This set of slides This set of slides covers the supernova of white dwarf stars and the late-in-life evolution and death of massive stars, stars > 8 solar masses, including supernova of these large mass stars. Units covered: 65, 66, 67.

If a white dwarf is in orbit around a red giant companion star, it can pull material off the companion and into an accretion disk around itself. Material in the accretion disk eventually spirals inward to the surface of the white dwarf. Mass Transfer and Novae

Novae If enough material accumulates on the white dwarf’s surface, fusion can be triggered anew at the surface, causing a massive explosion. This explosion is called a nova (new as in new star.) If this process happens repeatedly, we have a recurrent nova.

If the mass of one of these accreting white dwarfs exceeds 1.4 solar masses (the Chandrasekhar Limit), gravity wins! (momentarily) The additional gravity causes just enough compression… This compression causes the temperature to soar, and this allows carbon and oxygen to begin to fuse into silicon. The energy released by this fusion blows the star apart in a Type 1a Supernova. The Chandrasekhar Limit and Supernovae

Supernova! This is a SINGLE STAR with a luminosity of BILLIONS of stars!

The light output from a Type 1a supernova follows a very predictable curve. –Initial brightness increase followed by a slowly decaying “tail” All Type 1a supernova have similar peak luminosities, and so can be used to measure the distance to the clusters or galaxies that contain them. Type 1a Supernova – Another Standard Candle

Formation of Heavy Elements Hydrogen and a little helium were formed shortly after the Big Bang. ALL other elements were formed inside stars. Low-mass stars create carbon and oxygen in their cores at the end of their life, thanks to the high temperature and pressure present in a red giant star. High-mass stars produce heavier elements like silicon, magnesium, etc. up through iron, by nuclear fusion in their cores. –Temperatures are much higher. –Pressures are much greater. Highest-mass elements (heavier than iron) must be created in supernovae - the death of high-mass stars.

The Lifespan of a Massive Star

Layers of Fusion Reactions As a massive star burns its hydrogen, helium is left behind, like ashes in a fireplace. Eventually the temperature climbs enough so that the helium begins to react, fusing into carbon. Hydrogen continues to fuse in a shell around the helium core. Carbon is left behind until it too starts to fuse into heavier elements. A nested shell-like structure forms. Once iron forms in the core, the end is near…

Core Collapse of Massive Stars Iron cannot be fused into any heavier element, so it collects at the center (core) of the star. Gravity pulls the core of the star to a size smaller than the Earth’s diameter. The core compresses so much that protons and electrons merge into neutrons, taking energy away from the core. The core collapses, and the layers above fall rapidly toward the center, where they collide with the core material and “bounce”. The “bounced material collides with the remaining infalling gas, raising temperatures high enough to set off a massive fusion reaction – an enormous nuclear explosion. This is a Type II, Ib, or Ic supernova. (Ib, Ic subcatagories)

Light Curve for a Supernova The luminosity spikes when the explosion occurs, and then gradually fades, leaving behind a…

Supernova Remnant The supernova has left behind a rapidly expanding shell of heavy elements that were created in the explosion. Gold, uranium and all other heavy elements all originated in a supernova (Type II) explosion.

Types of Supernovae, Summary Type Ia: The explosion that results from a white dwarf exceeding the Chandrasekhar Limit (1.4 solar masses.) Type II: Supernovae resulting from massive star core collapse. Less common: –Type Ib and Ic: Result from core collapse, but lacks hydrogen, lost to stellar winds or other processes.

Stellar Corpses A type II supernova leaves behind the collapsed core of neutrons that started the explosion, a neutron star. If the neutron star is massive enough, it can collapse, forming a black hole…

Jocelyn Bell, a graduate student working with a group of English astronomers, discovered a periodic signal in the radio part of the spectrum, coming from a distant galaxy. Astronomers considered (briefly) the possibility of an alien civilization sending the regular pulses. More pulsating radio sources were discovered These were named pulsars. All pulsars are extremely periodic, like the ticking of a clock. In some cases, this ticking is amazingly fast! A Surprise Discovery

An Explanation An idea was proposed that eventually solved the mystery. A neutron star spins very rapidly about its axis, due to conservation of angular momentum. If the neutron star has a magnetic field, this field can form jets of electromagnetic radiation escaping from the star. If these jets are pointed at Earth, we can detect them using radio telescopes. As the neutron star spins, the jets can sweep past earth, creating a signal that looks like a pulse. Neutron stars can spin very rapidly, so these pulses can be quite close together in time.

The Crab Nebula Pulsar

Interior Structure of a Neutron Star Density approx. equal to atomic nucleus density.

Most pulsars emit both visible and radio photons in their beams. Older neutron stars just emit radio waves. Some pulsars emit very high energy radiation, such as X-rays. –X-ray pulsars. –Magnetars. Magnetars have very intense magnetic fields that cause bursts of x-ray and gamma ray photons. High-Energy Pulsars

10 15 gauss mag field strength. Earth’s field, about 1 gauss.