Transcript Supernova
Supernova Explosions • Stars may explode cataclysmically. – Large energy release (103 – 106 L) – Short time period (few days) • These explosions used to be classified as novas or supernovas. – Based on absolute magnitude • They are now all called supernovas. Hydrogen Lines • Supernovas are classified by their emission spectra. – Historical classification – Not related to mechanism • The initial classification is based on hydrogen. • Secondary classification is based on other elements. – Silicon absorption – Helium emission Mass Relations -20 • Stars on the HR diagram line up according to mass. -15 Abs. Magnitude -10 -5 10 M 0 3 M 0.5 M 5 10 0.02 M 15 20 O B A F G K M Spectral Type • The time on the main sequence is spent burning hydrogen. – Massive stars burn faster Giants -20 -15 Abs. Magnitude -10 -5 0 Capella giants Aldebaran • Helium fusion through triple alpha causes a helium flash. – Rapid expansion 100 x R 5 10 15 20 • When core hydrogen is exhausted helium burning begins. – Degenerate gas core 108 K O B A F G K M Spectral Type Degenerate electrons • The nuclei from fusion are separated from their electrons. – Filled fermi states with degenerate electrons – Provides opposing force to gravity • The energy of contraction blows off outer layers of star. inward force of gravity outward force of electrons Dwarves -20 • Giants that exhaust their core helium become white dwarves. – Planetary nebulas -15 Abs. Magnitude -10 -5 0 giants 5 10 15 20 white dwarves O B A F G K M Spectral Type • Isolated white dwarves slowly cool due to lack of further fusion. Binary Dwarves • White dwarves can occur in binary stars. – One star ages faster – Original detection • White dwarves continue gravitational pull on companion. – Tidal forces Sirius image from Chandra - NASA Binary Explosions • A binary can transfer gas from a giant to a white dwarf. • If the white dwarf exceeds MCH, gravity will exceed electron repulsion. • It will explode into a type I supernova. – Star-sized fusion bomb white dwarf giant star gas pulled to partner supernova Binary Life Cycle 1-3 M 4-9 M • Close binary stars will evolve at different times. • The massive star will form a white dwarf first. 1-3 M 1.5 M supernova • The second star goes giant and engulfs white dwarf. – Material from the second star is also blown away Core Fusion • For high mass stars fusion continues beyond helium fusion. • Each fusion stage requires higher temperatures and pressures and takes place in deeper layers. • Fusion steps – Hydrogen to helium – Helium to carbon – Carbon to oxygen – Oxygen to neon – Neon to silicon – Silicon to iron Supergiants -20 -15 Abs. Magnitude -10 Rigel -5 0 supergiants Betelgeuse 5 10 15 20 O B A F G K M Spectral Type • Massive stars can sustain helium burning and that are brighter than expected are large and are called supergiants. – M > 5-8 M Gravitational Binding • The change in gravitational energy is released during collapse. – From 1 M, r = 1000 km – To r = 10 km • The estimate is an order of magnitude greater than the amount needed for nuclear changes. – 90% available for release 2 M 10 km GM 2 E 3 10 46 J R M r sun Total Energy • The energy released by the collapse of a core is great. – Optical: 1042 J in weeks – About 1010 times the Sun – Equal to some galaxies Death of Supergiants -20 supernovae -15 Abs. Magnitude -10 • A supergiant with more than 8 M will oscillate in temperature becoming more luminous. -5 0 5 Sun 10 15 20 O B A F G K M Spectral Type • Eventually the core is so collapsed by gravity that the electrons cannot hold the core apart. • A star like this will become a type II supernova. Neutrino Production • The core can cool by producing neutrinos. – Plasma at 1011 K – Opaque to photons • Neutrinos can carry kinetic energy. – Hot enough for all three types – Pair production dominates Neutrino Observation Stellar Explosion • When gravitational force exceeds the electron repulsion, the core collapses immediately. • The energy is released as photons and mostly neutrinos. • The outward energy hits collapsing material and the star explodes. Supernova Remnants • The supernova core collapse is at 200 billion K. • The photons are energetic enough to break up iron nuclei. • The particles from the broken nuclei fuse with iron to create heavy elements. • This matter goes to form new stars and planets. Nuclear Force • Neutron stars forms when the core mass exceeds the Chandrasekar mass: 1.5 M. – Photodisintegration: 1.4 x 1045 J – Electron capture: 1.6 x 1045 J • Nuclear forces stop further collapse. – Reach nuclear density R r0 A1 3 r0 = 1.2 x 10-15 m nuc 3 AmN 3mN 3 3 4R 4r0 nuc = 2.3 x 1017 kg/m3 Pulsars • Neutron stars create very large magnetic fields. – Spin faster with collapse – Up to 30 Hz • They can be observed as repeating flashes of light as the magnetic poles point towards us. Rotation Time • Minimum period is found by balancing gravity and centripetal force. – Fast rotation from high density min 2 max min 11 • The period decreases with time. – Magnetic dipole radiation – Predict 1200 years for Crab pulsar R3 2 GM 12 M 1 2 h M G 0 . 6 ms M mN c 2 M dErot d 2 I 3 0 (m 2 sin ) 2 dt dt 3c 4 d C 3 dt X-rays • The surface gravity creates tremendous accelerations. – Accelerating electrons radiate photons – Radiate as x-rays • X-ray telescopes in orbit can spot neutron stars in supernova remnants. X-ray Pulsars • Pulsars also emit x-rays. – Blink at characteristic period – Crab nebula period 33 ms Crab nebula off Crab nebula on