Supernovae, neutron stars and black holes

1. Neutron stars and black holes:

  • After a massive star has blown itself apart in a supernova explosion, a neutron star is often left at the star’s core.
  • Neutron stars are even more dense than white dwarfs, as they are made only from highly dense nuclear material.
  • A neutron star of mass about 1.5 times that of the Sun has a radius of only about 12km.
  • Some very massive stars (in the region of 20 solar masses) collapse at the end of their lives in an even more spectacular fashion.
  • As their nuclear fuel runs out, the speed of that collapse is so fast that the gravitational tide even manages to collapse the neutrons at its core.
  • Figure 1 Neutron star collusion
  • Under these circumstances a black hole is formed (figure 2). A black hole is so dense that not even light can escape from it, because its escape velocity is higher than the speed of light.
  • Figure 2 Black holes formed

2. Gamma ray bursts

  • Neutron stars spin very rapidly on their axes.
  • Many such stars spin round several hundred times a second.
  • These rapidly spinning stars are known as pulsars because they emit radiation along their axes of rotation.
  • As supergiant stars collapse into neutron stars or black holes, they emit gamma-ray bursts.
  • As matter collapses into the centre of a very massive star, collisions between particles produce very energetic gamma rays, which are emitted along the axis of rotation of the star (figure 3).
  • Figure 3 Gamma ray burst
  • It is thought that the most energetic gamma-ray bursts are produced when a supermassive star (of some 50 solar masses) collapses into a black hole.
  • The fact that gamma-ray bursts last for a few seconds (or at the most a few minutes) indicates how rapidly larger stars collapse.
  • A gamma ray burst produced by a supergiant star, close to the Earth, could have catastrophic consequences.
  • The radiation dose, on the side of the Earth lacing the star could be lethal for all animals.
  • The fossil record shows that there was a mass extinction of animals on the Earth some 450 million years ago. One possible explanation is that this was caused by a gamma-ray burst.

3. Supernovae – glorious endings

  • Nuclear fusion does not continue beyond the elements iron and nickel. Figure 4(a) shows a large star towards the end of its life.


    Figure 4 (a) A large star, more than eight solar masses, has a layered structure, with iron at its core.

  • Owing to the various stages of nuclear fusion, the star is layered like an onion with shells of different nuclei – iron in the centre, with helium and hydrogen in the outer layers.
  • In Figure 4(b), which shows the core of the star, the nuclear fuel has just been exhausted, and without the outward pressure from the thermonuclear fusion process, the pull of gravity begins to collapse the star.
  • Under the intense gravitational forces, the core collapses in a matter of seconds. The outer part of the core can reach speeds as high as 20-30% of the speed of light, and the centre of the core rises to temperatures as high as 100 billion kelvin (101 K).
  • At these temperatures, the iron nuclei begin to dissociate into helium nuclei, protons and neutrons.


    Figure 4 (b) The core begins to collapse as the nuclear fuel runs out.

  • In such high temperatures and pressures, protons and electrons can combine, in a reverse heta decay, to form neutrons and neutrinos:
  • In this way the centre of the core turns into a ball of neutrons, which will become a neutron star, Figure 4(c).
  • At this point the core collapses no further and the infalling matter rebounds, producing a shock wave, which spreads outwards as shown in Figure 4(d).
  • The extremely high temperature in the centre of the star restarts the nuclear reactions in the outer layers of the star and a huge amount of energy, perhaps 1046J, is produced in a few seconds.

    Figure 4 (c) The rapid collapse produces a core of neutrons

  • The shock wave moving out from the centre of the star blows the outer layers apart, and energy moves out into space at an enormous rate. This is a supernova (a type 2 supernova).
  • Supernovae are amongst the brightest objects in the sky. They outshine an entire galaxy and in a matter of a few seconds emit more energy than the Sun does in its entire lifetime.
  • Supernovae are colossal events and highly significant for our existence. The energy produced in a supernova explosion produces heavy elements beyond iron, and it is from the remnants of a supernova that our Sun and our Solar System formed.


    Figure 4 (d) A shock wave rebounds off the neutron core.

4. Type 1a supernovae

  • Many stars exist as binary stars, which means that two stars rotate about a common centre of gravity. Such stars can coexist in stable orbits for millions of years.
  • However, if the stars are of different masses, they evolve at different rates.
  • Figure 5 shows a pair of stars that are a little more massive than the Sun.
  • The star A has passed through the main sequence and red giant stages and is now a white dwarf.


    Figure 5 a pair of stars that are a little more massive than the Sun

  • Later, star B moves into the red giant stage, and as it expands matter is pulled into the white dwarf.
  • If the mass of the white dwarf grows to be larger than 1.4 solar masses, the star collapses. At this point carbon and oxygen in the white dwarf suddenly begin to undergo nuclear fusion. Such a rapid collapse, followed by the reignition of nuclear fusion, can trigger a supernova explosion.
  • Type la supernovae are easily identified by astronomers for two reasons.
  • First, they have approximately the same absolute magnitude, because they always occur in stars with about 1.4 solar masses.
  • Secondly, the rapid onset of fusion in the collapsing star produces the nuclear isotope Ni.
  • As explained earlier, this isotope decays to cobalt-56 with a half-life of 6 days, and then cobalt-56 decays to iron-56 with a half-life of 77 days.
  • So, type la supernovae have a characteristic light curve (Figure 6), which decays on a time scale governed by the half-lives of the isotopes 2 Ni and Co.


    Figure 6 All type 1a supernovae produce light curves of a characteristic shape.

  • As the two isotopes decay, massive numbers of high-energy photons are emitted, which power the light emitted by the remnants of the expanding supernova.
  • Because type la supernovae have a characteristic absolute magnitude, they are used as standard candles. This means that astronomers can calculate the distance of a galaxy from Earth by measuring the apparent magnitude of a type la supernova in the galaxy.
  • This use of type 1a supernovae has led to a controversial result. Measuring the distance to very distant galaxies has led cosmologists to the conclusion that the Universe was expanding more slowly in the past.
  • For a long time, it was assumed that the action of gravity would cause the expansion of the Universe to slow down.
  • The idea of a Universe with accelerating expansion is a most controversial idea.
  • There is no firm explanation for this theory yet, but cosmologists suggest that some ‘dark energy’ in the Universe may be responsible for an accelerating expansion.

5. Supermassive black holes at the centre of galaxies:

  • Supermassive black holes (SMBHs) are massive black holes that reside at the centers of galaxies, including our own Milky Way. They have masses millions or even billions of times that of our sun.
  • Characteristics of SMBHs:
    – Massive: SMBHs have enormous masses, typically between [math] 10^5 \, \text{and} \, 10^{10}  [/math] solar masses (M).
    -Central location: They are found at the centers of galaxies, including elliptical, spiral, and dwarf galaxies.
    – Gravitational dominance: SMBHs dominate the gravitational potential of their host galaxies, influencing the motion of stars and gas.
    – Accretion disks: SMBHs are surrounded by accretion disks, where material is heated and emits intense radiation.
    – Quasars and AGN: SMBHs can power quasars (incredibly luminous objects) and active galactic nuclei (AGN), which are extremely bright and energetic.
  • Observational evidence for SMBHs:
    – Stellar motions: Stars near the galactic center move at high velocities, indicating a massive, unseen object.
    – Gas dynamics: Gas clouds and accretion disks near the galactic center show signs of strong gravitational influence.
    – X-rays and gamma rays: Telescopes detect X-rays and gamma rays from hot accretion disks and relativistic jets.
    – Gravitational waves: The detection of gravitational waves by LIGO and VIRGO provide strong evidence for SMBH mergers.
  • Figure 7 Supermassive black holes at the centre of galaxies

7. Schwarzschid radius:

  • The event horizon for a black hole can be described as ‘the point of no return’, that is the boundary beyond which the gravitational pull becomes so big that escape becomes impossible.
  • So, if you are in a spacecraft just outside the event horizon of a black hole you could escape with very powerful rockets.
  • However, once inside the event horizon the escape velocity is higher than the speed of light, and a spacecraft would be trapped (and of course torn apart by the immense gravitational forces).
  • We can calculate the approximate radius of the event horizon using Newton’s law of gravitation.
  • The gravitational potential energy of a spacecraft, of mass m, at a distance R from the centre of a black hole of mass M, is given by
  • [math] E_p = -\frac{GMm}{R} [/math]

  • If the spacecraft is to escape, its kinetic energy,

  • [math] K.E = \frac{1}{2} mv^2 \\ \frac{1}{2} mv^2 – \frac{GMm}{R} > 0 \\ \frac{1}{2} mv^2 > \frac{GMm}{R} [/math]

  • The radius of the event horizon is known as the Schwarzschild radius, , at which point the escape velocity is the speed of light, c, So

  • [math] \frac{1}{2} mc^2 = \frac{GMm}{R_s} \\ \text{and} \\ R_s c^2 = \frac{2GM}{c^2} [/math]
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