麻豆传媒

IN ABOUT 5 billion years, oursun 鈥� a pretty average-sized yellow star 鈥� will turn into a red giant, its outer layers expanding and consuming the solar system. Eventually, as the gas is blown off, it will become a planetary nebula and leave behind a very faint, very dense object called a white dwarf. A teaspoon of white dwarf material would weigh 5 tonnes here on Earth. Only quantum pressure between electrons stands between a white dwarf and black hole status.

While this might seem exotic, there is another, even more exciting end-of-life possibility for stars: the neutron star. These are the densest stars that we have ever seen, and are the last gravitational stop before black hole territory. A neutron star packs one-and-a-half times the mass of the sun into a space about the size of Los Angeles. One forms when a star at least 10 times bigger than the sun collapses into a white dwarf.

Because of the extra mass of the originating star, the quantum pressure that keeps a white dwarf stable isn鈥檛 powerful enough to prevent further collapse. The electrons and protons in the white dwarf will be forced to merge and become neutrons. Those neutrons can create a stable object because there is another quantum pressure that can kick in 鈥� neutron degeneracy pressure 鈥� and it too will prevent black hole formation within a certain mass range.

With both electron and neutron degeneracy pressure, the key is the Pauli exclusion principle. This says that particles that have a quantum rotation number of 陆, like electrons and neutrons, can鈥檛 share the same quantum state. What does this have to do with stars? In the case of the neutron star, it means that the neutrons have an energy that causes them to push against each other when they get too close. This pushing behaviour causes them to counterbalance gravity, which tries to force them closer together.

The consequences of this simple quantum mechanical principle are lovely. Neutron stars are, in my view, the most exciting objects made out of everyday mass we have ever observed in the universe. I might even say that they are more fascinating than black holes, but I don鈥檛 want to upset anyone! Neutron stars are sometimes described as giant atomic nuclei, and they do indeed have features in common with a tiny atom because they are made primarily of the same particles that exist in an atomic nucleus. But neutron stars remain stable because of gravity, while atomic nuclei are held together by the strong nuclear force, which is described by a particle physics theory called quantum chromodynamics (QCD).

鈥淣eutron stars are, in my view, the most exciting objects made out of everyday mass we have ever observed鈥�

Neutron stars are incredibly important laboratories for studying QCD, which presents us with problems on Earth because the conditions needed to really probe it are hard to come by. We can鈥檛 make a neutron star because the energy required is enormous. Even if we were to make one, the gravitational implications would be disastrous for Earth.

We know that the innards of neutron stars are comprised of what we call 鈥渘uclear pasta鈥�, although maybe it should be called 鈥渘uclear lasagne鈥� to give a clearer visual. At the surface, there is a mix of electrons and ionised atoms, beneath which there is a layer of neutrons, electrons and nuclei. Below that, there are quantum liquids, and then something we physicists refer to as 鈥渜uark soup鈥�, more formally known as quark-gluon plasma. This plasma is difficult to study in the lab, and neutron stars are probably key to understanding how well the particle physics theory matches reality.

Neutron stars can also exhibit unique astronomical behaviours that allow us to gain insight into their structure and the QCD behaviours that may underlie it. For example, some can have large magnetic fields and rotate very quickly. Together, these phenomena translate into an object that emits strong beams of light repeatedly, just like a lighthouse. We call these pulsars and, among other things, they can be used for calibrating clocks. They can also be easier to find than other types of neutron stars, due to the beams calling out so stridently.

Because we can鈥檛 build a neutron star in a lab, we have to look carefully at those we can see and come up with sophisticated ways to analyse that data. In my work with Anna Watts at the University of Amsterdam in the Netherlands, I give a particle physicist鈥檚 perspective on a model that Watts and her other collaborators have been developing of what we call the neutron star鈥檚 鈥渆quation of state鈥�. This is the relationship between how pressure and density vary in the star, and it is highly dependent on the quark soup we know so little about. With time, we hope that, using a combination of data and increasingly sophisticated models, we can gain insight both into neutron star structure and fundamental particle physics.