Neutron Stars & Black Holes (AQA A Level Physics)

Neutron Stars

• Neutron stars are objects which form after a supernova has ejected the outer layers of a star into space

  • A core which has a mass between 1.4 and 3 solar masses will become a neutron star

• A neutron star is defined as:

  An extremely dense collapsed star made up of neutrons

• Neutron stars are extremely small and dense (~10¹⁷ kg m⁻³) 

  • A neutron star with the mass of the Sun would have a diameter of about 30 km

  • A teaspoon of neutron star would have a mass of about 100 million tonnes

• The immense gravitational forces acting on the core crush the electrons and protons until they combine into neutrons, via reverse beta decay:

• Further collapse is prevented by neutron degeneracy pressure

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

  • These stars are called pulsars

What is a pulsar?

A fast-rotating neutron star is called a pulsar

• Pulsars are much easier to identify than slow, or non-rotating, neutron stars

  • This is because they emit radiation periodically which makes them easier to detect

  • In particular, they emit radio waves strongly, and sometimes X-rays and gamma rays

Black Holes

• After a supernova has ejected the outer layers of a star into space, the most massive cores can collapse into an infinitely dense point called a singularity

  • A core which has a mass greater than 3 solar masses will become a black hole

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

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

• The escape velocity beyond the event horizon is greater than the speed of light

  • This is why black holes cannot be seen directly, as photons cannot escape beyond the event horizon

What is a black hole?

A black hole is an object which is so dense that its escape velocity is greater than the speed of light

Schwarzchild Radius of a Black Hole

• The radius of a black hole’s event horizon is called the Schwarzschild radius and is given by:

• Where:

  • = the Schwarzschild radius (m)

  • = gravitational constant

  • = mass of the black hole

  • = speed of light

Supermassive Black Holes

• Observations of stars at the centre of the Milky Way suggest that a mass equivalent to millions of stars is contained in a very small volume

• Astronomers determined that the mass at the galactic centre is, in fact, a supermassive black hole 

  • Sagitarrius A*, the one in our galactic centre, has a mass of 4 million solar masses

• Since this discovery, over 150 supermassive black holes have been identified at the centres of other galaxies similar to the Milky Way

  • This is thought to be strong evidence that supermassive black holes exist at the centres of nearly all large galaxies