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The Death of Stars 12 April 2005 AST 2010: Chapter 22 1 Stellar Questions • What happens to old stars? • How does death differ for small and large stars? 12 April 2005 AST 2010: Chapter 22 2 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 (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 12 April 2005 AST 2010: Chapter 22 4 Stellar 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 12 April 2005 AST 2010: Chapter 22 6 Stellar Nucleosynthesis (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. 12 April 2005 AST 2010: Chapter 22 7 Consequences • 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! 12 April 2005 AST 2010: Chapter 22 10 White 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 12 April 2005 AST 2010: Chapter 22 11 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 12 April 2005 AST 2010: Chapter 22 12 Radius of a White Dwarf 12 April 2005 AST 2010: Chapter 22 13 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 12 April 2005 AST 2010: Chapter 22 14 From Giant to White Dwarf 12 April 2005 AST 2010: Chapter 22 15 White Dwarfs 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 12 April 2005 AST 2010: Chapter 22 16 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 12 April 2005 AST 2010: Chapter 22 17 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 12 April 2005 AST 2010: Chapter 22 18 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 12 April 2005 AST 2010: Chapter 22 19 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 nonthermal 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 12 April 2005 AST 2010: Chapter 22 20 Crab Nebula 12 April 2005 AST 2010: Chapter 22 • A famous supernova remnant is the Crab Nebula • Chinese astronomers recorded the explosion on July 4, 1054 • Anasazi Indians painted a picture of it 21 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. 12 April 2005 AST 2010: Chapter 22 22 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 as 1011 Suns – as bright as an entire galaxy! 12 April 2005 AST 2010: Chapter 22 23 SN 1987a 12 April 2005 AST 2010: Chapter 22 • 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 24 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 12 April 2005 AST 2010: Chapter 22 25 12 April 2005 AST 2010: Chapter 22 26 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 12 April 2005 AST 2010: Chapter 22 27 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 12 April 2005 AST 2010: Chapter 22 28 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 12 April 2005 AST 2010: Chapter 22 29 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) 12 April 2005 AST 2010: Chapter 22 30 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 lowerdensity surface of the expanding cloud • After a few days the gas has expanded and thinned out enough to just produce blueshifted emission lines 12 April 2005 AST 2010: Chapter 22 31 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 12 April 2005 AST 2010: Chapter 22 32 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 12 April 2005 AST 2010: Chapter 22 33 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 12 April 2005 AST 2010: Chapter 22 34 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 × 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 12 April 2005 AST 2010: Chapter 22 35 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”) 12 April 2005 AST 2010: Chapter 22 36 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 12 April 2005 AST 2010: Chapter 22 37 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 12 April 2005 AST 2010: Chapter 22 38 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 12 April 2005 AST 2010: Chapter 22 39