Our Place in the Cosmos

Lecture 13Supernovae, neutron stars

and black holes

Evolution of Massive Stars

• In the last lecture we followed the evolution of low mass stars (below about 3M) from the main sequence to planetary nebulae and white dwarfs• More massive stars spend a much

shorter time on the main sequence and end their lives in spectacular and dramatic fashion

CNO cycle

• In the hotter cores of massive main sequence stars hydrogen fusion can occur by an efficient mechanism known as the carbon-nitrogen-oxygen (CNO) cycle (as well as the less efficient proton-proton chain that occurs in low-mass stars)

• This explains the dramatically higher luminosity of high mass stars

• Note that carbon is not consumed in the CNO cycle - it acts instead as a catalyst

CNO Cycle

Energy Production by PP and CNO Chains

Proton-Proton CNOdominates in low-mass stars …. dominates in high mass stars

Post-Main Sequence Evolution

• The helium core of a massive star reaches a temperature of 108 K, at which point helium fusion can begin, before it becomes electron degenerate

• There is therefore no explosion of the core - the star makes a smooth transition from core hydrogen-burning to core helium-burning

• A massive star does not become a red giant but moves horizontally on the H-R diagram as it grows modestly in size while surface temperature falls

• It now has the structure of a horizontal branch star

After leaving the main sequence, massive stars move horizontally back and forth on the H-R diagram

The dotted region is the instability strip where stars pulsate in size

Nucleosynthesis

• When a high-mass star exhausts the helium in its core, the core shrinks until it reaches a temperature of 8 x 108 K

• At this point carbon can fuse into more massive nuclei [low-mass stars never get hot enough to do this]

• When the carbon is exhausted, core-burning of neon, oxygen and silicon successively occurs

• This synthesis of heavier nuclei from lighter ones is known as nucleosynthesis

Pulsating Variable Stars

• At this stage in their evolution, some massive stars pass through the instability strip on the H-R diagram

• Changes in the ionization state (what fraction of electrons are removed from atoms) alter the transparency of the star to escaping radiation

• When energy is trapped inside the star it expands• A change in ionization state then allows the trapped

radiation to escape and the star shrinks• This cycle continues - the star is a Pulsating Variable

Star• Examples include Cepheid and RR Lyrae variables

with periods of order days increasing in proportion to luminosity

Captions

Mass Loss

• Even main sequence massive stars are losing mass at up to 10-5 M per year due to radiation pressure

• The most massive stars (20 M or more) may lose 20% of their mass while on the main sequence and 50% over their entire lifetime

• An extreme example is Eta Carinae with a mass around 100 M but losing about 1 M every 1000 years

End of Fusion

• Once all fusable material in the core of a low-mass star is exhausted the star expires relatively gently as outer layers are ejected to form a planetary nebula leaving behind the degenerate core as a white dwarf

• Massive stars end their lives in a spectacular explosion known as a supernova

• Nucleosynthesis proceeds as far as iron, the most tightly bound atomic nucleus

• Whatever other nucleus one tries to fuse with iron, the product will have less binding energy then iron

• Therefore no element heavier than iron is fused within stars

1. Fusing elements lighter than iron releases energy

2. Fusing elements more massive than iron requires energy - iron and more massive elements do not burn

Neutrino Cooling

• Once carbon starts to burn, fusion proceeds extremely rapidly as neutrinos efficiently carry energy away from core - neutrino cooling

• Nuclear reaction rate must increase to balance energy lost by neutrinos

• Hydrogen burning lasts for millions of years• Helium burning lasts for ~ 100,000 years• Carbon burning lasts or ~ 1000 years• Oxygen burning lasts for ~ 1 year• Silicon burning lasts for a few days• By now the star is radiating nearly all its energy

in the form of neutrinos

Supernova

• Once silicon is all fused to iron in the star’s core no more fusion can occur

• The iron core then collapses beyond the electron-degenerate stage to densities of 10 tonnes per cubic cm and temperatures of 10 billion K

• A process called photodisintegration then kicks in, which breaks up the iron nuclei and squeezes electrons into nuclei to produce neutron-rich isotopes

• Within about 1 second the core is collapsing at a rate of of about one quarter of the speed of light

Supernova

• At very high densities, the strong nuclear force becomes repulsive, causing the collapsing core to “bounce”, sending a shockwave through the rest of the star

• Over the next couple of seconds, about one-fifth of the mass of the core is converted into neutrinos, some of which are trapped by the huge densities of material within the core, adding to the shockwave

• Within about one minute the shockwave has pushed pass the helium shell and within a few hours reaches the surface, heating it to 500,000 K

• The star has exploded in a supernova explosion

Captions

Supernovae

• This type of supernova is known as a Type II supernova

• Type I supernovae occur due to mass accretion in binary stars

• Either type shine with the luminosity of 100 billion Suns

• One hundred times more energy, however, is released in the form of the kinetic energy of the ejected gas

• One hundred times more energy again is released in the form of neutrinos

Supernova 1987A

• Supernova 1987A in the Large Magellanic Cloud - a companion dwarf galaxy - was visible to the naked eye in the Southern Hemisphere

• Neutrinos from this supernova had already - but unknowingly - been detected by neutrino telescopes, confirming theories of supernova explosions

Spreading it Around

• Supernovae are responsible for enriching interstellar space with the heavy elements synthesized within stars

• They are also responsible for synthesizing elements heavier than iron by the process of neutron capture

• Supernovae are thus essential for life

• We are literally made up of the material from exploding stars! Remnant of

SN 1987A

Neutron Stars

• The remaining core of a supernova has collapsed to the density of atomic nuclei

• If the remnant is no more massive than about 3 M further collapse is halted by neutron degeneracy

• The resulting star is known as a neutron star• Typical radii are only 10 km, but with a mass

more than 1.4 M

• Neutron stars are a billion times denser than white dwarfs and 1015 times denser than water

X-Ray Binaries

• If the neutron star is part of a binary system material may be transferred from the giant companion

• Tiny size but large mass of neutron star leads to large gravitational acceleration of infalling material onto an accretion disk

• Disk is heated to high temperatures so that it emits in X-rays - the most energetic form of electromagnetic radiation

• A relativistic jet may also form

X-Ray Binary

Pulsars

• Conservation of angular momentum means that many neutron stars are rotating at 10-100 times per second

• Any magnetic field is concentrated by the collapsing star to values trillions of times greater than Earth’s magnetic field

• Charged particles are accelerated along the field lines towards the magnetic poles

• Electromagnetic radiation is beamed out away from the poles like light beams from a lighthouse

• We can detect this radiation with radio telescopes - the star appears to “pulse” twice each revolution, hence the name pulsar

• The first pulsar was discovered in 1967

Black Holes

• Neutron stars are supported by neutron degeneracy

• Above about 3M the force of gravity can no longer be resisted

• As the neutron star collapses its surface gravity increases until the escape velocity vesc = [2G M/r] exceeds the speed of light

• Not even light can escape and we have a black hole

• A black hole will form if the stellar core left after a supernova explosion exceeds 3M or if the neutron star accretes sufficient mass from a companion to put it over the limit

Evidence for Black Holes

• If no radiation can escape from a black hole, how can we tell that they are there?

• Black holes are located via the effect of their gravity

• The size of a black hole is given by its Schwarzschild radius rS = 2GM/c2

• If the Sun were a black hole it would have a radius of only 3km

• Closely-orbiting objects or particles will be rapidly accelerated giving rise to X-ray radiation, as in Cygnus X-1 - a ~30M supergiant and ~10M black hole binary

Summary

• Massive stars “live fast, die young, and leave a beautiful corpse” (supernova remnants)

• They are responsible for synthesising all elements heavier than carbon, many of which are essential for human life

• Supernovae spread these heavy elements throughout space - they will be incorporated into future generations of stars and humans!

• The remnant cores are either neutron stars (below about 3M) or black holes (> 3M)