A typical neutron star has a radius of just a few kilometres and a mass around 1.5 times that of the Sun. A million ‘Earths’ squashed into a region the size of a city. Neutron stars tend to spin very fast, emitting bright beams of radio waves that illuminate the Universe like a lighthouse. The first observation of such a neutron star, known as a pulsar, was made by Jocelyn Bell Burnell and Antony Hewish in 1967. So regular is the pulse, which sweeps over Earth every 1.3373 seconds, that Bell Burnell and Hewish christened it Little Green Men-1. The fastest pulsar yet discovered, known as PSR J1748-2446ad, rotates 716 times every second. Neutron stars are extremely energetic celestial objects. On 27 December 2004, a burst of energy hit the Earth, blinding satellites and expanding our ionosphere. The energy was released by the rearrangement of the magnetic field around a neutron star called SGR 1806-20, which lies 50,000 light years from Earth on the other side of the galaxy. In a fifth of a second the star radiated more energy than our Sun emits in a quarter of a million years.
The gravitational pull at the surface of a neutron star is 100 billion times that of Earth. Anything that falls onto the surface is flattened in an instant and transformed into nucleon soup. If you were to fall onto the surface of a neutron star, the particles that were once a part of your voluminous atoms would be transformed into neutrons and squashed together so tightly that they would be jiggling around at near light speed in an attempt to avoid each other. This jiggling can support a neutron star with a mass of around two solar masses, but no more. Beyond this limit, gravity wins. If a little more mass were poured onto its surface, the city-sized star would collapse to form a spacetime singularity. Georges Lemaître, a Catholic priest and one of the founders of modern cosmology, described the Big Bang singularity at the origin of our Universe as a day without a yesterday. A singularity formed by gravitational collapse is a moment with no tomorrow. What remains outside is a dark imprint of what once shone: a black hole.
Today, we have concrete observational evidence that our Universe is populated by black holes. The images shown in Figure 1.1 were obtained by the Event Horizon Telescope Collaboration, a network of radio telescopes located across the Americas, Europe, the Pacific, Greenland and Antarctica. The left-hand image shows the central supermassive black hole in the galaxy M87, which lies 50 million light years from Earth. As is so often the case in science, this fuzzy image from far away grows increasingly wonderful as you learn more about what you are looking at.
This black hole has a mass 6.5 billion times that of our Sun and lies within the dark central region of the image, known as the shadow. This region is dark because gravity is so strong that light cannot escape, and since nothing can travel faster than light, nothing can escape. Inside the shadow lies the event horizon of M87’s black hole, a sphere in space of diameter 240 times the distance from the Earth to the Sun. It shields the external Universe from the singularity. The bright disk surrounding the shadow is formed mainly by rays of light emitted from gas and dust spiralling around and into the black hole, their paths twisted and forged into a distinctive doughnut shape by the hole’s gravity.
Figure 1.1. Left: The supermassive black hole at the centre of the galaxy M87. Right: Sagittarius A*, the black hole at the centre of our own galaxy. Both as imaged by the Event Horizon Telescope Collaboration. (European Southern Observatory/EHT Collaboration/Science Photo Library)
The right-hand image is the supermassive black hole at the centre of our own galaxy, Sagittarius A*. At a mere 4.31 million solar masses, it is a minnow by comparison. The glowing disk would fit comfortably within the orbit of Mercury. Its presence was first inferred indirectly, by observing the orbits of stars around it. These stars are known as the ‘S Stars’. The star S2 orbits particularly close to the black hole, with a period of just 16.0518 years. The precision is important, because the detailed observations of S2’s orbit were compared with the predictions of general relativity and used to infer the presence of a black hole well before it was photographed. S2 was observed to make its closest approach to Sagittarius A* in 2018, when it passed within just 120 Astronomical Units of the event horizon.† At closest approach, it was travelling at 3 per cent of the speed of light. Reinhard Genzel and Andrea Ghez received the 2020 Nobel Prize for these high-precision observations performed over many years. They were proof that there is a ‘supermassive compact object at the centre of our galaxy’, in the words of the Nobel Prize committee. They shared the prize with Sir Roger Penrose for his mathematical demonstration ‘that black hole formation is a robust prediction of the general theory of relativity’.
We’ve also detected numerous smaller, stellar mass black holes by detecting the ripples in space and time caused when they collide with each other. In September 2015, the LIGO gravitational wave detector registered the ripples in spacetime caused by a collision between two black holes that occurred 1.3 billion light years from Earth. The black holes were 29 and 36 times the mass of the Sun and collided and merged in less than two tenths of a second. During the collision, the peak power output exceeded that of all the stars in the observable Universe by a factor of 50. By the time the ripples reached us, over a billion years later, they shifted the distance measured along LIGO’s 4-kilometre-long laser ruler arms by one thousandth of the diameter of a proton in a fleeting, wiggling pattern that exactly matched the predictions of general relativity. LIGO and its sister detector Virgo have since detected a host of mergers between black holes. The 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Barry Barish and Kip Thorne for their leadership in designing, building and operating LIGO. The known ‘Stellar Graveyard’ of stellar mass black holes and neutron stars at the time of writing is shown in Figure 1.2.
Taken together, these observations, using different telescopes and techniques, demonstrate beyond reasonable doubt that neutron stars and black holes exist. Science fiction becomes science when experimental observations confirm theories, and as our theoretical voyage takes us along ever stranger paths into ever more tangled intellectual terrain, we should keep reminding ourselves that these absurd things are real. They are a part of the natural world, and we should therefore attempt to understand them using the known laws of Nature. If we fail, we have the chance to uncover new laws of Nature, and this has most assuredly turned out to be the case, beyond even the wildest dreams of the early pioneers.
Figure 1.2. The known stellar mass black holes and neutron stars arranged with the smallest mass objects at the bottom. The smallest circles are the neutron stars and the arrows indicate observed collisions and mergers between pairs of black holes or neutron stars. The numbers on the left are solar masses (mass of Sun = 1 solar mass).