FOR all the talk about a mysterious big bang at the start of the universe, we actually don’t have to go back too far in history to see big bangs. Some stars, like our sun, will end their lives rather quietly, slowly blowing off layers, possibly destroying solar systems in their wake, and leaving behind beautiful structures that garnered the name “planetary nebulae” before we understood what they were. But other, more massive, stars will go out in a fabulous phenomenon called a supernova, where the outer layers of the star collapse onto its core, igniting an explosion.

Supernovae are quite sudden and have at points in history been observed with the naked eye. The most famous example is Supernova 1006, so named because it occurred in the year AD 1006. Records from across Asia and North America indicate that communities around the world noted its occurrence. These explosions are so powerful that they can produce elements that can’t be made in stars, which can only make atomic elements as heavy as iron.

Supernovae can also occur when a white dwarf ends up in a binary orbit with what we call a companion star. White dwarfs are themselves the remnants of long-gone stars – our sun is expected to leave behind a white dwarf one day. A typical white dwarf will have about 70 per cent of the mass of the sun, squeezed into at most 2 per cent of the sun’s radius. They are held together by gravity, but don’t collapse into a black hole because of quantum pressure between their many electrons.

As these little ghosts wander through their home galaxies, they sometimes cross paths with regular stars and become gravitationally entangled, forming a binary relationship. The white dwarf’s gravitational pull can begin to rip gas away from its companion, ultimately grabbing on to more than it can handle, leading to an explosion. This is another kind of supernova – a type Ia supernova – to be distinguished from the collapse of supergiant stars described above, which are type II supernovae.

“Supernovae are so powerful that they can produce elements that can’t be made in stars”

As these explosions occur, multiple transitions are happening: the gases and plasma in the explosion are being blown off at high speeds and also at such high energy that they can engage in forms of nuclear fusion that can’t happen in their progenitor (ancestor) stars.

One of two things is thought to happen. In one scenario, a black hole forms at the centre of the supernova, a phenomenon in which there is such an enormous concentration of mass that the structure of space-time is radically different from what we consider to be normal. These black holes can consume all forms of matter and energy without restriction, even light.

The alternative possibility is the formation of a neutron star. These are the most compact and dense non-space-time phenomena in the universe, even more so than black holes. Think fitting the mass of the sun into London’s city centre. A very tight squeeze! Like white dwarfs, they are held together by gravity, but don’t collapse under their own mass thanks to the quantum properties of particles that comprise them.

Reading all of this, you might have the impression that we have a pretty good grasp of the physics that underpins supernovae. But actually, many mysteries remain, for example the abundances of atomic elements that we expect to be fused in the explosions.

When it comes to neutron stars, we are still confused about fundamental properties like the state of matter inside them and what the properties of the star are when it exists on the boundary between becoming a neutron star and being massive enough to collapse into a black hole.

Observations of neutron star PSR J0740+6620 over the past few years are challenging our understanding. Using radio telescopes at Green Bank Observatory in West Virginia and Arecibo in Puerto Rico, astronomers have found that this particular neutron star has a mass more than twice the sun’s but it is only some 20 or 25 kilometres in diameter. This is so dense that it is close to the boundary of where we might expect a black hole to form, yet there it is, a stable neutron star.

Follow-up work with the Neutron Star Interior Composition Explorer X-ray (NICER), an X-ray telescope on the International Space Station, is affirming that this star is quite dense. Two recent preprints from the NICER team, including one for which I am a co-author, give estimates for the mass and radius of the star. But our papers disagree slightly on some of these values.

It isn’t clear why this is, although it is probably due to differences in the data analysis techniques. Some might view this as upsetting, but I think it is exciting. With neutron stars, we are just getting started.