Chapter 23 Neutron Stars Neutron stars Inspired by

Chapter 23 Neutron Stars

Neutron stars • Inspired by the discovery of the Neutron in 1932, two Astronomers Fritz Zwicky (Clatech) and Walter Baade (Mount Wilson Observatory) proposed the following – When a massive star reaches the end of its life, the gravitational contraction can overcome the degenerate electron pressure that held the core during the last stages of the star. – Then the electrons and protons can combine under this pressure and transform into neutrons.

Neutron stars • What would be left is a core that consists of closely packed neutrons - A Neutron Star • Further compression will be resisted by the degenerate neutron pressure (due to Pauli exclusion principle which prohibits two neutrons from occupying the same energy state). • Since this pressure is much stronger than the degenerate electron pressure (that keeps white dwarfs intact) it could support more massive stellar corpses than white dwarfs.

Discovery of Neutron Stars(1960 s) • No one took much notice of Zwicky & Baade’s postulate of 1930 s • Until a grad. Student at Cambridge university in U. K. working on an array of radio antennas discovered in 1967 a very regular pulsating radio source (period 1. 3373011 sec) coming from one location in the sky. The came to be known as Pulsars. • Initially, it was thought to be “E. T”.

Pulsars are rotating neutron stars • Astronomers tried to find what pulsars were but eventually had to rule out all known candidates including white dwarfs. • Finally astronomers had accepted the more exotic idea that pulsars are actually neutron stars. • The discovery of the Crab pulsar in the center of the Crab nebula (a supernova remnant) convinced astronomers that stellar corpses can take two forms -White dwarfs and Neutron stars – Crab Pulsar Period = 0. 33 seconds

Rapidly Rotating Neutron stars • Why do neutron stars emit radiation at all? Why is it pulsed? How can it pulse so rapidly (30 times per second in the case of Crab Pulsar) • Neutron star spins rapidly due to conservation of angular momentum during shrinking • When a star many times bigger than our Sun collapses to a Neutron star (~ 30 km diameter) it will rotate extremely rapidly (due to conservation of angular momentum).

Rapidly Rotating Neutron stars • All stars possesses magnetic fields. • When a star collapse and form a neutron star, the magnetic field surrounding the neutron star becomes extremely intense – One trillion times(1012) that of the Sun. • So, a neutron star is rapidly spinning dense object with an intense magnetic field. • But, why does it radiate and why does it pulsate?

Pulsars: Rapidly rotating Neutron Stars

Pulsars: Rapidly rotating Neutron Stars • Consider a neutron star whose magnetic axis is inclined to the rotation axis (true for all planets in the solar system) • The magnetic field is so strong that it spontaneously creates pairs of electrons and positrons(anti-electrons) and these particles are pushed into the magnetic fields. • As these particles spiral along the magnetic field lines they emit EM radiation. • This produces two narrow beams of radiation streaming out of the magnetic poles.

Pulsars: Rapidly rotating Neutron Stars • If the neutron star is oriented in such away that one of the beams sweep across Earth, then we will detect a brief flash (pulse) as it sweeps across Earth • A radio telescope therefore, will detect regular pulses of radiation and the period of the pulses equal to the period of rotation. • This is very similar to how a ship at sea will see the beam of light from a lighthouse.

Pulsars Crab pulsar

Pulsars

Neutron Stars • Why is the Crab nebula so luminous? • Usually emission nebulae(or H II regions) shine due to the presence of an O or B type star close by. • The Crab pulsar(the neutron star at the center) is the energy source that make the Crab nebula shine. • Some of the rotational energy of the neutron star must get transferred to the surrounding gas. – As a result the rate of rotation of the neutron star is decreasing - it is slowing down!

Neutron Stars • This slowing down is observed in the Crab nebula • How does this energy transfer take place? • The answer is Synchrotron Radiation: – Radiation produced when relativistic (high speed) electrons ejected by the neutron star (forming immense jets at the poles) spiral their way through the strong magnetic field. • Spinning neutron stars slow down as it radiated its rotational energy. – Old neutron stars spin more slowly

Neutron Stars • X-Ray image shows where the relativistic electrons are ejected from the neutron star.

Neutron Star Interior • Although the light coming from pulsars provide information as to the surrounding, scientist construct theoretical models to study the internal structure.

Neutron Star Interior • The interior of a neutron star is a sea of densely packed degenerate neutrons. • These neutrons exhibit exotic physical properties: – Superfluidity: degenerate neutrons can flow without any friction. – Superconductivity: electrical currents inside the neutron star can travel around without experiencing any electrical resistance. Therefore, once you start an electrical current it keeps going forever. This drives the large magnetic field.

Millisecond Pulsars • Pulsars with rotation periods in the millisecond region (i. e. they spin several hundred times a second) are called millisecond pulsars. • The extremely rapid rotation and the very gradual slowing down observed in these pulsars lead to one conclusion: – Mass transfers from a close by binary companion star(probably a redgiant) feeds matter into the neutron star making it spin faster. – Therefore, a slow aging pulsar can be “spun-up” by mass transfer from its bloated companion.

Neutron Star Interior • The interior of a neutron star is a sea of densely packed degenerate neutrons. • These neutrons exhibit exotic physical properties: – Superfluidity: degenerate neutrons can flow without any friction. – Superconductivity: electrical currents inside the neutron star can travel around without experiencing any electrical resistance. Therefore, once you start an electrical current it keeps going forever. This drives the large magnetic field.

Pulsars The Black Widow Pulsar: the black widow pulsar is slowly eating away its companion star by blasting it with energetic particles.

Pulsating X-ray sources

Pulsating X-ray sources • If material from the companion star is drawn into the magnetic poles, then there will be large quantities of charged particles spiraling in the intense magnetic field • This will produce intense hot spots that will emit copious amounts of X-ray radiation. – X-ray luminosity of such a Pulsating X-ray source is 100, 000 time the luminosity of the Sun (over all wavelengths)

Upper mass limit of Neutron Stars • In Neutron stars the gravity is balanced by two forces. – Degenerate neutron pressure – Strong nuclear force. • If the mass of a neutron star has a mass larger than the maximum limit of 2 to 3 M then the gravitational collapse will be too strong to be resisted by these two forces. • There is no force in nature known to physicists that could stop a massive star from collapsing…. into a Black Hole !!!