12 april 2005ast 2010: chapter 221 the death of stars

12 April 2005 AST 2010: Chapter 22 1

The Death of The Death of StarsStars


Stellar QuestionsStellar Questions• What happens to old stars?• How does death differ for small and

large stars?


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

• The outer layers are ejected as the core shrinks to its most compact state

• A large amount of mass is lost at this stage as the outer layers are returned to the interstellar medium

• For the common low-mass stars (with masses of 0.08 to 5 times the mass of the Sun during their main sequence stage), the increased number of photons flowing outward from the star's hot, compressed core will push on the carbon and silicon grains that have formed in the star's cool outer layers to eject the outer layers and form a planetary nebula


Stellar NucleosynthesisStellar Nucleosynthesis• Helium and heavier elements produced in

stars through nuclear fusion• When the outer layers of a star are thrown

back into space, the processed material can be incorporated into gas clouds that will later form stars and planets– Source for the stuff our Earth is made of– All of the atoms on the Earth except

hydrogen and most of the helium are recycled star material -- they were created in stars


Stellar Nucleosynthesis Stellar Nucleosynthesis (Cont’d)(Cont’d)• Atoms heavier than He up to the iron atoms made in

Star cores• Low mass stars can only synthesize helium• Stars similar to our Sun can synthesize He, C, O

• Massive stars (M* > 5 solar masses) can synthesize He, C, O, Ne. Mg, Si, S, Ar, Ca, Ti, Cr, Fe

• Elements heavier than iron are made in supernova explosions from the combination of the abundant neutrons with heavy nuclei– Synthesized elements are dispersed into interstellar medium

by the supernova explosion. – Elements later incorporated into giant molecular clouds.– Eventually become part of stars and planets.


ConsequencesConsequences• Increasing the mass of the stellar core

increases the compression of the core• The degenerate particles are forced

closer together, but not much closer together because there is no room left

• A more massive stellar core remnant will be smaller than a lighter core remnant– This is the opposite behavior of

regular materials: usually adding mass to something makes it bigger!


White DwarfsWhite Dwarfs• They form as the outer layers of

a low-mass red-giant star puff out to make a planetary nebula

• Since the lower-mass stars make the white dwarfs, this type of remnant is the most common endpoint for stellar evolution

• If the remaining mass of the core is less than 1.4 solar masses, the pressure from the degenerate electrons (called electron degeneracy pressure) is enough to prevent further collapse


White Dwarfs DensityWhite Dwarfs Density• Because the core has about the mass of the

Sun compressed to something the size of the Earth, the density is tremendous– around 106 times denser than water (one sugar-

cube volume's worth of white dwarf gas has a mass > 1 car)!

• A higher-mass core is compressed to a smaller radius so the densities are even higher

• Despite the huge densities and the “stiff” electrons, the neutrons and protons have room to move around freely---they are not degenerate


Radius of a White DwarfRadius of a White Dwarf


White Dwarf’s Cooling (1)White Dwarf’s Cooling (1)• White dwarfs shine simply from the release of

the heat left over from when the star was still producing energy from nuclear reactions

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

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


From Giant to White From Giant to White DwarfDwarf


White Dwarfs White Dwarfs Cooling (2)Cooling (2)

• Their rate of cooling and the distribution of their current temperatures can be used to determine the age of our galaxy or old star clusters that have white dwarfs in them– However, their small size makes them extremely difficult to

detect

• Because it is above the atmosphere, the HST can detect these small dead stars in nearby old star clusters called globular clusters

• Analysis of the white dwarfs may provide an independent way of measuring the ages of the globular clusters and provide a verification of their very old ages derived from main sequence fitting


Death of Massive StarsDeath of Massive Stars• Rare high-mass stars (masses of 5 - 50 times

the Sun's mass in main sequence stage) end their life in a different way

• When a massive star's iron core implodes, the protons and electrons fuse together to form neutrons and neutrinos

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

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


Death of Massive Stars: Death of Massive Stars: SupernovaSupernova

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

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


Supernova Explosion (1)Supernova Explosion (1)• The superheated gas is blasted into space

carrying a lot of the heavy elements produced in the stellar nucleosynthesis process

• This explosion is a supernova• Expanding gas crashes into the surrounding

interstellar gas at thousands of kilometers/second, – the shock wave heats up the interstellar gas to

very temperatures and it glows

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


Supernova Explosion (2)Supernova Explosion (2)• Planetary nebulae and H II regions are

lit up by the action of ultraviolet light on the gas, while supernovae glow from shock-wave heating

• Gas from supernova explosions also has strong radio emission with a non-thermal continuous spectrum that is produced by electrons spiraling around magnetic field lines

• Gas from recent explosions (within a few thousand years ago) are visible with X-ray telescopes as well


Crab NebulaCrab Nebula• A famous

supernova remnant is the Crab Nebula

• Chinese astronomers recorded the explosion on July 4, 1054

• Anasazi Indians painted a picture of it


Vela Vela SupernovaSupernova• Occurred long before the

Crab Nebula• Much more spread out. • Parts have run into

regions of the interstellar medium of different densities.

• For that reason and because of turbulence in expanding

supernova gas, the remnant seen today is wispy strands of glowing gas.


Supernova OutputSupernova Output• Neutrinos formed when the neutron

core is created fly away from the stiff core, carrying most of the energy from the core collapse away with them

• Some energy goes into driving the gas envelope outward

• The rest of the energy goes into making the supernova as bright as 1011 Suns– as bright as an entire galaxy!


SN 1987aSN 1987a• Supernova

occurred in satellite galaxy of the Milky Way at beginning of 1987

• Called SN1987a• Kamiokande

neutrino detector saw a burst of neutrinos

• Confirmation of supernova models

• Left image shows star before it went supernova


HST Images of SN1987aHST Images of SN1987a• The material from

the explosion is expanding outward at over 9.5 million km/hr preferentially into two lobes that are roughly aligned with the bright central ring

• Central bright ring and two outer rings are from material ejected by the star before its death


Supernova Rate in the Supernova Rate in the UniverseUniverse• Supernovae are very rare

– about one every hundred years in any given galaxy

– because the stars that produce them are rare.

• But… there are billions of galaxies in the universe, – simple probability says that there should be a few

supernovae happening somewhere in the universe during a year and that is what is seen!

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


Stage 9: Core RemnantStage 9: Core Remnant• Core mass < 1.4 solar masses,

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

• Core 1.4 < mass <3 solar masses,– Neutrons bump up against each other to form a

degenerate gas– Forms a neutron star about the size of small city. – Neutrons prevent further collapse of the core

• Core > 3 solar masses : Complete collapse– As it collapses, it may momentarily create a

neutron star and the resulting supernova rebound explosion

– Gravity finally wins. Nothing holds it up– Becomes a black hole


Novae and Supernovae Novae and Supernovae Type IType I• An isolated white dwarf has a boring future: it

simply cools off, dimming to invisibility• White dwarfs in binary systems where the

companion is still a main sequence or red giant star can have more interesting futures

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

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


Novae (1)Novae (1)• Eventually the hydrogen gas gets dense and

hot enough for nuclear reactions to start– The reactions occur at an explosive rate

• The hydrogen gas is blasted outward to form an expanding shell of hot gas

• The hot gas shell produces a lot of light suddenly

• From the Earth, it looks like a new star has appeared in our sky

• Early astronomers called them novae (“new” in Latin)


Novae (2)Novae (2)• They are now known to be caused by old,

dead stars• The spectra of a nova shows blue-shifted

absorption lines showing that a hot dense gas is expanding towards us at a few thousands of kilometers per second

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

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


Novae (3)Novae (3)• After the nova burst, gas from the regular star begins

to build up again on the white dwarf's surface• A binary system can have repeating nova bursts• If enough mass accumulates on the white dwarf to

push it over the 1.4 solar mass limit, the degenerate electrons will not be able to stop gravity from collapsing the dead core

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


Novae (4)Novae (4)• A huge amount of energy is released very

quickly with such power that the white dwarf blows itself apart

• This explosion is called a type I supernova to distinguish them from the supernova (called a type II supernova) that occurs when a massive star's iron core implodes to form a neutron star or black hole

• Type I supernovae are several times brighter than type II supernovae

• Tycho’s supernova was a type I


Neutron StarsNeutron Stars• If the core mass is between 1.4 and 3 solar

masses, the compression from the star's gravity will be so great the protons fuse with the electrons to form neutrons

• The core becomes a super-dense ball of neutrons

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

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


Neutron StarsNeutron Stars• The neutrons are degenerate

and their pressure (called neutron degeneracy pressure) prevents further collapse

• Neutron stars are about 30 kilometers across, so their densities are much larger than even the incredible densities of white dwarfs: – 2 × 1014 times the density of water

• Recently, the Hubble Space Telescope was able to image one of these very small objects

• Even though it is over 660,000 K, the neutron star is close to the limit of HST's detectors because it is at most 27 kilometers across


Pulsars (1)Pulsars (1)• In the late 1960's astronomers discovered

radio sources that pulsated very regularly with periods of just fractions of a second to a few seconds

• The periods are extremely regular– only the ultra-high precision of atomic clocks can

show a very slight lengthening in the period

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

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


Pulsars (2)Pulsars (2)• Normal variable stars (stars near the end of their life

in stages 5 to 7) oscillate in brightness by changing their size and temperature

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

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

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


Pulsars (3)Pulsars (3)• A rapidly rotating object with a

bright spot on it could produce the quick flashes if the bright spot was lined up with the Earth

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

• Neutron stars are compact enough and strong enough to rotate that fast– The pulsar at the center of the Crab Nebula rotates 30 times

every second

• In the figure it is the left one of the two bright stars at the center of the HST image


Pulsar SizePulsar Size• The 1/1000th of second burst of energy means that

the pulsars are at most (300,000 kilometers/second) × (1/1000 second) = 300 kilometers across

• This is too small for normal stars or white dwarfs, but fine for neutron stars

• When neutron stars form they will be spinning rapidly and have very STRONG magnetic fields (109 to 1012 times the Sun's)

• The magnetic field is the relic magnetic field from the star's previous life stages

• The magnetic field is frozen into the star, so when the core collapses, the magnetic field is compressed too

• The magnetic field becomes very concentrated and much stronger than before

12 april 2005ast 2010: chapter 221 the death of stars

Documents

large stars

death of stars

form stars

stars cool outer layers

old stars

common lowmass stars

helium stars similar

star cores low mass