Neutron Stars & Black Holes (OCR A Level Physics): Revision Note

Neutron Stars & Black Holes

• In the final evolutionary stages of a massive star's life cycle, the remnant left after a supernova explosion will either be:

  • A neutron star

  • A black hole

Characteristics of a Neutron Star

• Neutron stars are objects formed in stars with cores which have masses greater than the Chandrasekhar limit

• Neutron stars are:

  • Extremely dense

  • Very small

• To put into perspective how dense and small a neutron star is:

  • The density of a neutron star is 4 × 10¹⁷ kg m⁻³

  • The density of the Earth is 5 × 10³ kg m⁻³

  • The density of a white dwarf is 1 × 10⁹ kg m⁻³

  • A neutron star with the mass of the Sun would be only 30 km in diameter

• Some neutron stars rotate rapidly (up to 600 times per second) emitting bursts of highly directional electromagnetic radiation

  • These stars are called pulsars

Characteristics of Black Holes

• The core collapses into an infinitely dense point called a singularity

• A singularity is defined as:

  A theoretical point at which matter is compressed to an infinitely small point and the laws of physics, as they are currently understood, break down

• The gravitational field is so strong that nothing, not even light, can escape it

  • This region is known as a black hole

• The boundary at which light and matter cannot escape the gravitation pull of the black hole is called the event horizon

• The escape velocity beyond the event horizon is greater than the speed of light, hence photons cannot escape from a black hole

Characteristics of a Black Hole