From White Dwarfs to Black Holes: Lively Quantum Matter in Dead Stars

By Dainius Kilda

Throughout their cosmic lives, stars ‘feed’ on the energy produced by nuclear reactions that fuse lighter elements into heavier ones. Nucleosynthesis in the cores of stars calls for infernal temperatures – the inferno starting at around 10 million K. In this heat, matter exists in the state of plasma: a mess of fully ionized atoms, all their electrons unbound from the nuclei. Thermal energy that fusion produces is also what a star needs to sustain the outward pressure in its core to remain stable against the gravitational force. Just like in a normal gas, the pressure in a stellar plasma comes from ions and electrons whizzing and bouncing around at high speeds, and becomes more intense as they are heated.

After about ten billion years of shining glory, a star like our Sun will exhaust its nuclear fuel and become unable to support itself against its own gravity. It will end its life by shedding the outer layers, and the dead core will succumb to a rapid gravitational collapse, as if being crushed under its own weight. Fortunately, the quantum nature of its tiny constituents – electrons – saves the star from impending catastrophe. On quantum scales, electrons behave as particles and waves (known as de Broglie waves) at the same time. When packed closely together, de Broglie waves of electrons begin to overlap and start interacting in a quantum way. Electrons belong to a class of particles called fermions, so their way of interaction needs to comply with the rules of a fermionic society. To see how this works, imagine fermions inside a box where they can only occupy specific quantized energy states – which is exactly what electrons experience in real matter. But fermions are socially awkward beings who avoid each other: two fermions cannot occupy the same quantum state in the same volume simultaneously. This is the Pauli exclusion principle, one of the fundamental principles of quantum mechanics. The very same exclusion principle guides electrons in atoms to form well-organized orbitals, leading to the chemical elements and the periodic table.

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Mega Magnets

By Maja Bachmann

When discussing extremes in physics, this magazine edition would not be complete without taking a look at the most extreme and powerful magnets that exist. They enable physicists to study the properties of complex materials under extreme conditions and sometimes even new phases of matter are discovered!

The most intense magnets can reach up to 3’500’000 times the strength of the Earth’s natural magnetic field. This would easily bend a bar of steel and so one of the challenges in building these magnet systems is to find materials strong enough to withstand the enormous forces that are generated while using the magnet.

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