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

1015 gauss mag field strength. Earth’s field, about 1 gauss.