Stellar Explosions Explosions

Chapter 21: Stellar Explosions Explosions

© 2017 Pearson Education, Inc. Chapter 21: Stellar Explosions • The Final Fate of Stars • Low Mass Stars – Forming White Dwarfs – The Fate of a White Dwarf • Novae • High Mass Stars – Fusion of Heavy Elements – Collapse of the Iron Core – Photodisintegration • Supernovae • Making elements heavier than Iron • The Cycle of Stellar Evolution

© 2017 Pearson Education, Inc. Final Fate of Stars • A low mass star (up to ¼ solar masses) cannot achieve the temperatures required to fuse He. After the main sequence 1 2 + – 4 1H → 4He + 2 e + 휈푒 It forms a Helium white dwarf.

© 2017 Pearson Education, Inc. • A star of mass greater than ¼ solar masses but less than 8 solar masses will achieve the temperatures required to fuse He to C and O 2 6 – 3 4He → 12C 2 6 8 – 4He + 12C → 16O and forms a Carbon-Oxygen white dwarf

© 2017 Pearson Education, Inc. Final Fate of Stars • Even larger stars, of mass greater than 8 solar masses but less than 12 solar masses can go on to fuse O with He to form Ne 2 8 10 – 4He + 16O → 20Ne ending up as Neon-Oxygen white dwarfs. • Stars of mass greater that 12 solar masses will have a very different history. Electron degeneracy pressure cannot sustain the core • This leads to a catastrophic explosion that leaves behind a neutron star. • The explosion is called a supernova.

• As the white dwarf cools, its size does not change much; it simply gets dimmer and dimmer, and should finally cease to glow (black dwarf). White dwarfs are sustained by electron degeneracy. • However, we sometimes see white dwarfs suddenly brighten, i.e., turn nova. • Nova Persei, a WD that suddenly brightened by a factor of 40,000! • In general, a nova is a star that flares up very suddenly and then returns slowly to its former luminosity.

© 2017 Pearson Education, Inc. White Dwarfs, Binaries and Novae • We know today that a nova is a white dwarf that is undergoing an explosion on its surface • The explosion results in a rapid and temporary increase of its luminosity • What causes a white dwarf to undergo surface explosions?

© 2017 Pearson Education, Inc. White Dwarfs, Binaries and Novae • Material falls onto the white dwarf from its main- sequence companion. • This forms a swirling disk of matter around the white dwarf. The disk gets hotter and hotter (by friction or viscosity) even as it falls in and becomes more and more luminous. • The gas becomes denser as it builds up on the surface because of the great mass and small radius of the white dwarf. • When enough material has accreted, fusion can reignite very suddenly, burning off the new material.

© 2017 Pearson Education, Inc. White Dwarfs, Binaries and Novae • The sudden start of fusion can generate shock waves that blow the surface layers into space. • Material keeps being transferred to the white dwarf, and the process repeats. • A nova represents one way in which a dead star in a binary system can extend its life.

© 2017 Pearson Education, Inc. Supernovae • Supernovae are far more catastrophic and luminous explosions than novae. • Supernovae are not “super” versions of novae. These explosions have a very different origin. • They can achieve luminosities that are over millions of times that of the sun. • They are one-time events. Once they occur, there is little or nothing left of the progenitor star. • Supernovae are classified by duration (light-curves) and by their absorption spectra. The most important consideration is the presence or absence of Hydrogen.

© 2017 Pearson Education, Inc. Supernovae • There are two types of supernovae, both equally common, which exhibit very different light-curves: – Type I, is a carbon-detonation supernova – Type II, is a core-collapse supernova that occurs at the death of a high-mass star.

© 2017 Pearson Education, Inc. Type I: Carbon Detonation Supernova Carbon-detonation supernova: • Each time a white dwarf in a binary goes nova, it ejects some of the matter it has collected from its companion. • Still, not all of it is eliminated and the white dwarf grows in size. • In time this white dwarf will accumulate too much mass from its binary companion. • If the white dwarf’s mass exceeds 1.4 solar masses (Chandrashekar limit), electron degeneracy can no longer keep the core from collapsing. • Carbon fusion begins throughout the star almost simultaneously, resulting in a carbon explosion.

• Nothing is left of the white dwarf after the Carbon Detonation. Type I supernovae are Hydrogen poor. • In this way, the heavier elements are transferred back to the Interstellar medium

© 2017 Pearson Education, Inc. Type I: Carbon Detonation Supernova • On the right is an image of the supernova remnant G 299, which resulted from a Type I supernova explosion. • (Type I supernovae are used to measure the expansion of the universe) • The supernova remnant SN 1572. It resulted from a Type I supernova, visible to the naked eye and observed in 1572, in particular by Tycho Brahe. © 2017 Pearson Education, Inc. Larger Mass Stars • Larger mass stars are able to fuse heavier elements. The sequence goes like this 2 10 12 – 4He + 20Ne → 24Mg 2 12 14 – 4He + 24Mg → 28Si • This is the He capture sequence. It requires higher temperatures at every step and each step occurs faster. The last two stages occur appreciably only in stars of mass > 12 solar masses. Advanced nuclear fusion of the heavier elements then starts to occur: 6 8 14 – 12C + 16O → 28Si 2 14 16 – 4He + 28Si → 32S 14 14 28 – 28Si + 28Si → 56Fe

© 2017 Pearson Education, Inc. Larger Mass Stars • Core temperatures of stars of mass greater than 12 solar masses allow the fusion of elements up to Fe. • Other pathways also occur, e.g., an intermediate mass element like Nitrogen is formed during the following reaction: 1 6 7 6 + – 1H + 12C → 13N → 13C + n + 푒 + 휈푒 6 1 7 – 13C + 1H → 14N • All these reactions release energy. • Iron is the heaviest element that is energetically favorable to fuse. • Fusion to even heavier elements obviously does occur, but now it removes energy from the star.

© 2017 Pearson Education, Inc. Larger Mass Stars • But He capture continues, releasing energy, as well: 2 14 16 – 4He + 28Si → 32S 2 16 18 – 4He + 32S → 36Ar 2 18 20 – 4He + 36Ar → 40Ca 2 20 22 – 4He + 40Ca → 44Ti 2 22 24 – 4He + 44Ti → 48Cr 2 24 26 – 4He + 48Cr → 52Fe • and… at the cost of energy: 2 26 28 – 4He + 52Fe → 56Ni 2 28 30 – 4He + 56Ni → 60Zn – Etc.

© 2017 Pearson Education, Inc. Fate of the Iron Core • Iron lies at the lowest point of the curve, so with the appearance of substantial amounts of iron, fusion ceases. • The iron core has no way to generate the energy required to sustain itself and it begins to collapse. • With the collapse, the core temperature rises to about 10 billion degrees K. • Gravitational energy is released as heat in the form of photons. • According to Wien’s law these photons have a very small wavelength

© 2017 Pearson Education, Inc. Photodisintegration • Wien’s Law: 2.898 × 10−3 휆 = = 2.898 × 10−13 m 푇 • They are smaller than the nucleus and extremely energetic. They act as high energy bullets and begin a process of photodisintegration of the heavy elements in the core. • In less than one second the process undoes all the effects of fusion, splitting the Iron nuclei into smaller and smaller pieces. • This process cools the core, reduces the pressure and accelerates the core’s collapse.

© 2017 Pearson Education, Inc. Photodisintegration • Soon the core ends up where it began, made of protons, electrons and neutrons, but at much higher densities. • The gravitational force (weight of the core) is far too much for electron degeneracy to counterbalance it. • The electrons then begin to combine with the protons to form neutrons. This process is called neutronization − 푝 + 푒 → 푛 + 휈푒 • Since there are equal numbers of protons and electrons the central region of the core is now made of neutrons.

© 2017 Pearson Education, Inc. Type II: Core Collapse Supernova • When the neutronization process begins, the core density is about 1012 kg/m3 • A very large number of neutrinos are produced in this process. These neutrinos carry out about 1046 Joules of energy (in about 10 seconds) because they interact weakly with matter. • As they get closer together, the neutrons begin to exert an outward degeneracy pressure on the core. • The core continues to collapse, compressing the matter to a density of 1017 kg/m3, at which point the neutron degeneracy pressure causes the collapse to halt. The outer layers rebound off the inner core.

© 2017 Pearson Education, Inc. Type II: Core Collapse Supernova • A massive shock wave sweeps through the star, ejecting all the outer layers (including the heavy elements just formed) into space. This is a Core- Collapse Supernova.

• A neutron core survives this type of a supernova explosion. Type II supernovae are Hydrogen rich.

© 2017 Pearson Education, Inc. Supernova Remnants • Supernovae leave remnants—the expanding clouds of material from the explosion. • The core is bounded by an expanding shock wave, which is expanding into space and sweeping up material from the ISM Electrons ejected from the core are in the middle, The Crab nebula emitting radio synchrotron resulted from a Type II radiation. Supernova (2 kpc). © 2017 Pearson Education, Inc. Supernova Remnants There are in general three kinds of remnants: • Crab-like remnants (Crab) • Shell-like remnants like Cassiopeia on the right (3.4 kpc, Type II). A shock wave plows through space. Sometimes the shells contain a central neutron core like Vela, on the right (294 pc, Type II). • Composites: appear shell-like in radio and crab-like in X-rays, or crab-like in both frequencies but also have shells.

• This is the most common process, so these elements will be more abundant than others. • But it is not the only way elements can form.

© 2017 Pearson Education, Inc. Formation of Elements • Elements can also form by direct capture of free protons and neutrons by heavy nuclei to form even heavier nuclei. 14 • By the time 28Si appears the temperature in the core is about 1 billion K. At this point, the photons are so energetic that photodisintegration begins and there is a competition between forming heavier nuclei and breaking them up. 14 • The result is that some heavy nuclei (like 28Si) are 2 destroyed creating new 4He and promoting He capture to form even heavier elements. • The presence of a variety of elements, He, protons and neutrons then produces other, less common, elements.

© 2017 Pearson Education, Inc. Formation of Elements Heavier than Iron • He capture cannot proceed after Iron is formed. Making elements heavier than iron occurs via neutron capture. • Neutrons have no charge, so they are easier for heavy elements to capture. • Neutron capture is a very slow process and is often referred to as the s-process. • Being slow, the heavier nuclei have time to decay into lighter nuclei, so it ends up being a competition between capture and decay. • This is why there are only trace quantities of the heaviest elements.

© 2017 Pearson Education, Inc. Formation of the Heaviest Elements • Neutron capture, or the s-process, can function to produce only relatively stable nuclei. • Any element with an atomic mass greater than 209 (Bismuth) cannot be produced by neutron capture because the decay rate is greater than the capture rate. • However, in the last 15 minutes of a supernova explosion there are so many neutrons that the capture rate exceeds the decay rate as the neutrons are jammed into the nuclei. • This is known as the rapid process (r-process) because it proceeds very rapidly. • So the heaviest elements are formed after the star has died!

© 2017 Pearson Education, Inc. The Cycle of Stellar Evolution • Star formation is cyclical: Stars form, evolve, and die. • In dying, they send heavy elements into the interstellar medium. • These elements then become parts of new, next generation stars. • And so it goes.

© 2017 Pearson Education, Inc. Summary • Once hydrogen is gone in the core, a star burns hydrogen in the surrounding shell. The core contracts and heats; the outer atmosphere expands and cools. • Helium begins to fuse in the core as a helium flash. The star expands into a red giant as the core continues to collapse. The envelope blows off, leaving a white dwarf to gradually cool. • A nova results from material accreting onto a white dwarf from a companion star. • The same white dwarf may undergo many novae.

© 2017 Pearson Education, Inc. Summary, cont. • A Type I supernova is a carbon explosion, occurring when too much mass falls onto a white dwarf. • Very massive stars become hot enough to fuse carbon, then heavier elements, all the way to iron. At the end, the core collapses and rebounds as a Type II supernova. • All heavy elements are formed in stellar cores or in supernovae. • The heaviest elements are formed after the star is dead. • Stellar evolution can be understood by observing star clusters.