Neutron Stars and Black Holes

When stars run out of hydrogen to fuse in their cores, stars below about 10 times the mass of the Sun become red giants and then white dwarfs. More massive stars will become supergiants, and then undergo a supernova, becoming either a neutron star or a black hole[1].

1. Supergiants

Stars over about 10 times the mass of the Sun are massive enough to continue fusion once they have used up all the helium in the shell around their core. Carbon and oxygen fuse into magnesium, neon, and other elements, and this extra radiation pressure causes these stars to expand even further, becoming red or blue supergiants.

Red supergiants are much larger than red giants, and so appear much brighter. Red supergiants can occasionally contract because different elements in the core fuse at different rates. When they contract, they become hotter and therefore bluer, and are then known as blue supergiants. The most massive stars evolve directly into blue supergiants, but many stars oscillate between the two phases[2].

2. Supernova

Energy continues to be released by fusion until the star's core is made of iron nuclei (made of 26 protons and 26 neutrons). Iron nuclei consume, rather than release, energy when they fuse, and so fusion ceases, and the core suddenly collapses under the pull of gravity.

Photograph of a supernova remnant.

Supernova SN 1987A. Image credit: ESA/Hubble and NASA/CC-A.

The cores of these stars are so massive that not even electron degeneracy pressure can contain them. Electrons and protons combine to form neutrons, and the inner core is held together by neutron degeneracy.

Matter falls onto the inner core at about a quarter of the speed of light and bounces off, producing a shock wave. This is known as a core-collapse supernova, and for about a month, the star will be brighter than a whole galaxy[3].

Diagram showing the evolution of different types of stars.

Stellar Evolution. Image credit: NASA/CXC/M.Weiss/Public domain.

3. Neutron stars

In stars that were originally about 10 to 25 times the mass of the Sun, the core remains stable, and is now known as a neutron star[4]. Neutron stars are typically about 1.4 times the mass of the Sun, and are about 20 km wide[5]. They can rotate hundreds of times a second, and have a large magnetic field because they also contain charged particles, like electrons and protons.

Some neutron stars have magnetic fields so high that quantum field theory needs to be invoked to describe their behaviour. These may be the most magnetic objects in the universe[6a].

German astronomer Walter Baade and Swiss astronomer Fritz Zwicky first proposed that supernovae could transform main sequence stars into neutron stars in 1934[7].

Artist's impression of a neutron star, the magnetosphere extends well beyond the radius of the neutron star.

Artist's impression of a neutron star and its magnetic field. Image credit: ESO/L.Calçada/CC-A.

Polish astronomer Aleksander Wolszczan and Canadian astronomer Dale Frail discovered the first extrasolar planets around neutron star PSR B1257+12 in 1992[8]. PSR B1257+12 is now thought to have at least three planets. These range from being about 2% as massive as Earth, to being about four times as massive[9]. Any life that may have existed on these planets would almost certainly have been destroyed by the supernova that created the neutron star.

The atmosphere of a neutron star is very thin, and its movement is fully controlled by the neutron star's magnetic field. Charged particles are accelerated by the magnetic field lines, and are drawn to the magnetic poles where they emit radiation, usually in the form of radio waves. This is known as synchrotron radiation.

From our perspective, the radio waves appear as pulses that occur when beams of radiation pass the Earth, as the star rotates. This is why neutron stars are sometimes known as pulsars.

The surface of a neutron star is probably composed of a lattice of iron. Below this, nuclei increase in neutrons and, even further down, neutrons exist freely of nuclei, in a superfluid[10]. A superfluid is a fluid that behaves as if there is no friction, due to the laws of quantum mechanics.

The core of a neutron star exists under such extreme forces that its composition is not known.

If a neutron star is in a binary system, and gains matter from its companion, the matter is drawn to the magnetic poles, releasing enormous amounts of energy in the form of X-rays[6b].

4. Black holes

The cores of the most massive stars can't even be held together by neutron degeneracy and collapse in on themselves, becoming black holes. Black holes have such a strong gravitational field that almost nothing can escape after it reaches a certain point. This is known as the event horizon, which occurs at the Schwarzschild radius.

The only thing that appears to escape a black hole is Hawking radiation, which is a consequence of quantum mechanics[11].

In 1973, Israeli-American physicist Jacob Bekenstein and American physicist John Archibald Wheeler devised the no-hair theorem, which states that only three properties can be attributed to black holes: mass, charge, and angular momentum[12].

Artist's impression of a black hole surrounded by a disc of material.

Artist's impression of a black hole surrounded by a disc. Image credit: XMM-Newton, ESA, NASA/Public domain.

If a person were to approach the event horizon of a black hole, then what would happen next is a matter of perspective. For those watching from beyond the event horizon, gravitational time dilation means that an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it. Light also appears redder near the black hole, and so before an object reaches the event horizon it becomes too dim to be seen[13a].

From the perspective of a person falling into a black hole, they would cross the event horizon in a finite time, although they would not be aware of when this was, as they would not be able to determine its exact location. In most cases, they would be drawn to the centre where they would probably be torn apart in a process known as spaghettification[13b].

One day, it may be possible for people to use black holes to travel to a different space or time, although this idea is still highly speculative because no one knows what's at the centre of a black hole. Classical relativity predicts a singularity, a region of zero volume containing all of the star's mass, but this is probably not correct because a theory of quantum gravity is needed to explain the behaviour of objects so small and heavy.

When objects fall into a black hole, they add to its mass. A non-rotating black hole about 10 times the mass of the Sun will have an event horizon about 30 km in diameter, this diameter will increase by about 3 km for every solar mass that falls into it*.

The most massive black holes are known as supermassive black holes, and these are thought to exist within the centre of most galaxies. The Milky Way contains the supermassive black hole Sagittarius A*, which is about 4 million times as massive as the Sun[14].

Using:

2GM/c2
≥ RSch,

where RSch is the Schwarzschild radius:

A black hole 1 times the mass of the Sun will have a Schwarzschild radius of:

2 × 6.674×10-11 × (1×1.99×1030)/299,792,4582
≥ 2,956 m ≳ 3 km.

For a black hole 2 times the mass of the Sun:

2 × 6.674×10-11 × (2×1.99×1030)/299,792,4582
≥ 5,911 m ≳ 6 km.

For a black hole 3 times the mass of the Sun:

2 × 6.674×10-11 × (3×1.99×1030)/299,792,4582
≥ 8,866 m ≳ 9 km.

...

For a black hole 10 times the mass of the Sun:

2 × 6.674×10-11 × (10×1.99×1030)/299,792,4582
≥ 29,555 m ≳ 30 km.

5. References

  1. Chandra, 'Supernovas and Supernova Remnants', last accessed 15-02-16.

  2. Saio, H., Georgy, C. and Meynet, G., 2013, 'Evolution of blue supergiants and alpha Cygni variables: puzzling CNO surface abundances', Monthly Notices of the Royal Astronomical Society, 433, pp.1246-1257.

  3. Burrows, A., 2000, 'Supernova explosions in the Universe', Nature, 403, pp.727-733.

  4. Cambridge Physics, 'Neutron Stars', last accessed 15-02-16.

  5. NASA: Imagine the Universe, 'Neutron Stars and Pulsars', last accessed 15-02-16.

  6. (a, b) Klus, H., 2015, 'Breaking the quantum limit: the magnetic field of neutron stars in extra-galactic Be X-ray binaries', PhD thesis.

  7. Baade, W., and Zwicky, F., 1934, 'On super-novae', Proceedings of the National Academy of Sciences, 20, pp.254-259.

  8. Wolszczan, A., and Frail, D. A., 1992, 'A planetary system around the millisecond pulsar PSR 1257+ 12', Nature, 355, pp.145-147.

  9. Konacki, M. and Wolszczan, A., 2003, 'Masses and orbital inclinations of planets in the PSR B1257+ 12 system', The Astrophysical Journal Letters, 591, pp.147-150.

  10. Chamel, N. and Haensel, P., 2008, 'Physics of Neutron Star Crusts', Living Reviews in Relativity, 11, pp.17-22.

  11. Hawking, S. W., 1975, 'Particle creation by black holes', Communications in mathematical physics, 43, pp.199-220.

  12. Misner, C. W., Thorne, K. S., Wheeler, J. A., 1973, 'Gravitation', W. H. Freeman.

  13. (a, b) University of California, Santa Barbara, 'What happens beyond an event horizon? ', last accessed 15-02-16.

  14. Schödel, R., et al, 2002, 'A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way', Nature, 419, pp.694-696.

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