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Chapter 22: The Death of Stars. What happens to old stars? How does death differ for small and large stars?. Stage 8: Planetary Nebula or Supernova . The outer layers are ejected as the core shrinks to its most compact state.
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Chapter 22:The Death of Stars What happens to old stars? How does death differ for small and large stars? Astronomy 2010
Stage 8: Planetary Nebula or 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 (I.e 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. Astronomy 2010
Stellar Nucleosynthesis • H, He, some Li, Be, B produced during the Big Bang. • Other elements produced in stars through nuclear fusion. • When the outer layers of a star are thrown back into space, the new, heavy elements can 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 not created in the big bang. They were created in stars. Astronomy 2010
Stellar Nucleosynthesis (Cont’d) • Atoms from helium to iron are 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. Astronomy 2010
Degenerate matter • When atoms become super-compressed, particles bump right up against each other to produce a kind of gas, called a degenerate gas. • Normal gas exerts higher pressure when it is heated and expands, but the pressure in a degenerate gas does not depend on the temperature. • The laws of quantum mechanics must be used for gases of ultra-high densities. Astronomy 2010
Degenerate Gas • Only certain energies are permitted in a closely confined space. • The particles are arranged in energy levels like rungs of an energy ladder. In ordinary gas, most of the energy levels are unfilled and the particles are free to move about. But in a degenerate gas, all of the lower energy levels are filled. • Only two particles can share the same energy level in a given volume at one time. • For white dwarfs the degenerate particles are the electrons. For neutron stars the degenerate particles are neutrons. • How close particles can be spaced depends inversely on their masses. • Electrons are spaced further apart in a degenerate electron gas than the neutrons in a degenerate neutron gas because electrons are much less massive than neutrons. Astronomy 2010
Consequences (1) • Degenerate gases strongly resist compression. • Degenerate particles (electrons or neutrons) locked into place because all of the lower energy shells are filled up. • The only way they can move is to absorb enough energy to get to the upper energy shells. • This is hard to do! • Compressing a degenerate gas requires a change in the motions of the degenerate particle. But that requires A LOT of energy. • Degenerate particles have no ``elbow room'' and their jostling against each other strongly resists compression. The degenerate gas is like hardened steel! Astronomy 2010
Consequences (2) • The pressure in a degenerate gas depends only on the speed of the degenerate particles NOT the temperature of the gas. • But to change the speed of degenerate particles requires A LOT of energy because they are locked into place against each other. • Adding heat only causes the non-degenerate particles to move faster, but the degenerate ones supplying the pressure are unaffected. Astronomy 2010
Consequences (3) • 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! Astronomy 2010
White Dwarfs • Form as the outer layers of alow-mass red giant star puffout 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. Astronomy 2010
White 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 sugarcube 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. Astronomy 2010
Radius of a White Dwarf Adding more mass causes the radius to decrease! At about 1.4 solar masses, the size becomes zero! Astronomy 2010
White Dwarf Cooling • 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. Astronomy 2010
From Giant to White Dwarf Astronomy 2010
White Dwarf 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. • The HST can detect these small dead stars in nearby old star clusters called globular clusters. • Analysis of the white dwarfs provides 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. Astronomy 2010
Death 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. Astronomy 2010
Death of Massive Stars - Supernova • 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. Astronomy 2010
Supernova Explosion • 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. Astronomy 2010
Supernova Explosion (cont’d) • Also, the ratio of the strengths of the individual doubly-ionized oxygen is that expected from shock-wave heating. • Planetary nebulae and H II regions are lit up by the action of ultraviolet light on the gas, while supernova 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. Astronomy 2010
Crab 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. Astronomy 2010
Vela Supernova • 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. Astronomy 2010
Supernova 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 as1011 Suns • as bright as an entire galaxy! Astronomy 2010
SN 1987a • Supernova occurred in satellite galaxy of the Milky Way at beginning of 1987 • Called SN1987a. • Kamiokande detector (Japan) saw a burst of neutrinos. • Confirmation of supernova models. after before Astronomy 2010
HST 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. Astronomy 2010
Supernova Rate in the Universe • 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. Astronomy 2010
Stage 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 Astronomy 2010
Novae and Supernovae Type 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. Astronomy 2010
Novae • 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). • They are now known to be caused by old, dead stars. Astronomy 2010
Novae • The spectra of novae show blue-shifted absorption lines from hot dense gas expelled 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. • After a 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. Astronomy 2010
Type Ia Supernovae • 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. Astronomy 2010
Type Ia Supernovae • 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 Ia supernova to distinguish it from the supernovae (called type II supernovae) that occur when a massive star's iron core implodes to form a neutron star or black hole. • Type Ia supernovae are several times brighter than Type II supernovae. • Tycho’s supernova was a type Ia. • Type Ia supernovae are used as “standard candles”. Astronomy 2010
Tycho’s Supernova and Companion Astronomy 2010
Neutron 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. Astronomy 2010
Neutron 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 x 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. Astronomy 2010
Pulsars • 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”). Vela pulsar Astronomy 2010
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. Astronomy 2010
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 Crab pulsar Astronomy 2010
Pulsars (4) • Another clue comes from the length of each pulse itself. • Each pulse lasts about 1/1000th of a second (the time between pulses is the period mentioned above). • An important principle in science is that an object cannot change its brightness faster than it takes light to cross its diameter. • Even if the object could magically brighten everywhere simultaneously, it would take light from the far side of the object longer to reach you than the near side. Fastest known pulsar B1937 Astronomy 2010
Pulsars (5) • Observed change in brightness to be smeared out over a time interval equal to the time it would take the light from the far side of the object to travel to the near side of the object. • If the object did not brighten everywhere simultaneously, then a smaller object could produce a pulse in the same interval. The brightness fluctuation timescale gives the maximum size of an object. Astronomy 2010
Pulsar 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. Astronomy 2010
Summary Astronomy 2010
