Neutron Stars & Black Holes

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.
• Neutron stars are extremely small and dense (approximately $10^{17}$ 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 material 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.

Pulsars

• Some neutron stars rotate rapidly (up to 600 times per second) emitting bursts of highly directional electromagnetic radiation. These stars are called pulsarsA rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles. As the star rotates, the beams sweep across space like a lighthouse, producing periodic pulses when detected from Earth..
• Pulsars are much easier to identify than slow, or non-rotating, neutron stars 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 horizonThe boundary around a black hole inside of which light cannot escape. At this boundary, the escape velocity equals the speed of light..
• 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.

Schwarzschild radius

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

• Where $R_s$ = the Schwarzschild radius (m), $G$ = gravitational constant, $M$ = mass of the black hole (kg), and $c$ = speed of light (m s$^{-1}$).

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. Sagittarius A*, the one in our galactic centre, has a mass of about 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. Crucially, this is thought to be strong evidence that supermassive black holes exist at the centres of nearly all large galaxies.