First of all, I must apologise to you, my faithful few, for the lack of an article last week. Firstly, I was away visiting a friend in Spain and hadn’t got an article close enough to finishing for publishing. Secondly, I am still suffering with my back, which has been making it somewhat difficult to sit and do the necessary research to make these articles as entertaining and explanatory as I try and do.
Anyway, all that aside, I’m sort of coming in a day early this week, and I’m heading back to the cosmos in an attempt to explain Neutron Stars and Pulsars. What they are, where they come from, and what they do. Hopefully. As ever, you’re the judge, have a read and let me know your thoughts.
Among all the objects in the observable universe, neutron stars are arguably the most extraordinary. They are the collapsed remnants of massive stars that have ended their lives in titanic supernova explosions, compressing more mass than our Sun into a sphere only about twenty kilometres (about 12.5 miles) across. Now then, these astonishing objects are so incredibly and immensely dense that they challenge our understanding of matter itself and offer scientists a glimpse into physical conditions that can never be recreated here on Earth.
So, where do they come from? Well, the story of a neutron star begins with a massive star, typically one containing at least eight times the mass of our own Sun. Throughout most of its life, a star generates energy through nuclear fusion, using immense temperatures and pressures in its core to combine lighter elements into heavier ones. During the longest phase of a star’s existence, hydrogen nuclei fuse together to form helium, releasing enormous amounts of energy in the process. This energy creates an outward pressure that balances the inward pull of gravity that keeps the star stable and shining brightly.
As the supply of hydrogen gradually becomes depleted, the star begins fusing progressively heavier elements. Helium fusion produces carbon and oxygen. In stars that are sufficiently massive, carbon is then fused into elements such as neon, sodium and magnesium. Neon fusion creates oxygen and magnesium, while oxygen fusion produces silicon, sulphur and a variety of other elements, too many to list here, and, if I’m honest, I don’t know all of them and if I carry on researching them, this article is never going to see the light of day (see what I did there?). Eventually, silicon fusion forms iron and nickel in the star’s core, with each successive stage requiring temperatures far greater than the last, which creates a layered structure within the star somewhat like the layers of an onion.
This process effectively turns a massive star into a cosmic factory. Over millions of years it manufactures increasingly heavier elements, transforming hydrogen into helium, helium into carbon and oxygen, and then producing many of the elements that make up planets, oceans and living things. The carbon in your cells, the oxygen you breathe, the calcium in your bones and much of the iron in your blood were all forged within stars long before our Solar System existed. Cool, yeah? We really are made from stardust!
Anyway, this process continues until iron accumulates in the core, which in turn marks a critical turning point in a star’s life because, unlike lighter elements, fusing iron does not release energy. Instead, it requires energy, and as a result, the star suddenly loses its ability to generate the outward pressure needed to oppose gravity. Once the core becomes dominated by iron, the balance that has sustained the star for millions of years collapses as gravity gains the upper hand and the core implodes in just a fraction of a second.
This collapse is so incredibly violent that the outer layers of the star are blasted into space in a supernova explosion, while deep within the collapsing core, electrons and protons are crushed together to form neutrons. What remains is an incredibly compact object composed almost entirely of neutron matter. If the remaining core contains too much mass, the collapse continues and a black hole forms. If the mass falls within the appropriate range, however, the core stabilises as a neutron star.
A useful analogy is to imagine gravity and quantum forces playing a tug-of-war.
As gravity is trying to crush the star, a quantum effect known as neutron degeneracy pressure is pushing back. If the core’s mass is within the appropriate range, that is 1 to 3 times the mass of the Sun, the two balance and a neutron star survives. If gravity is too strong, gravity wins and a black hole forms. So basically that means, if the collapsed core is heavy enough to avoid becoming a white dwarf, but not so heavy that it must become a black hole, the inward pull of gravity is balanced by neutron pressure and the object settles into a stable neutron star.
That is, if the remaining core is relatively small, it becomes a white dwarf, and this happens to stars less massive than those that produce neutron stars. If the core is roughly between about 1 and 3 times the mass of our Sun, the collapse can be halted by quantum-mechanical forces between neutrons, and the core stabilises to become a neutron star. If the core is more massive than the maximum mass a neutron star can support, as even neutron pressure cannot resist gravity, the collapse continues indefinitely and creates a black hole. Simple stuff, isn’t it!
It is the supernova itself which is responsible for creating and dispersing many heavy elements into interstellar space, enriching future generations of stars and planets. Even heavier elements, including gold and platinum, are now thought to be produced primarily during neutron-star mergers, where two neutron stars collide in one of the most energetic events in the universe.
The resulting neutron star is almost impossibly compact. Although it typically measures only about twenty kilometres in diameter, it contains roughly one and a half times the mass of the Sun. Some neutron stars are even more massive. Which means that a quantity of neutron-star material no larger than a sugar cube would weigh around a billion tons on Earth, and make a decent meal for Nibbler and his kind from Futurama, remember I mentioned this in a previous article? Another comparison offered by NASA states that a teaspoon of neutron-star matter would weigh millions of tons. If our own planet were compressed to the same density, Earth would shrink to a sphere only a few hundred metres across.
Such enormous density gives rise to equally extraordinary gravity. At the surface of a neutron star, gravity can be approximately one hundred billion times stronger than it is on Earth, and an object falling towards the surface would accelerate to a significant fraction of the speed of light before impact. The gravitational field is so intense that it bends light itself, allowing an observer to see more than half of the star’s surface simultaneously because light from the far side can be curved around the object on its journey through space. Remember my article on spacetime?
Despite decades of study, scientists still do not fully understand what lies inside a neutron star. The outer layers are thought to consist of an ultra-dense crust of atomic nuclei immersed in a sea of electrons. Deeper inside, neutrons dominate the structure, and are packed together far more tightly than in ordinary atomic nuclei. At the very centre, conditions may be so extreme that matter exists in exotic forms unknown elsewhere in the universe. The core may contain superfluid neutrons, superconducting protons, strange particles called hyperons, or even quark matter, in which the fundamental constituents of protons and neutrons exist freely. Determining the true nature of neutron-star interiors remains yet another one of the greatest unsolved problems in modern physics.
As is the way with all things cosmic, some neutron stars can reveal themselves in a more dramatic fashion. These objects, known as pulsars, are rapidly rotating neutron stars that emit beams of electromagnetic radiation from regions near their magnetic poles. The magnetic axis is usually tilted relative to the spin axis, so as the star rotates the beams sweep through space in much the same way that the beam from a lighthouse sweeps across the sea. Whenever one of these beams crosses Earth’s line of sight, astronomers detect a pulse of radiation, that is, the object appears to be flashing on and off, even though what we are actually witnessing is the star’s rapid rotation. Every pulsar is a neutron star, but only those whose beams happen to point towards Earth can be observed as pulsars.
The incredible spin rates of pulsars originate during the collapse of their parent stars. As the original star’s core shrinks from hundreds of thousands of kilometres across to only a few tens of kilometres, angular momentum is conserved, similar to a figure skater spinning faster when pulling their arms inward, and as a result, neutron stars can rotate at extraordinary speeds. While many pulsars rotate once every few seconds, others complete hundreds of rotations every second. The fastest known pulsar, PSR J1748−2446ad, spins approximately 716 times per second, equivalent to about 43,000 revolutions every minute.
The discovery of pulsars was one of the most remarkable moments in twentieth-century astronomy. In 1967, while analysing data from a radio telescope at the University of Cambridge, graduate student Jocelyn Bell Burnell noticed a highly regular series of radio pulses arriving from space. The signals were so precise that, for a short time, members of the research team jokingly referred to the source as LGM, standing for Little Green Men! Further observations soon showed that the signals originated from a natural astronomical phenomenon, leading to the discovery of the first known pulsar and providing the first direct evidence for the existence of neutron stars.
One of the most remarkable characteristics of pulsars is their astonishing precision. The regularity of their pulses makes them among the best natural timekeepers in the universe. In some cases, their long-term stability rivals that of atomic clocks here on earth. This precision enables astronomers to use pulsars for testing Einstein’s theories of relativity, searching for gravitational waves, creating highly accurate timing systems and even investigating methods of navigating future spacecraft using pulsars as galactic positioning beacons!
While pulsars are impressive, there is an even more extreme variety of neutron star that exists. These objects, known as magnetars, possess the strongest magnetic fields known in nature. The standard unit for measuring magnetic fields is the Tesla (T), and according to the European Space Agency, their magnetic fields can reach around one hundred billion Tesla. For comparison, Earth’s magnetic field is only a tiny fraction of a Tesla, that is 0.00005 T, and even powerful MRI scanners operate at only 1 – 3 T. The magnetic field of a magnetar is powerful enough to distort atomic structures, fracture the star’s crust and release immense bursts of X-rays and gamma rays. Only a few dozen magnetars have been identified, which makes them among the rarest of known stellar remnants.
Neutron stars also play a crucial role in some of the most violent events in the cosmos. In binary systems, two neutron stars can orbit one another for millions or even billions of years, and as they do so, they gradually lose energy through the emission of gravitational waves causing them to spiral closer together. Eventually they collide and merge, and these mergers generate enormous bursts of energy, produce ripples in spacetime and create spectacular explosions known as kilonovae (article on kilonova vs supernova coming soon). They are also believed to be responsible for producing many of the universe’s heaviest elements, including gold and platinum, as mentioned above.
The landmark detection of GW170817 in 2017, the first-ever joint detection of gravitational waves and electromagnetic radiation from a binary neutron star merger, represented humanity’s first direct observation of gravitational waves generated by a neutron-star merger and marked the beginning of a new era in astronomy.
What makes neutron stars so fascinating is that they exist at the boundary between the familiar and the extreme. They are not as mysterious as black holes, whose interiors remain hidden behind event horizons, yet they exhibit physical conditions far beyond anything achievable in terrestrial laboratories. They allow scientists to study gravity, nuclear physics, magnetism and relativity under the most demanding conditions imaginable. Every new observation brings us closer to understanding the behaviour of matter at densities and energies that push the laws of nature to their limits.
So, in many ways, a neutron star can be thought of as a giant atomic nucleus floating through space, an object the size of a city, containing more mass than the Sun, spinning hundreds of times each second, powerful enough to bend light and warp spacetime, and still holding secrets that challenge some of the brightest minds in modern science!