as “standard Type Ia supernovae are used as “standard candles”.candles”.

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Tycho’s Supernova and Tycho’s Supernova and CompanionCompanion

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Neutron StarsNeutron StarsIf the core mass is between 1.4 and 3 solar If the core mass is between 1.4 and 3 solar masses, the compression from the star's masses, the compression from the star's gravity will be so great the protons fuse gravity will be so great the protons fuse with the electrons to form neutrons. with the electrons to form neutrons.

The core becomes a super-dense ball of The core becomes a super-dense ball of neutrons. neutrons.

Only the rare, massive stars will form Only the rare, massive stars will form these remnants in a supernova explosion. these remnants in a supernova explosion.

Neutrons can be packed much closer together Neutrons can be packed much closer together than electrons so even though a neutron than electrons so even though a neutron star is more massive than a white dwarf, it star is more massive than a white dwarf, it is only about the size of a city. is only about the size of a city.

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Neutron StarsNeutron StarsThe neutrons are degenerate and The neutrons are degenerate and their pressure (called their pressure (called neutron neutron degeneracy pressuredegeneracy pressure) prevents ) prevents further collapse. further collapse. Neutron stars are about 30 kilometers Neutron stars are about 30 kilometers across, so their densities are much larger across, so their densities are much larger than even the incredible densities of white than even the incredible densities of white dwarfs: 2 x 10dwarfs: 2 x 101414 times the density of water. times the density of water.Recently, the Hubble Space Telescope was Recently, the Hubble Space Telescope was able to image one of these very small able to image one of these very small objects. objects. Even though it is over 660,000 K, the Even though it is over 660,000 K, the neutron starneutron star is close to the limit of HST's is close to the limit of HST's detectors because it is at most 27 detectors because it is at most 27 kilometers across.kilometers across.

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PulsarsPulsarsIn the late 1960's astronomers discovered In the late 1960's astronomers discovered radio sources that pulsated very regularly radio sources that pulsated very regularly with periods of just fractions of a second to with periods of just fractions of a second to a few seconds.a few seconds.

The periods are extremely regular---only the The periods are extremely regular---only the ultra-high precision of atomic clocks can show ultra-high precision of atomic clocks can show a very slight lengthening in the period. a very slight lengthening in the period.

At first, some thought they were picking up At first, some thought they were picking up signals from extra-terrestrial intelligent signals from extra-terrestrial intelligent civilizations. civilizations.

The discovery of several more pulsars The discovery of several more pulsars discounted that idea---they are a natural discounted that idea---they are a natural phenomenon called phenomenon called pulsarspulsars (short for (short for “pulsating star”). “pulsating star”). Vela pulsar

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Pulsars (2)Pulsars (2)Normal variable stars (stars near the end of Normal variable stars (stars near the end of their life in stages 5 to 7) oscillate in their life in stages 5 to 7) oscillate in brightness by changing their size and brightness by changing their size and temperature. temperature.

The density of the star determines the The density of the star determines the pulsation period--denser stars pulsate more pulsation period--denser stars pulsate more quickly than low density variables.quickly than low density variables.

However, normal stars and white dwarfs are However, normal stars and white dwarfs are not dense enough to pulsate at rates of not dense enough to pulsate at rates of under one second.under one second.

Neutron stars would pulsate too quickly Neutron stars would pulsate too quickly because of their huge density, so pulsars because of their huge density, so pulsars must pulsate by a different way than normal must pulsate by a different way than normal variable stars. variable stars.

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Pulsars (3)Pulsars (3)A rapidly rotating object with a A rapidly rotating object with a bright spot on it could produce bright spot on it could produce the quick flashes if the bright the quick flashes if the bright spot was lined up with the Earth. spot was lined up with the Earth.

Normal stars and white dwarfs cannot rotate fast Normal stars and white dwarfs cannot rotate fast enough because they do not have enough gravity enough because they do not have enough gravity to keep themselves together; they would spin to keep themselves together; they would spin themselves apart. themselves apart.

Neutron stars are compact enough and strong Neutron stars are compact enough and strong enough to rotate that fast. The enough to rotate that fast. The pulsar at the center of the Crab Nebulapulsar at the center of the Crab Nebula rotates rotates 30 times every second. 30 times every second.

In the figure it is the left one of the two In the figure it is the left one of the two bright stars at the center of the HST imagebright stars at the center of the HST imageCrab pulsar

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Pulsars (4)Pulsars (4)Another clue comes from the length of each Another clue comes from the length of each pulse itself. pulse itself. Each pulse lasts about 1/1000th of a second Each pulse lasts about 1/1000th of a second (the time (the time betweenbetween pulses is the period pulses is the period mentioned above). mentioned above). An important principle in science is that An important principle in science is that an object cannot change its brightness an object cannot change its brightness faster than it takes light to cross its faster than it takes light to cross its diameter. diameter.

Even if the object could magically brighten Even if the object could magically brighten everywhere simultaneously, it would take light everywhere simultaneously, it would take light from the far side of the object longer to reach from the far side of the object longer to reach you than the near side. you than the near side.

Fastest known pulsar B1937

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Pulsars (5)Pulsars (5)Observed change in brightness to be Observed change in brightness to be smeared out over a time interval equal smeared out over a time interval equal to the time it would take the light to the time it would take the light from the far side of the object to from the far side of the object to travel to the near side of the object. travel to the near side of the object. If the object did not brighten If the object did not brighten everywhere simultaneously, then a everywhere simultaneously, then a smallersmaller object could produce a pulse object could produce a pulse in the same interval. The brightness in the same interval. The brightness fluctuation timescale gives the fluctuation timescale gives the maximummaximum size of an object. size of an object.

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Pulsar SizePulsar SizeThe 1/1000th of second burst of energy means The 1/1000th of second burst of energy means that the pulsars are that the pulsars are at mostat most (300,000 (300,000 kilometers/second) kilometers/second) ×× (1/1000 second) = 300 (1/1000 second) = 300 kilometers across. kilometers across. This is too small for normal stars or white This is too small for normal stars or white dwarfs, but fine for neutron stars. dwarfs, but fine for neutron stars. When neutron stars form they will be spinning When neutron stars form they will be spinning rapidly and have very STRONG magnetic fields rapidly and have very STRONG magnetic fields (10(1099 to 10 to 101212 times the Sun's). times the Sun's).The magnetic field is the relic magnetic field The magnetic field is the relic magnetic field from the star's previous life stages. from the star's previous life stages. The magnetic field is frozen into the star, so The magnetic field is frozen into the star, so when the core collapses, the magnetic field is when the core collapses, the magnetic field is compressed too. compressed too. The magnetic field becomes very concentrated The magnetic field becomes very concentrated and much stronger than before.and much stronger than before.

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SummarySummaryInit. Init. MassMass

(M(Msunsun))

Final Final MassMass

(M(Msunsun))

Final dispositionFinal disposition

< 0.01< 0.01 < 0.01< 0.01 PlanetPlanet

0.01 to 0.01 to 0.080.08

0.01 to 0.01 to 0.080.08

Brown dwarf (H and Brown dwarf (H and He)He)

0.08 to 0.08 to 0.250.25

White dwarf, mostly White dwarf, mostly HeHe

0.25 to 0.25 to 1010

< 1.4< 1.4 White dwarf, mostly White dwarf, mostly C & OC & O

10 to 1210 to 12 < 1.4< 1.4 White dwarf, O, Ne, White dwarf, O, Ne, MgMg

12 to 4012 to 40 < 3< 3 Supernova Supernova neutron neutron starstar

> 40> 40 > 3> 3 Supernova Supernova black black holehole