Neutron Stars and Pulsars
A neutron star packs more than the mass of the Sun into a sphere the size of a city, supported by neutron degeneracy and nuclear forces; when it spins and beams radiation, we see it as a pulsar.
Definition
A neutron star is a compact stellar remnant a few times denser than an atomic nucleus, supported chiefly by neutron degeneracy pressure and nuclear forces, and a pulsar is a rapidly rotating, strongly magnetized neutron star observed as pulses of radiation.
Scope
The topic covers the formation of neutron stars in core-collapse supernovae, their internal structure and the poorly known dense-matter equation of state, the maximum neutron-star mass, rotation-powered pulsars and their use as precise clocks, and the extreme magnetic fields of magnetars.
Core questions
- How are neutron stars formed and what supports them?
- What is the matter inside a neutron star like?
- Why do pulsars emit regular pulses?
- What is the maximum mass a neutron star can have?
Key concepts
- neutron degeneracy
- equation of state
- pulsar
- magnetic dipole
- spin-down
- magnetar
- glitch
Key theories
- Neutron degeneracy and the dense-matter equation of state
- Neutron stars are supported by neutron degeneracy pressure stiffened by the repulsive nuclear force; their structure follows from the equation of state of matter beyond nuclear density, which sets the relation between mass and radius and the maximum mass.
- The rotating magnetic dipole model of pulsars
- A pulsar is a rapidly spinning neutron star whose strong, misaligned magnetic field channels beams of radiation along its poles; as the star rotates the beam sweeps past Earth, producing the observed clock-like pulses while magnetic braking gradually slows the spin.
Mechanisms
When a massive star's iron core collapses, electrons combine with protons to form neutrons and the core rebounds into a neutron star roughly twenty kilometers across. Conservation of angular momentum and magnetic flux leaves it spinning rapidly with an enormous magnetic field; charged particles accelerated along the field lines produce beamed radiation seen as pulses, while magnetic torques slowly drain its rotational energy.
Clinical relevance
Neutron stars are natural laboratories for matter at supranuclear densities and for strong-field gravity; millisecond pulsars rival atomic clocks and are used to test general relativity and to search for gravitational waves, and neutron-star mergers produce heavy elements and detectable gravitational-wave signals.
History
Baade and Zwicky proposed neutron stars in 1934, Oppenheimer and Volkoff modeled them in 1939, and Jocelyn Bell Burnell discovered the first pulsar in 1967; Pacini and Gold soon identified pulsars as rotating magnetized neutron stars, a picture confirmed by the pulsar in the Crab supernova remnant.
Debates
- The neutron-star equation of state and maximum mass
- The behavior of matter above nuclear density, and hence the maximum neutron-star mass and the possible presence of exotic phases such as quark matter, remain uncertain; mass and radius measurements and gravitational-wave observations are steadily narrowing the possibilities.
Key figures
- Jocelyn Bell Burnell
- Antony Hewish
- Fritz Zwicky
- Franco Pacini
Related topics
Seminal works
- hewish1968
- shapiro1983
Frequently asked questions
- How can something so small be so heavy?
- In a neutron star, gravity has crushed matter to densities comparable to or exceeding that of an atomic nucleus, so a teaspoon of the material would weigh billions of tons; this lets more than a solar mass fit within a radius of only about ten kilometers.
- Why do pulsars pulse so regularly?
- A pulsar emits radiation in narrow beams from its magnetic poles, and because the neutron star rotates rapidly and steadily, each time a beam sweeps across Earth we record a pulse, making pulsars among the most precise natural clocks known.